Several nano-carriers including protein, casein, chitosan, gelatin, zein, polyethylene glycol, arabinogalactan, poly-D, L-lactide-co glycolide, poly L-lysine and polyaniline are used for encapsulation. According to literature, chitosan-based glycolipid nano-carriers have the tendency to enhance the anticancer activity of fucoxanthin by 25.8-fold, as they help to keep the compound active for a longer period of time [131]. In different studies, rutin was encapsulated using poly (lactic-co-glycolic acid) and zein as a wall matrix, which resulted in a slow delivery rate (25% after 60 h) of rutin at the targeted location in the human body [132]. Presently, one of the researchers used chitosan and alginate to encapsulate bioactive compounds from the waste of grapes, in which, interestingly, the bioactivity of the targeted compound was enhanced and the encapsulation protected the compound from degradation in the gastrointestinal tract [133]. Besides, compounds including peptides are encapsulated using lipid-based nano-carriers or liposomes, which are comprised of double-layer protection of surfactant molecules and aqueous fluid [134]. Furthermore, nano-carriers of hybrid structure (combination of liposome and chitosan) are also in practice to encapsulate compounds like caffeine to enhance the encapsulation efficiency [135].

The techniques employed to encapsulate these compounds are divided into categories based on power consumed for encapsulating, such as top-down and bottom-down techniques as well as their combined treatment [136]. In the case of the top-down method of encapsulation, high power consumption takes place as it is equipped with instruments like spray-drying, ultra-sonication, homogenizer and many more, while the bottom-down method is practiced in minimum consumption of energy; for example, precipitation, micro-emulsification, conjugation, interchange of atoms, etc. Major factors which play a vital role in selecting a particular technique for nanoencapsulation are delivery motive, delivery rate of release, solubility and stability of the nano-carrier, as well as cost of production [131].

#### *4.1. Encapsulating Carriers for Bioactive Compounds*

#### 4.1.1. Polymeric Nano-Carriers

Polymeric nano-carriers are considered to be extremely suitable material for encapsulation and delivery of bioactive compounds. Presently, natural-based nano-carriers including casein, starch, chitosan, whey protein and albumin are maximum used. In 2018, Ravi et al. [132] used chitosan as a wall material to encapsulate marine carotenoid fucoxanthin. It resulted in enhanced anticancer activity of the bioactive compound and increased the caspase-3 activity 25.8-fold. Further, Gagliardi et al. [133] carried out a comparative study using synthetic and natural-based nanoparticles, namely, poly (lactic-co-glycolic acid) and zein for the encapsulation of rutin. The results indicated that zein loaded with 0.8% rutin concentration reflected slower release (25%) after 60 h as compared to poly

(lactic-co-glycolic acid) (100%). Further, in 2021, Portugal researcher Costa et al. [134] encapsulated bioactive grape pomace extract using chitosan and alginate nanoparticles, which protected the bioactive compounds from hydrolysis in the gastrointestinal tract as well as enhanced its bioactivity.

#### 4.1.2. Lipid-Based Nano-Carriers

Lipid-based nano-carriers, known as vesicular carriers, include nano-liposomes, niosomes and particulate carriers (solid lipid nanoparticles and nano-structured lipid carriers). It is a spherical bilayer developed by the reaction between surfactant molecule and aqueous fluid. It is used to encapsulate various bioactive compounds including peptide. Solid lipid nanoparticles are fabricated by mixing solid lipid in internal phase, whereas nano-lipid particles are developed by mixing liquid and solid lipid together [137]. Chaudhari et al. [138] encapsulated piperine and quercetin using Compritol as a solid lipid, while squalene as liquid lipid and span 80 as well as tween 80 as emulsifiers and co-emulsifier. These encapsulated bioactive compounds reflected slower release due to slower erosion of lipid wall matrix (12 h). Another study conducted by Abd-Elhakeem et al. [139] illustrated about improving the bioavailability and oral target delivery of eplerenone by using lipid-based nanoencapsulation. Eplerenone-loaded nano-lipid capsules reflected in improved permeability up to two folds higher compared to conventional aqueous drug in rabbit intestine after the period of 24 h.

#### 4.1.3. Hybrid Nano-Carriers

Hybrid nano-carriers consist of two main networks including internal (metallic materials and polymers) and external (single/multi-lipid layer) networks. The outer layer of this nano-particle acts as a protection against deterioration and water diffusion. These organic–inorganic and lipid-polymer carriers are basically developed for the treatment of cancer cells with controlled release of bioactive compounds. Seyedabadi et al. [136] developed a slow-release encapsulated caffeine using chitosan coated in nano-liposomes as compared to nano-liposome without chitosan covering; hence, combination of chitosome proved better for the encapsulation of caffeine.

#### *4.2. Nanoencapsulation Techniques for Encapsulation of Bioactive Compounds*

Nanoencapsulation of bioactive compounds is significantly more complex as compared to the micro-encapsulation process. It is divided into three main categories, which consist of top-down, bottom-down and a blend of both [131]. Top-down techniques require high-energy-efficiency instruments including spray-drying, ultra-sonication, high-pressure homogenization, etc., while the bottom-down technique is limited to low-energy-consumption techniques, such as precipitation, micro-emulsification, conjugation, layer-wise accumulation, interchange of atoms and molecular into nano-size level, etc. [137]. Several factors influence the choice of a particular technique for tailoring nano-capsules, which include delivery motive, delivery rate of release, solubility and stability of the nano-carrier, as well as cost of production.

In the present review, several encapsulation techniques including ultra-sonication, high-pressure homogenization, microfluidization, nano-fluidics, nano-spray-drying, electrospinning, electro-spraying, milling and vortex fluidic have been discussed.

#### 4.2.1. Electrospinning

In electrospinning, fluids with electric charges are processed by passing highvoltage electricity to polymeric fluid, which results in the development of dry microand nano-structures. Basically, the instrument comprises three major parts including a syringe pump, stainless steel electrified needle, and the collector plate inside a chamber. The feed fluid is pushed with a definite flow-rate inside the nozzle/needle, which is subjected to high-voltage electricity. The experiments are carried out at ambient

temperature and after the nano-fibers are collected on the electric collector plate, they are kept in a desiccator prior to packaging [140]. The principal element responsible for the characteristic of nano-fiber includes operating conditions such as spinning fluid properties, polymer attributes and mechanical parameters of the instrument with its several nozzle setups [141]. The diameter of nano-fibers ranges from 1 μm to numerous nano-meters. These nano-fibers are in demand as a reinforced material in food packaging, drug delivery, and biosensing due to its substantial surface area, flexibility to develop into several structures and magnified porosity [142]. There are five different types of electrospinning strategies including blend electrospinning, coaxial electrospinning, emulsion electrospinning, high-throughput electrospinning and polymer-free electrospinning. In the blend and emulsion electrospinning method, the core (bioactive compound) and the wall (polymeric) solutions are blended together for electrospinning using single-nozzle, which works efficiently in controlling the release of bioactive compounds. Hydrophilic and hydrophobic molecules can be easily encapsulated using this technique [143]. On the contrary, in the case of a coaxial electrospinning setup, the component consists of a syringe with a twin-compartment, where two different nozzles are attached to one syringe outlet pump for electrospinning the core and wall solution together, which results in achieving encapsulated fibers [144]. However, highthroughput electrospinning is a needless technique applicable for the fabrication of ultrathin fibers via emulsions subjected to centrifugal pressure of polymeric fluid. In 2018, Kutzli produced glycoconjugates using high-throughput electrospinning. Apart from all these techniques, polymer-free electrospinning is an enhanced version of electrospinning, wherein the polymeric fluid with high molar mass is injected through a pump on the surface to achieve higher yield than the conventional techniques [145]. Xiao et al. [146] indicated that this technique was able to fabricate nano-fibers of a diameter ranging from 87 to 57 nm, whereas other reports by Moreira et al. [147] indicated that polymer-free electrospinning is useful for the food industry to improve the uniformity of nano-fibers derived from spirulina. Further, Poornima et al. [148] used electrospinning to encapsulate resveratrol with poly(ε-caprolactone) and poly (lactic) acid and the results proved to be effective in controlled drug release delivery, while another recent study by Leena and Anandharamakrishnan et al. [149] achieved the highest encapsulation efficiency (96.9%) of resveratrol using Zein. Researchers also indicated that these nano-fibers are useful as edible nano-films for oral delivery.

#### 4.2.2. Electrospraying

Elcetrospraying, popularly known as electro-hydro-dynamic atomization (EHDA), is an alternate solution for the drying-encapsulation technique. It runs on high-voltage electric current at ambient temperature. The primarily principle for both the techniques (electrospinning and the electrospraying) is similar, the unique difference between these technologies lies in intermolecular cohesion of polymeric fluid which is remarkably low in the case of electrospraying, and thus results in breakage of jet into fine droplets. The jet particles, when exposed in air, gain a spherical shape in view of the surface tension. A report by Bhushani and Anandharamakrishnan [140] revealed that electrospraying helps to enhance the permeability and bioactive releasing attributes in catechins from green tea with the help of zein as a wall material. Later in 2019, Jayan and Anandharamakrishnan also revealed that resveratrol, when nanoencapsulated using the same wall material, results in 68% encapsulation efficiency [150].

#### 4.2.3. Nano-Spray Dryer

The nano-spray-drying technique is quite similar to that of conventional spray-drying, where the fluid is subjected to droplet formation and further dried by the heated drying gas to form dry particles. In the case of nano-spray, the little modification has been made which includes a particular nozzle, which fabricates nano-droplets using a constant flow of drying gas used from laminar. Further, the nano-droplets are subjected to vibrating mesh of 4.0, 5.5, and 7.0 μm size holes. The fragment size of the spray-dried sample totally depends on the concentration of the fluid, temperature of drying gas, spray velocity and the size of the droplets [151]. Adel et al. [152] encapsulated curcumin in hydroxypropyl beta-cyclodextrin using the nano-spray-drying technique for the pulmonary delivery of curcumin in lung tissues and the results indicated significant reduction of proinflammatory cytokines compared to the pure drug. Further, Mozaffar et al. [153] dried nano-structured lipid carriers of 3% palm oil and 3% tween 80 (60 g/L) mixed with sodium chloride. The results revealed that the presence of sodium chloride protected nano-structured lipid carriers from aggregation during the process of spray-drying and about 50% of lipid molecules were encapsulated in salt particles.

#### 4.2.4. Micro-/Nano-Fluidics

The primary concept of micro-/nano-fluidics is based on interfacial interaction between the core and wall fluids. It helps in the formation of spherical drops and slows down the release of the bioactive compounds. In addition to this, it helps in the production of similar-size accurate nano-droplets [154]. It consists of components including a molded set of channels fabricated on a base of polydimethylsiloxane (PDMS) of glass. The fluid is passed through this channel as these are interconnects from all directions. Gas and liquid are injected from a syringe using hydrostatic pressure. Generally, this technique incorporated four types of emulsion devices including single-, double-, multi- and flow-focusing nano-/microfluidic devices. It is employed for the fabrication of nano-emulsions, nano-liposomes and nanoencapsules. Jafari et al. [155] developed an oil-in-water nano-emulsion using microfludization at an optimized condition of 42–63 MPa microfluidization pressures with 1–2 cycles. The results of the study indicated smaller droplets of fish oil in developed nano-emulsion. After a couple of years, Wang et al. [156] modified citrus pectin using microfluidation, and the results revealed that properties of nano-emulsion were enhanced, thereby protecting cholecalciferol from UV degradation as compared to the original pectin. In addition, the molecular weight and hydrodynamic diameter of modified pectin was also reduced to 237.69 kDa and 418 nm, respectively.

#### 4.2.5. High-Pressure Homogenization

High-pressure homogenization refers to the production of nano-fragments of a homogenous size in a fluid under a specific high pressure. It comprises 10–15-fold higher pressure (100–400 MPa) than conventional homogenizer. According to the research, this technique is excessively utilized by the milk and milk products industries to upgrade its texture, taste, flavor, and improved shelf life characteristics with enhanced anti-microbial quality (in activation of Salmonella spp., Listeria monocytogenes, Staphylococcus aureus, and Escherichia coli). About 3 and 4 log cycles reduction was reported at 200 and 300 MPa homogenization pressure at 30 ◦C and 40 ◦C inlet temperature [157]. It also acts as a substitute against thermal processing, as it is significantly effective for the inactivation of enzymes and microbial activity [158]. Additionally, it also helps in the development of nano-emulsion, which is stable at ambient condition. Fernandez-Avila et al. [159] developed a soy protein isolate-stabilized emulsion employing high-pressure homogenization, and the results revealed that emulsion treated with 100 to 200 MPa with 20% soybean oil was most stable with its improved physical stability in terms of particle size and rheology.

#### 4.2.6. Ultrasonication

"Ultrasound" is a cluster of sound waves beyond human hearing frequency (>16 kHz). Basically, it is divided into two streams, low- and high-intensity waved; low sound waves are generally used for the detection purpose (sonography), whereas high sound waves are used for the modification in molecules including its size reduction, it also helps in the development of emulsification and extensively utilized by the food industries [160]. In nanotechnology, it is basically utilized for the development of several genres of nano-structure. The components of ultrasound comprise an electric generator, piezoelectric transducer for transforming electrical energy into sound energy and a sound emitter of titanium horn shape for conveying the ultrasonic waves into the sample or medium [161]. Various ranges of nano-delivery techniques have been developed, incorporating lipid and surfactant molecules as a wall material for the fabrication of nano-emulsions, nano-liposomes, niosomes, etc., using ultrasound [162–164]. Nano-carriers such as biopolymeric and polymeric have also been developed for the encapsulation of various food bioactive compounds by the food industry.

#### 4.2.7. Supercritical-Based Technologies

It is an alternate green method for the development of nano-particles [165]. Principally, the temperature of critical solvent is above its critical limit or at single phase irrespective of pressure. Carbon dioxide is commonly used as a supercritical fluid as it is nontoxic, low cost and non-flammable. In this process, a liquid solvent is employed, which could absolutely mix with the super critical fluid such as carbon dioxide so as to liquefy the solute subjected to micro-ionization. Now, due to the insolubility of solute in the super critical fluid, instant precipitation takes place, which further brings out the outcome of developed nano-particles [166]. Presently, methods including micronization via rapid expansion of supercritical (RESS) solution, supercritical antisolvent (SAS), supercritical melt micronization (ScMM), spray coation, supercritical CO2 coating, etc. are employed for encapsulation of bioactive compounds using supercritical system. Several researchers indicate that this technology can be useful in various ways including development of encapsulated products depending upon properties of wall matric and the active ingredients. Based on the behavior of the core and wall material to be encapsulated, the processing technique using supercritical CO2 is decided. For example, the interaction of CO2 with the active material, wall material and the solvent used. In case of a biopolymer drug delivery system, the interface between the supercritical CO2 and the polymer used plays a vital role in encapsulation process. Whereas polymer including polylactide (PLA) is good for SAS treatment, in the case of RESS, it is difficult to solubilize in supercritical CO2 [167,168].

#### 4.2.8. Polymerization

In this technique of nano- encapsulation, firstly, to form nano-particles, monomers of an aqueous fluid are polymerized then the bioactive compound is mixed in it. Further, when the encapsulation takes place, the nano-particles are purified by removing extra stabilizer and surfactant settled on the surface of the nano-particles. This technique is generally incorporated for developing poly butylcyanoacrylate nano-particles [169,170].

#### 4.2.9. Coacervation or Ionic Gelation

A technique with wide range of hydrophilic compounds (gelatin, sodium alginate and chitosan) was incorporated for the development of nano-particles. Two kinds of aqueous phase fluids are prepared using chitosan polymer (propylene oxide) and polyanion sodium tripolyphosphate. A nano-size coacervation is formed when the tripolyphosphate (negative charged) reacts with chitosan (positively charged) and, due to the reaction of ions, liquid is converted into gel [171].

#### **5. Physicochemical Properties of Encapsulated Bioactive Compounds**

#### *5.1. Particle Size*

The foremost properties of nano-particles are the size of its particle and its distribution. These properties are responsible for overall quality including its delivery ability, stability, viscosity, etc. [172]. Due to the relative mobility and significantly tiny size, the intracellular holding capacity is relatively higher in nano-particles than micro-particles. Reports reveal

that nano-particles of 100 nm reflected 2.5 times higher holding capacity compared to 1 μm micro particles [173].

Several types of microscopes are used to detect the size and structure of the nanoparticles. Optical properties of nano-particles including single/double/multi-emulsions and micro/nano-capsules capsule are measured by scanning electron microscope/laser diffraction based on its properties (wet/dry). To determine the number of pores, surface study is required which needs extremely strong analyzers such as transmission electron microscopy. Several compound locations can be detected using confocal or fluorescence microscopy by mixing fluorophores to dye the bioactive compound. Dynamin light scattering, also known as photon correlation spectroscopy, is extensively utilized to detect size of the nano-particles, which are in the range of 1000 nm. It helps in finding the range of particle size along with its concentration in the given matrix [174]. It is operated to identify the charge attributes of the nano-particles. It indicates the electrical capacity of the nano-particles, which can further be modified by changing the composition of compounds mixed in the aqueous fluid. Zeta potential more than (+/−) 30 mV of a nano-particle is known to be stable. Interestingly, with the help of the Zeta potential test, it can be identified whether the wall material has been encapsulated inside the nano-capsule or covering its outside structure [175]. Desai et al. [176] reported that nano-particles can diffuse through the submucosal layers in rats while a micro-particle is only limited till the epithelial lining.

#### *5.2. Stability of Encapsulated Bioactive Compound*

"Stability of nano-particles" refers to the strength and balance needed by nanoparticles to remain intact inside the wall matrix until the desired time and place of release. Nano-emulsions have better stability due to their morphological structure of tiny droplets. Additionally, the strength of bioactive compound to remain stable can be examined by placing them in a modified environment including high/low temperature, different ionic charge fluids and different pH range [177].

#### *5.3. Encapsulation Efficiency and Loading Capacity*

It is defined as the amount of bioactive compound encapsulated inside the wall matrix. The amount of compound encapsulated can be quantified by using techniques including high-performance liquid chromatography, UV-Vis spectroscopy and UV-Vis spectroscopy [178]. A perfect nano-particle is the one which has maximum compound loading capacity with minimum quantity of wall material. Loading of bioactive compounds can be carried out using two methods including incorporation and absorption methods. The capacity of entrapment is basically dependent on solubility of the compound encapsulated in the wall material, specifically the interaction in the molecules of bioactive compound-polymer, molecular weight and the availability of functional groups [179]. Proteins and macromolecules at isoelectric degree are reported to possess maximum holding capacity. Moreover, in the case of smaller molecules, the ionic reactions between compound and polymer can help in increasing the holding capacity of the matrix.

#### *5.4. Control Release*

The release of the bioactive compounds encapsulated in a particular matrix depends on several aspects including compound solubility, surface bound/adsorption, and diffusion from the matrix, matrix degradation and combination of both diffusion and degradation. Nano-spheres with even distribution of bioactive compounds tend to release by erosion of wall material. The process of release is totally controlled by diffusion if degradation of wall material takes place at a slow rate. The quick release of the compound results in poor wall material or low bounding capacity of the compound [180]. It was reported that the mixing method practiced plays a vital role on release profile of nano-capsules as it will slow the release of compounds [181]. In contrast

to this, if the compound is protected by the polymer coating, then the release takes place by the diffusion method from inside to matrix outside. Further, there are several techniques including ultra-filtration, reverse dialysis bag, dialysis bag and diffusion of cell with synthetic of artificial membrane that is practiced for the compound release in vitro.

#### **6. Application of Nanoencapsulation in Food Industries**

Application of nanoencapsulation in several industries including nutraceutical, pharmaceutical, food, packaging and preservation has exponentially expanded in the last few years. It is extremely promising for the fragrance and flavor industries as the volatile compounds tend to evaporate during processing time. Besides, these compounds undergo chemical changes due to oxidation at atmospheric condition, which results in degradation of the compounds, thus encapsulation helps to stabilize as well as retain the natural color, flavor and fragrance while enhancing the shelf life of the compound. An excellent example is retaining the fresh aroma of brewed coffee through microencapsulation of flavor compounds such as ketones, pyrazines, furans, pyridines, etc., using food starch derived from waxy maize as encapsulating material. Nanoencapsulation of tea compounds (caffeine, theanine and catechins) using various proteins, lipids and carbohydrates have been reported to increase the effectiveness of their health imparting properties such as anticancer, antidiabetic and anti-inflammatory [182–184].

It has been researched and reported that during gastrointestinal digestion, encapsulated catechin showed improved retention of its biological properties as compared to free catechin [185]. Rojas-Graü et al. [186] revealed that the nanoencapsulation technique can be utilized as an anti-browning technique for food industries. Nanoencapsulated plant-based compounds are tagged as anti-browning compounds. Tyrosinase enzyme, also known as catalase B, accelerates the unwanted chemical reaction during food processing including enzymatic browning of fruits, vegetables and beverages that causes adverse effects on its organoleptic characteristics (Polyphenol Oxidase + O2 → Melanin). Zheng et al. [187] isolated tyrosinase inhibitors from Artocarpus heterophyllus to retard browning reacting in fresh-cut-apple slices. The results revealed that apple slices treated with Artocarpus heterophyllus extract along with 0.5% ascorbic acid reflected no browning reaction after 24 h.

Further, nanoencapsulation is popularly appreciated for improving the water stability, solubility and bio-accessibility of hydrophobic bioactive compounds including curcumin by binding it with naturally existing proteins (legume oligomeric globulins, ferritin and casein micelles) by a hydrophobic interactions bond which acts as a nanocarrier for hydrophobic nutraceuticals in drug delivery [188]. Human Serum Albumincurcumin nano-particles can be used for cancer treatment as they reflect effective antioxidant activity with enhanced antitumor properties [189]. Luo and co-workers [190] carried out ex vivo and in vivo adhesion experiments employing Tannic acid/IR780 nano-particles, which were tailored with anti-ulcerative colitis properties (encapsulated Curcumin). Reports reveal that tannic acid-loaded curcumin nano-particles can be utilized for drug delivery to treat ulcerative colitis as tannic acid possesses degradable adhesive properties which can accumulate on the surface of inflamed mucosa. In Figure 4, ulcerative colitis mice were orally ingested with three different solutions including free IR780, IR780 nano-particles and tannic acid/IR780- nano-particles. A gradual decrease in adherence level was noticed in all the three samples at different time intervals (3 h, 6 h, 12 h, and 24 h). Mice with tannic acid/IR780-nano-particles reflected the maximum adherence towards inflamed mucosa. Additionally, nanoencapsulation is also considered as an effective technique for encapsulating antidiabetic synthetic compound insulin. In 2021, Hadiya et al. [191] encapsulated insulin using chitosan to 170–800 nm size of spherical shape and about 15–52% of delivery efficiency was achieved.

Nanoencapsulation is also useful in active packaging, which helps to prolong shelf life and preserve quality of the food products. It is employed to retain and enhance the nutritional and organoleptic attributes of the product with extended shelf life [192]. Encapsulated bioactive compounds rich in antioxidants and antimicrobials including vitamin C, Vitamin E, carotenoids, etc., are incorporated to edible films as active compounds during the fabrication as it helps in maintaining the level of freshness of the products. The organoleptic attributes of the coated product can be maintained by incorporating additional flavor, colors and sweeteners. Nano-fibers of cinnamon EO encapsulated in polyvinyl alcohol/β-cyclodextrin are obtained to use as a film for coating the packaging box. The film coating (1.5 cinnamon EO-β-cyclodextrin) acts as an antimicrobial barrier to suppress bacterial and fungal spoilage of the food sample while extending its shelf life by 5 d at 10 ± 0.5 ◦C [193]. Adel et al. [194] prepared a bio-composite using β-cyclodextrin citrate (50%), and oxidized nano-cellulose (7%) in chitosan solution. The prepared film reflected in reduced water vapor permeability 2.09 ± 0.08 (10–11 g m−<sup>1</sup> s−<sup>1</sup> Pa<sup>−</sup>1). In addition, the film was fortified with clove EO nano-particles, which resulted in higher activity of Gram-negative bacteria than Gram-positive. Further, Xiao et al. [195] developed a nano-composite film by encapsulating pesticide/insecticide (iprodione) using poly (ethylene glycol)-poly (ε-caprolactone and chitosan. The encapsulation of iprodione reflected in its improved efficacy by 2-fold along with its reduced dosage.

#### **7. Conclusions and Prospects**

At present, due to the tremendous growth of deadly diseases, it has become necessary for humankind to build up a strong immune system and this could be only possible though ingestion of bioactive compounds extracted from plant sources. This scenario has encouraged researchers to search for the conventional and sustainable extraction method for collection of bioactive compounds from plant matrixes. Nevertheless, application of these novel extraction techniques still remains a challenge due to its working principal as it works on several parameters; therefore, in-depth knowledge is required to follow this technique on the larger scale at the industrial level to achieve a promising output. However, due to the evolution of these novel techniques, the extraction has become easy and the extraction yield has been significantly enhanced in maximum bioactive compounds. Novel technologies have helped in enhancing the extraction of bioactive compounds without significant degradation. Moreover, encapsulation techniques offered to deliver the bioactive compounds at the site where it is needed. The combination of these two techniques offers a synergistic effect for extraction as well as targeted delivery.

**Author Contributions:** S.N., writing—original draft preparation; N.K.R., writing—review and editing; C.O., writing—review and editing; and B.T., writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors and their work were supported by the Irish Department of Agriculture, Food and the Marine, under the Food Institutional Research Measure to the U-Protein project under grant No. 2019PROG702.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Entry Link on the Encyclopaedia Platform:** https://encyclopedia.pub/13442 the entry published on the encyclopedia platform.

#### **References**

## *Entry* **High-Speed Railway**

**Inara Watson**

School of Engineering, London South Bank University, London SE1 0AA, UK<EMAIL_ADDRESS>
**Definition:** Union Internationale des Chemins (UIC) defines the high-speed railway (HSR) as a highspeed railway system that contains the infrastructure and the rolling stock. The infrastructure can be newly built dedicated lines enabled for trains to travel with speed above 250 km/h or upgraded conventional lines with a speed up to 200 or even 220 km/h. HSR requires specially built trains with increased power to weight ratio and must have an in-cab signalling system as traditional signalling systems are incapable of above 200 km/h.

**Keywords:** speed; infrastructure; rolling stock; in-cab signaling system; absence of level crossing

#### **1. HSR Technologies**

HSR systems were divided into four groups depending on their relationship with conventional rail [1]; dedicated line, mixed high-speed line, conventional mixed line, and fully mixed [2]. Each of these types of HSR has some advantages and disadvantages.

Dedicated HSR represents a line that is fully separated from a conventional line, has a high capacity, high safety, and no level crossings. The line has fences all along the line, often built on viaducts or in long tunnels, and has a high construction cost, such as the case in Japan and Taiwan.

Mixed HSR lines have a wider area to serve, increased accessibility as high-speed trains run on dedicated and conventional lines, high capacity of HS lines stretch over larger areas, reduced building costs. HSR trains can use conventional rails in city centres in areas where land is more expensive to build dedicated lines. However, stretches of conventional lines have less capacity and can be a bottleneck for increased traffic, reduces safety, increases maintenance costs, whilst the rolling stock must be equipped with two signalling systems for HSR and conventional rails, such as the case in France and China.

Mixed conventional rail represents lines that are used by HSR trains and by conventional trains. Mixed traffic reduces the capacity of the line because of big differences in the speed of trains, and it also reduces safety. It can be a suitable solution if a country has a different gauge from the standard gauge size to be part of the European railway network and supports interoperability of international services, such as the case in Spain. This type is more difficult and expensive to maintain, needs special rolling stock, which is also more expensive to purchase and maintain.

Fully mixed lines represent lines used by all types of trains, including freight, have maximum flexibility to be used to full capacity, reduces safety, reliability, and punctuality, and increases maintenance costs. An example of such lines as those used in Germany.

There are two ways to develop the HSR system: build new systems or upgrade conventional railways. Building lines, operating, and maintaining them is an expensive business, but it gives an opportunity to develop a system that can operate at a higher speed and with bigger time savings [3].

Each new project includes planning, land purchasing, infrastructure building, and rolling stock costs. Upgrading existing lines will exclude the need for land purchasing, which may bring big savings. Upgrading conventional rail creates lots of disruption for traffic, and it does not allow reaching the required speed on the new lines. However, it is less expensive as it costs approximately US\$4.37 million/km in 2007 prices [4]. Table 1

**Citation:** Watson, I. High-Speed Railway. *Encyclopedia* **2021**, *1*, 665–688. https://doi.org/10.3390/ encyclopedia1030053

Academic Editors: Raffaele Barretta, Ramesh Agarwal, Krzysztof Kamil Zur and Giuseppe Ruta ˙

Received: 14 June 2021 Accepted: 19 July 2021 Published: 27 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

shows the HSR technologies in selected countries. Despite this difference, there is a lot in common in all HSR systems. All are powered by electricity, have a continuous welded rail, which reduces the noise level and the track vibration, in addition to being built on ballast track or on concrete slab tracks.

**Table 1.** High-speed rail technologies in selected countries Adapted from ref. [5].

All HSRs must have advanced signalling systems and automated train control systems. The Automated Train Control (ATC) systems were first developed in Japan and introduced for Shinkansen trains. The system was named Digital Communication and Automatic Train Control (DS-ATC) system. In Europe, it is the European Train Control System (ETCS). The next step in the development of the control systems was the introduction of ERTMS. The ERTMS system was first introduced in Italy on a 204 km line between Rome and Naples. The ERTMS combines GSM-R (communication) and ETCS (signalling) systems. With rapid progress from 2G to 5G network UIC is working on developing the successor of GSM-R, the Future Rail Mobile Communication System (FRMCS) [4]. The system can be introduced to railways as early as 2025. Another common thing for all HSRs is that they are very expensive to build [5] and only two of them recovered construction costs, Shinkansen in Japan, and Paris-Lyon line in France [6].

Rolling Stock is another part of the HSR systems. High-speed trains have a large variety in axle loading ranging from 11.4 t for the Hitachi train to 23 t axle loading for Bombardier and Acela Express [7]. This large difference can be explained by the type of railway that uses this rolling stock. The Shinkansen line that uses Shinkansen-Series 700 is fenced throughout to secure the entire length of the track. In contrast to these, the Acela Express operates on an upgraded line with level crossings. Amtrak trains are equipped with an anti-collision structure to meet USA crash standards. Zefiro, a high-speed train manufactured by Bombardier, is one of the most efficient and advanced trains in the world.

#### **2. Differences between HSR and Conventional Rail**

The fundamental principle is the same, but the biggest difference lies within speed and capacity. To increase the capacity of the line, it is very important that trains that operate on the line must not have a big difference in operational speeds.

HSR and conventional rail have the following technological differences:

In track quality: HSR requires a specific design, higher standards of surface, welded rails, and a different and more advanced signalling system. Most HSRs are built on slab tracks. HSR has a larger curve radius than conventional rail. For a speed of 300 km/h, the minimum radius is 4000 m, grades of 3.5% rather than 1–1.5% for existing lines with mixed traffic [8]. HSR track is protected by fences from wild and farm animals. HSR lines do not have any level crossings and have fewer stops than conventional lines.

In traction power: For higher speeds, there is a need to have a more powerful rolling stock.

In signalling system: The signalling system depends on the speed of trains. Railway lines with speeds under 160 km/h use trackside signals to control the safe movements of trains, but if speeds exceed 160 km/h, the driver cannot reliably read signals placed on the trackside. So, for speed above 160 km/h, use onboard signalling [9]. When the speed of the HSR is above 200 km/h, the traditional signalling systems become inefficient [1]. HSR trains require in-cab signalling systems.

In power supply: HSR needs at least 25 kV, but conventional rail voltage can be lower. There are three main electrification systems that are in use for HSR, and they are: 3 kV DC, 15 kV AC, and 25 kV AC. The lines electrified at a higher voltage have lower losses during the transformation and transmission of the energy from the power station to the train. It was found that for trains on lines electrified by 3 kV DC it is necessary to produce 22.6% more than the energy received by the pantograph. For lines electrified by 25 kV AC, it is necessary to produce only 8% more energy than received by a pantograph. Another advantage of using 25 kV AC is the possibility to supply power to high-speed trains with a greater distance between substations, which means a reduction in construction and maintenance costs [10]. HSR can be powered by lower voltage, but it will increase the amount of energy that it needs to produce to power high-speed trains, increase the carbon dioxide emissions, and increase the cost of construction and maintenance of HSR lines.

Technological innovations can promote a faster and less costly transition to the decarbonisation of railways. To speed up the decarbonisation of HSR, the technical innovation will play a significant role. One of these innovations relates to using the Flexible Medium Voltage DC Electric Railway System (MVDC-ERS). It will increase the efficiency of the power distribution and support and consolidate the renewable energy sources [11].

#### **3. Selected HSRs from Europe, Asia, and USA**

According to the UIC on 2020, there were 20 countries worldwide that have HSR in operation: 13 countries in Europe (Austria, Belgium, Denmark, Finland, France, Germany, Italy, Poland, Czech Republic, Spain, Switzerland, the Netherlands, and the U.K.), 3 countries in Asia (China, Taiwan-China, Japan, and South Korea), 4 other countries (Saudi Arabia, Morocco, Turkey, and the USA), and another 24 countries worldwide planning to build HSR. The total length of HSR in operation, under construction, and planned to build is 104,413 km worldwide [7]. Table 2 shows all countries with HSR systems in operation worldwide in 2020.

#### *3.1. HSR in Japan*

The first high-speed line was opened in Japan on 1 October 1964. It took five years to construct a 515 km line between Tokyo and Osaka. It provided a tremendous boost to the economy in Japan and encouraged countries around the world to develop HSR in their own countries. The core of the success of the Shinkansen lies in the decision to build dedicated lines for high-speed trains. A new double-track high-speed standard-gauge line for speeds up to 250 km/h. It was innovative to operate commercial services at this speed. Because of geological and geographical conditions in Japan, HSR requires many long tunnels and bridges. The disadvantage of this decision was the very expensive civil engineering work. Approximately 13% (86 km) of the line is in tunnels, and 33% (174 km) on bridges and viaducts [12] (p. 14). HSR infrastructure in Japan must be resistant to earthquakes, floods, and deep snow.

From the beginning of building Shinkansen, it was decided not to use outside signals to provide information for the driver but to provide indications inside the cabin. It was achieved by transmitting coded signals to the train through track circuits and receivers installed at intervals along the track. In-cab signalling is mandatory today for all highspeed systems around the world. The track of Shinkansen was divided into sections, and if only one section ahead is clear, the train can move forward. This means that the train ahead was only one block away. In this case, the train can move only with reduced speed, and if the train ahead is two or more blocks ahead, then the train behind can move on at full speed [12] (p. 18). This system increases the capacity of a track.

**Table 2.** High-speed lines in operation worldwide Adapted from ref. [7].

Along with the new infrastructure, there is also a new generation of a train that has been built. One innovation of the Shinkansen rolling stock was that instead of putting traction equipment in a locomotive, it was spread along the length of the train. This reduces the maximum axle load to 16 tonnes [12] (p. 20). This increases the speed of the train, minimises infrastructure maintenance, and reduces energy consumption. However, with distributed power, the noise in passenger saloons is higher than with concentrated power (locomotive) trains [13]. The maximum length of trains in Japan is the same as in Europe, which is 400 m. In Japan, high-speed rolling stock (HSRS) was designed for a 15–20-year life cycle, but in Europe, it is around 30 years [13], and because of this, HSRSs in Japan do not need major renovation or technological innovations within their life cycle. This is an example and a suitable balance between economic benefits, shorter asset life, and low maintenance costs.

In 1964, it took four hours to travel from Tokyo to Osaka. By 2021, it reduced the time to 2 h 30 min after speeds were increased to 285 km/h [14]. In 2015, the number of departures of the Shinkansen bullet from Tokyo had increased to 14 times per hour (358 departures per day), and daily ridership increased to 445,000 with an average delay per train of 0.2 min. Totally 355 million passengers use a high-speed railway every year [15]. E5 series Shinkansen train has 10 cars with a capacity of 731 seats, that is more seats per train than in Germany or France [16]. This is one reason for the profitability of the Shinkansen. Figure 1 shows the plan of the Shinkansen HSR lines in Japan.

**Figure 1.** HSR network in Japan, in 2015 Reprinted from ref. [7].

The safety records and punctuality on Shinkansen lines are outstanding. The HSR carried out thousands of scheduled journeys per day, and the average delay across all high-speed lines is 0.2 min per train, including weather-related delays. For over 50 years of operation and carrying over 10 billion passengers without an accident in which passengers could be killed or injured, Shinkansen trains are the most safe and reliable in the world. Shinkansen is the great economic success of Japan, and today the Shinkansen has an operational revenue of around US\$19 billion a year [17]. This success has many reasons, and one of them is that the railway company owns the entire infrastructure, stations, the rolling stock, the track, and land around the railways. There is less bureaucracy, less management, and decisions are taken more quickly. With the future decline in Japan's population, there will be a slowdown in new developments of HSR, as there will not be enough demand for high-speed trains.

#### *3.2. HSR in France*

The first high-speed trains were introduced in Europe, in France over three decades ago in 1981, and they have carried over 2 billion passengers during these years [18]. It was an immediate successful implementation of TGV. It caused a decrease in air and road traffic, especially for flights. TGV offered a shorter trip time, higher comfort, frequent services, and competitive prices. Figure 2 shows the HSR network in France in 2015.

**Figure 2.** HSR network in France, in 2015 Reprinted from ref. [7].

France has 450 TGV trains, and they are serving around 230 destinations, operating in France and outside to Belgium, Germany, Spain, Italy, Luxembourg, and Switzerland [19]. Around 130 million passengers use the HSR in France every year [20]. From the beginning, the French railways had a very substantial government investment, and it was the government's determined ambition to build a railway corridor to connect the south of France with Paris. This corridor, Paris to Marseille via Lyon, is the most important one in France and serves around 40% of the population [21].

After more than 30 years of operating high-speed trains in France, almost 40 percent of these trains travel on conventional track [19]. Around 60 percent of TGV trains are travelling on new lines designated only for TGV, and all other traffic was prohibited. The new lines have higher gradients unsuitable for freight traffic. In France, the new HSRs were designed to avoid tunnelling, and this gives the benefit of the possibility to implement the double-deck trains. The TGV Duplex was introduced in 1996. This train can travel at speeds up to 300 km/h [22]. The big achievement of the HSR system in France is that the TGV system is compatible with existing conventional railways.

National Society of French Railways (SNCF) has one of the fastest train services in the world. In April 2007, TGV test train reached 574.8 km/h [23]. The latest TGV Duplex Oceane trains have a maximum operational speed of 320 km/h, and they comprise two powered cars, one at each end and eight carriages with a capacity of 556 seats, with the same number of staffs on the train as the TGV. The length of the TGV Duplex is 200 m. These two trains can be coupled together to increase the capacity of the train on busy lines [7].

There are two different ways to power high-speed trains: it can be as in Japan, with distributed traction, or as in France, with TGV, centralised traction. With increasing awareness about the damaging effect caused by transport on the environment and looking for ways to increase the efficiency of transport, it looks more economically appropriate to use distributed traction to power high-speed trains.

There are other ways to increase the capacity of trains, and one of them can be double-deckers (France, Germany, and Japan) or the widening of carriages (Sweden, Japan). Increasing the capacity of trains gives the possibility to reduce operational costs, and this can reduce the railways' fares. To increase the capacity of railway lines, it is needed to electrify the line, implement more advanced signalling systems, increase the speed of trains, and have dedicated lines for high-speed trains only.

One of the most important developments in the construction of TGV-PSE was the introduction of articulated suspension between passenger vehicles. Using this innovation can reduce the weight of the train, reduce the aerodynamic drag, decrease the level of noise from the train, and improve passenger comfort [24]. Figure 3 shows non-articulated and articulated bogies that are used in high-speed rolling stock.

**Figure 3.** Non-articulated and articulated bogies Reprinted from ref. [12]. (**a**) Non-articulated cars. (**b**) Articulated cars.

A traditional coach has two bogies, and each bogie has two axles; on TGV-PSE coaches, each bogie supports two coaches. Advantages of articulated trains are a more comfortable ride, passengers on the train are less affected by running noise, but the downsides of articulated trains are increased axle load, and that maintenance of these bogies is more difficult. TGV uses the 25 kV AC electrification system, but it can also work on 1500 DC. All TGV trains have at least two electrification systems. To extend track formation life and increase the speed of trains, the weight on one axle was restricted to 17 tonnes [7]. Minimising the axle load will reduce infrastructure and other structural maintenance, reduce construction costs, and reduce the noise level.

The new generation of TGV trains are lighter, have a 15% lower energy consumption, and are designed to be 98% recyclable with a maximum speed of 360 km/h [3]. Reducing the weight has been achieved by using aluminium instead of steel and by using articulated bogies. The train has a regenerative braking system that recovers 8% to 17% of electricity

and puts it back in the network [25]. It sufficiently reduces CO2 emissions and increases the efficiency of high-speed trains.

TGV trains have been installed with Automatic Protection System (APS) and been fitted with in-cab signalling system TVM430. The tracks have been divided into 1500-metre sections, and the in-cab signalling system TVM430 informs the driver of the maximum speed possible on any section. If the train's current speed is higher than the speed limit for that section, then ATP applies the brakes automatically. The TVM430 signalling system allows three minutes of headway, and this increases the capacity of the track to 22,000 passengers in one hour in one direction [26]. This capacity of rail track is equal to a six-lane motorway.

The safety of passengers for any railway network is a crucial requirement. To improve the safety of high-speed trains in France, the lines were redesigned without level crossings. In addition, lines were fully fenced, and advanced equipment was fitted to detect obstructions that occur on a railway line.

From 2008, the profitability of TGV steadily declined, and it was pronounced that there was a need to reduce several stations served by TGV to make the HSR network profitable. HSR services carry only 7% of passengers but account for 61% of the total French rail network traffic [27]. Since the economic crisis in 2007, the number of passengers using high-speed trains continues to decline.

Most HSRs around the world are not profitable but need to look at what benefit they can bring in areas where they pass through, and one example of this can be the city of Lille. It is a suitable example of how a city can flourish from the bypassing of high-speed trains. The development of the TGV HSR and TGV station brought prosperity to the city, blooming commercial activities, and tourism [3].

High-speed trains are the most efficient mode of transport. This is one reason that society continues to fund HSR services. HSR saves time and energy, improves accessibility, increases economic activity, and generates employment [20]. However, with a low-density population in France and only a few larges populated urban areas, it looks unlikely that in the current condition, the HSR will be profitable.

#### *3.3. HSR in Spain*

Construction of the HSR in Spain began in 1989 in the corridor between Madrid and Seville, and high-speed trains (AVE) started to run in 1992. Spain's HSR network is one of the widest in the world. In 2020, the length of the HSR network was 3330 km, 1293 km under construction, and 676 planned to build [7], with an average cost of €14 m per kilometre. HSR in Spain has a fleet of 229 trains with an average age of 11 years and in 2015 carried over 35 million passengers [28].

The Spanish HSR has a standard gauge of 1435 mm and is electrified with 25 kV AC and represents 16% of the total Spanish network [29]. Figure 4 shows the HSR network in Spain in 2015.

One of the advantages of 25 kV AC is the possibility to supply power to high-speed trains with a greater distance between substations (approximately 50 km apart), which means reduced construction and maintenance costs. In Spain, they have 59 AC substations and 384 DC substations [30].

The Spanish railway network has two different sizes of rail line gauges: standard gauge (1435 mm) and Iberian gauge (1668 mm). Talgo trains have automatic gauge changing equipment, which allows for the change from one type of gauge to another without stopping at speeds up to 15 km/h and have operational speeds of 220 km/h [31]. Spanish National Railway Network (RENFE) has a punctuality level of 98%. If a train arrives with a delay of over five minutes, all passengers will receive a 100% refund of the ticket price [32].

The Talgo 350, class S102 has the nickname "The Duck", which has served since 2005 and has an operating speed of up to 300 km/h. It is one of the fastest trains in Europe.  The new generation of Talgo trains is Talgo Avril. This train

is designed for speeds up to 380 km/h, with a low floor to improve the accessibility for vulnerable passengers [33].

**Figure 4.** HSR network in Spain, in 2015 Reprinted from ref. [7].

This train will consume less energy because of its lightweight construction, will produce less noise and vibration, and generate less carbon dioxide emissions. The Talgo Avril train comprises two powered cars, one in the front and one at the back, and 12 carriages with a capacity of up to 600 passengers. The train can automatically switch between different track gauges (1435 or 1668 or 1520 mm) and can be run on diesel or electric or both [34]. In addition, this train can use AC or DC electrification systems to run the train.

HSR in Spain on many routes is not profitable because of the low occupancy of trains. There are a few reasons for low occupancy: high unemployment, high ticket prices, many towns with HSR stations are small and can only generate a few passengers. Spanish Government subsidises HSR heavily, and the cost is around US\$3 billion per year [35].

#### *3.4. HSR in Germany*

Germany has 1571 km HSR in operation, 147 km under construction, and 291 km in the planning stage [7]. The development of HSR in Germany relieved the increasing demand for air and auto travel. The HSR now connects all the largest cities of Germany, and it is in the centre of the country's transport system. Germany has twice the density of the population of France and taking into consideration that the territory of Germany is smaller than that of France, this can be a suitable foundation for the success of the HSR system.

The Intercity Express (ICE) trains were designed and built by a Siemens-led consortium and are operated by the German Federal Railways (DB). ICE1 was introduced into service in 1991, and it composes of 2 power cars, one in the front and one at the back, and 12 carriages between them. It has a maximum axle load of 19.5 tonnes, 358 m long, and has a capacity of 703 seats. Germany has 59 sets of ICE1. The train has a 280 km/h maximum operational speed, powered by electricity from the overhead catenary 15 kV. ICE1 has three signalling systems, LZB, PZB, and ZUB, which are suitable for traffic to Switzerland [7].

The latest ICE3 trains were introduced in 2000 with a maximum operational speed of up to 300 km/h. The power system of ICE3 has been moved from the two ends to the underside of the cars, which is the same system as used in Japan. The train comprises eight coaches, four of which are powered. As trains do not have a power car, it increases the capability of trains as more seating is available for passengers; it has 429 seats per train [7]. The distributed power system has other advantages, and one of them is the low axle load, which is 16 tonnes per axle [7], and this will reduce maintenance costs.

ICE3 has one of the best braking systems. Braking equipment on ICE3 comprises three systems, and one of them is the regenerative system. In addition, ICE3 has a smaller loading gauge than previous ICE1 and ICE2. These changes have been made to allow operation on the European network [36]. For example, France did not allow ICE trains on their network before as ICE was too wide and too heavy. French railways have a restriction of 17 tonnes on one axle for HSR lines.

All railway lines in Germany have a standard gauge of 1435 mm and are electrified at 15 kV AC 16.7 Hz. The new lines are designed for speeds up to 300 km/h [7]. Figure 5 shows the HSR network in Germany in 2015.

**Figure 5.** HSR network in Germany and France Adapted from ref. [37].

From the beginning, DB allowed all categories of trains, including high-speed trains and freight trains to use the conventional lines and HSR lines also, but only some trains must run at a lower speed. This decision to allow freight trains to use the HSR was due to the amount of income that freight transportation brought into Germany. This decision differed from Japan and France, where HSR lines are dedicated only to passenger traffic. The mix of traffic brings some disadvantages. For example, if trains with different speeds use the same line, it will decrease the line capacity and can increase safety problems.

It is problematic to produce a timetable that satisfies both passenger traffic and freight traffic because of the significant speed differences. Most of the daylight time was allocated to passenger trains, but during the night, the line needed to be maintained. This caused some serious delays for freight transport. Increasing the number of real-time sensors to monitor the condition of infrastructure and rolling stock and implementing proactive maintenance instead of reactive will decrease the cost and time that needs maintenance.

Many railways' tracks have been upgraded in Germany, but with the mix of traffic, German HSR lines cannot compare with the French network. To travel fast, there is a need to have not only advanced rolling stock but modern infrastructure too. Travelling time in France will be half of that in Germany for the same length of a journey on railways. For example, the travelling time from Paris to Marseille (661 km) is 3 h and 17 min [38], but a similar distance between Hamburg and Munich (791 km) will take 6 h [39]. Another reason that affects travelling time on DB is that there are too many stops for high-speed trains in rural areas with not enough demand for passengers travelling on trains.

The biggest environmental impact from HSR is noise pollution. In Germany, noise legislation for railways has been in force since 1974 [40]. The maximum noise levels for the new build or upgraded transport infrastructure in Germany are as follows in Table 3.

**Table 3.** German maximum noise level in dB(A) for newly built or upgraded transport infrastructures Reprinted from ref. [41].

Germany spends annually €150 million to mitigate noise pollution from railways, and by 2020, they reduced the noise level by 10 dB [42]. Because of this legislation, some parts of newly built high-speed lines have been built in cut-and-cover tunnels to reduce the noise level and visual impact for areas with a high density of population.

The safety of passengers travelling on the railways is paramount for every railway. At the present time, only ICE trains and Eurostar have been fitted with a warning system that can detect damages in bogies and wheels early. ICE has been equipped with a train control system (LZB) [14]. This signalling system provides the driver with information for several kilometres ahead. The LZB system improves passenger safety and allows an increase in track capacity. Similar signalling systems for high-speed trains have been developed in Japan and France. Apart from the advanced train control systems, passenger safety in Germany was secured by not having level crossings on HSR lines.

#### *3.5. HSR in Italy*

Italy was the second country after Japan that introduced high-speed trains, and the first train went into operation in 1977, but the line was only completed in 1992. Italy has 921 km of HSR in operation and 327 km under construction [7]. Italy was the only country in the world that opened the HSR network for competition. From 2012, NTV (Nuovo Transporto Viaggiaton), a private HSR company, began to operate [43].

ETR460 Pendolino, a tilting train that went into service in Italy in 1988 [7]. For a country with many mountains, it was convenient to use tilting technology on conventional lines. By introducing tilting technology, the train can travel around 30% faster. This active tilting technology soon spread around the world, and now around 70% of all high-speed trains are using it [44]. Pendolino is electric powered with 3 kV DC with a designed maximum operational speed of up to 250 km/h [7].

The train is formed from nine cars. The maximum axle load of an unloaded train is 13.5 tonnes, a train length of 237 m with 480 seats [7]. Different variations of this train are now used in Germany, Spain, and the USA. The ETR500 high-speed train went into service in 1995. It was the first high-speed train that was designed in Italy and only to be used inside Italy. It has concentrated power, two locomotives, one at each end and 12 trailers, and the total length of the train is 354 m. There were 59 sets of trains manufactured [7]. There was another version of ETR500 designed and built for the Italian and French railway systems. The train was designed for maximum operational speeds up to 300 km/h with improved aerodynamic performances, and the maximum capacity of the train is 671 passengers. The train can run on 3 kV DC and 25 kV AC [7]. There was some back down in the integrated Italian HSR network in the European system, as Italy has some HSR lines using 3 kV DC electrification instead of the standard European system of 25 kV AC [7].

The latest high-speed train in Italy, the ETR1000, went into operation in 2015. Trenitalia is investing €1.5 billion in these trains and will build 50 sets of them. At that moment, they produced 13 sets of these trains. Trains can travel up to 400 km/h with a maximum operational speed of 300 km/h, and it is the fastest train in Europe [45]. ETR1000 is 202 m long with distributed traction along the carriages, four motored coaches, and four trailer coaches [7].

The train has been designed to be compatible with different signalling systems and different electrification systems within the European HSR network. The train has compatibility with the European Traffic Control System (ETCS). The ETR1000 can carry 457 passengers. The cost of this train is around US\$40 million [46].

With expanding the HSR network and upgrading conventional lines, HSR in Italy is getting more attractive to customers. HSR lines in Italy run from Turin to Salerno, and Italy has more HSR lines in development: from Milan to Venice, from Milan to Genoa, and from Naples to Bari. Figure 6 shows the HSR network in Italy in 2015.

**Figure 6.** HSR network in Italy in 2015 Reprinted from ref. [7].

On the HSR line from Rome to Florence operated by the "Frecciarossa", ETR500 trains have four departures every hour from Rome with a maximum operational speed of 300 km/h. The Italian railway has a standard track gauge of 1435 mm and is electrified by 3 kV DC. It will take only 1 h and 30 min on HSR to travel from Rome to Florence compared to a conventional train that takes 3 h and 20 min [47]. The majority of the lines were built close to existing corridors to reduce the environmental impact of the projects. The Italian government had lots of investments put into developing the HSR system in Italy. Table 4 shows the construction costs of the selected HSR lines in Italy.

**Table 4.** Capital costs of HSR in Italy Reprinted from ref. [1].

The Rome-Florence line is mostly straight and with no level crossings, with one line in each direction. It is planned to upgrade this line by changing the electrification system from 3 kV DC to the European standard of 25 kV AC [1]. The same trains also operate on the Rome to Naples line, which is 205 km with two departures every hour and a top speed up to 300 km/h [7]. This line has 39 km of tunnels and 39 km of viaducts and bridges. Rome-Florence line was the first to introduce the ERTMS [1].

ERTMS is the most advanced signalling technology in the world. This system uses wireless technology to replace the signals along the railway track. A computer inside the train cabin controls the speed limit of the train and braking distance. The ERTMS system can automatically reduce the speed of the train if it exceeds the maximum allowed speed on the line [48].

One of the significant features of the Italian HSR network is the wide introduction of ERTMS that it integrated into interconnection with the European railway network. It opened the possibility to drive the same rolling stock with the same team around Europe with no need to change on the border and carry on at speeds up to 300 km/h. With building the HSR lines in Italy, they delegated conventional railways to freight transport and to serve the regional passengers. This system increases the safety of passengers travelling by train as it prevents human error. It reduces operational and maintenance costs, as there is no need to install and maintain signals along the railway tracks, and it increases the capacity of tracks. This system is known around the world as the most advanced and safe signalling system for high-speed trains.

#### *3.6. HSR in USA*

USA has only one HSR, the Northeast Corridor (NEC) from Boston to Washington D.C., 735 km long, and the same track is shared by freight and passenger trains with much lower speeds. In the future, HSR networks in the USA will expand, with 763 km under construction, and is planning to build another 2108 km [7]. There are many reasons the USA is behind other countries by implementing the HSR system. One of those reasons is that in the USA, the land that the tracks are on is regulated by the individual states, but transportation decisions are regulated through federal policy. The development of land and infrastructure is not run by one department, and often the local interest opposes national interest. Another reason is that policies in the USA encourage car ownership: larger subsidies in highway construction, low density of suburban housing, and cheap fuel.

It was December of 2000 when Amtrak introduced a new train, Acela Express, the first high-speed train in the USA that can travel at a maximum speed up to 240 km/h. Amtrak is the private company that owns the line and provided the railway service from 1971 with very limited federal subsidies. The Acela Express runs between Washington D.C. and Boston. This area has a very high density of population. At the end of 2012, Acela Express brought around 25% of the total Amtrak's service revenue [49]. On the northeast corridor from Washington, D.C. to New York, high-speed trains carried over 3.5 million

rail passengers every year and have 76% percent of the market share between Washington, D.C., and New York [1].

The track Washington D.C.-Boston stretches through areas with a very high density of population and does not have fences to protect trains from frequently encountered debris. In addition, the line between Washington D.C. and Boston has many level crossings. The trains in these circumstances must be built to ensure the safety of drivers and passengers. As the Amtrak needs to have an anti-collision structure, trains are 45% heavier than the similar French TGV trains [7]. Figure 7 shows the service map of Acela Express.

**Figure 7.** Acela Express service map Adapted from ref. [50].

The Acela Express train comprises two powered cars, one at each end, and six passenger carriages, and in use are 20 sets of trains. The maximum axle load is 23 tonnes, a length of 203 m with a seating capacity of 304 seats. A total of 44 of those are first class, and 260 are second class [7]. The Acela Express uses the conventional line that has been upgraded, and this limits the maximum speed to 240 km/h, but by using the tilting-train technology, it provided the possibility to cut journey time [51]. Avelia Liberty, the new trains, will start operating in 2021, and there have been orders to manufacture 28 new trains. Avelia Liberty will comprise 2 locomotives and 10 passenger cars with a total seat capacity of 512 [7].

Avelia Liberty will be a tilted train with concentrated power and a maximum operational speed of 257 km/h [7]. The new trains will cut travel time by approximately 30 min. Tilting-track technology can be an alternative option for building new tracks or straightening existing ones. The tilting technology prevents passengers from having some discomfort from the lateral acceleration, also much cheaper than having to build new tracks [52]. This technology allows the train to have higher speeds in curves, which can reduce the journey time. Figure 8 shows the tilting bogie technology called passive tilt.

When the train runs on a curve, the tilting system tilts the body of the carriage up to a 5◦ gradient [12]. In addition, another tilting system has been developed, the controlled tilt system, called active tilt (Figure 9). Active tilt technology was first implemented in the United Kingdom. The onboard train computer stores all information about the curve radius, alignment, elevation, and the railway line where the train will run. Tilting the carriages start approximately from 30 to 40 m before the carriages enter the curve as it was found that it will reduce the passengers feeling the sense of motion sickness [12]. The Acela Express has active tilt technology [7].

**Figure 8.** Roller-type tilt system Adapted from ref. [12].

**Figure 9.** Principles of controlled tilt systems Reprinted from ref. [12].

#### *3.7. HSR in China*

The first HSR line, 405 km between Qinhuangdao and Shenyang North, went into operation in 2003, with a maximum speed of 250 km [7]. By 2020, China has the largest HSR system in the world, approximately 60% of the length of the world's HSR network. In 2020, China had 35,388 km of newly built passenger dedicated HSR lines, 5250 km under construction, and is planning to build another 1328 km [7]. HSR network in China will consist of eight horizontal and eight vertical HSR lines, which will connect the largest cities of China that have a population of 500,000 or more [53]. It will connect the south and north of China and west and east. The next step will be to connect China with Taiwan by an underwater tunnel [54].

China has some unproportioned population density, with more people living in the east part of China than in the west. The development of a new HSR will help to satisfy increasing demand in travel to the east of China. The most famous line in China is a line between Beijing and Shanghai. The line's length is 1318 km, it cost around €28 billion to build, and it is the third longest HSR in the world [55]. It took less than two years to build this line. HSR started operating in 2011. Trains can run at speeds up to 350 km/h. The line has 23 stops, and in the first year after opening, over 52 million passengers travelled on it [53].

Around 90 trains depart every day between Beijing and Shanghai. There are two types: super-fast with speed up to 300 km/h and just fast with speed up to 250 km/h. The

journey time by the fastest train from Beijing to Shanghai is 4 h 48 min [55]. HSR lines can transport freight during night-time. Figure 10 shows the HSR network in China in 2015.

**Figure 10.** HSR network in China, in 2015 Reprinted from ref. [7].

China has around 2500 high-speed train sets. The train CRH380-AL has been designed and manufactured in China, and it first went into service in 2011. It has a maximum operational speed of up to 300 km/h. CRH380-AL can be formed from 8 to 16 carriages with a capacity of up to 1061 passengers. The 16-carriage train has distributed power with 14 motored coaches and 2 trailer coaches, and it is 403 m long [7].

This train, equipped with a 25 kV AC 50 Hz electric system, has two braking systems, and one of them is the regenerative system, which generates electricity when the braking system slows the train down [7]. The train can accelerate up to 380 km/h [56].

#### *3.8. HSR in Turkey*

The first HSR line, 221 km between (Ankara-)Sincan and Eskisehir, went into operation in 2009 with a maximum speed up to 250 km [7]. With the opening of HSR, the travel time between Ankara and Istanbul has been reduced by more than half to just 3 h from 6 h and 30 min. The HSR lines are equipped with an ETCS Level 1 signalling system. In 2016, there were 38 high-speed trains per day, but by 2023, it will increase to 300 services per day, which will carry 120,000 passengers, and passenger numbers can increase to 945 million per year [57]. The development of HSR is funded mostly by the government and by credits from foreign banks [58]. Turkey has the fifth largest HSR network in Europe and the ninth largest in the world. Figure 11 shows the HSR network in Turkey in 2015.

**Figure 11.** HSR network in Turkey, in 2015 Reprinted from ref. [7].

Ankara-Eskisehir CAF (Spanish railway vehicle constructor) manufactured trains Class HT65000. The trains have a maximum operational speed of 250 km/h, have distributed power, comprising four motor coaches and two trailer coaches, and have a capacity of 419 seats [7]. In 2013, the State Railways of the Republic of Turkey (TCDD) ordered Siemens to manufacture 17 EMU (Electric Multiple Unit) Velaro-D Class HT80000, with distributed power, with four motored coaches and four trailer coaches, and with a maximum operational speed of 300 km/h [59]. In addition, Turkey intends to purchase another 106 high-speed trains [60]. However, one of Turkey's targets is to manufacture all HSR stock in-house and convert existing RS to be compatible with HSR.

In 2020, Turkey had 594 km of HSR in operation, 1652 under construction, and is planning to build another 5173 km [7]. HSR will connect 16 of the largest cities, and the government is planning to connect 55% of the population within an HSR network by 2023 [61]. Turkey wants to integrate into the EU and be a part of the European Single Market and part of the European Transport Network.

#### *3.9. HSR in Taiwan*

Figure 12 shows the route of HSR in Taiwan. The HSR in Taiwan comprises one double-track line 354 km long [7]. The line runs along Taiwan's western corridor from the capital of Taiwan (Taipei) to the main industrial city of Kaohsiung on the south of the island. These are the two largest cities in Taiwan. The first 345 km of HSR in Taiwan went into operation on 5 January 2007 with a maximum operational speed of 300 km/h from Taipei to Kohsiung [7]. In 2016, an additional 9 km was built from Taipei to Nangang [7]. In September 2018, the government announced that by 2029 they will build a new HSR line from Kaohsiung to Pingtung 17.5 km long. The estimated cost of a new line is US\$1.78 billion [62].

**Figure 12.** HSR network in Taiwan Adapted from ref. [63].

The line has a standard gauge of 1435 mm and is built on the slab track. To avoid crossing roads on the same level, 90.7% of the line was built in tunnels and on viaducts. The total cost of HSR was estimated to be US\$15 billion [64]. Figure 13 shows the ratio of HSR structure type by length.

**Figure 13.** Ratio of HSR structure type by length Adapted from ref. [65].

This line links towns and cities with 94% of the total population in Taiwan. The development of this line reduced the number of car journeys and the number of flights. As a result, it reduced the dependency of Taiwan on the export of petroleum and reduced CO2 emissions from the transport sector.

In 2013, ridership in a single day reached 200 thousand, and in 2018 it increased to 285 thousand. In 2017, the number of passengers using HSR increased to 60.57 million with 198 daily services. The total number of trains reached 51,751 with a 65.16% occupancy rate [63]. THSRC has an excellent punctuality rate of 99.72% [64].

The HSR in Taiwan is based on the technology of the Shinkansen. This is a new line, and as often to reduce the cost of the project, the stations are located outside of cities. Passengers can reach a station by a free shuttle bus from the centres of towns. In 2017, it provided 393,819 free shuttle journeys [64].

According to UIC HS Rolling Stock tables, Taiwan has in operation 34 trains, Class T700. The design of trains is based on the Series 700 Shinkansen and modified for THSR. Trains went into operation in 2007 [7]. The number of trains will increase to 54 by 2033. Each train comprises nine motor coaches and three trailer coaches. The maximum operational speed is 300 km/h. Each EMU has a length of 304 metres and has 989 seats, one coach first class with 66 seats and 11 coaches second class with 923 seats [7]. The train is powered by an overhead electric line with a voltage of 25 kV 50 Hz, equipped with a signalling system that allows operation on both tracks in both directions. The HSR line is equipped with an Automatic Train Control (ATC) system. The trains are equipped with an earthquake early warning system. This system allows the train to stop or slow down 10 s before an earthquake hits [64].

To cover 354 km without stops, it takes only 96 min, and in 2007 there were 61 trains every 24 h in each direction, from 7 am to 9:06 pm. In 2015, the number of stations reached 11, and the running time of trains stopped at each station was reduced to 138 min. In 2017, the number of stations increased to 12 [66]. Table 5 shows the increased ridership on HSR in Taiwan from 2013 to 2017.

#### **Table 5.** Operational statistics Reprinted from ref. [64].

#### *3.10. HSR in South Korea*

Figure 14 shows the HSR network in South Korea. In 2020, the total length of HSR lines in operation was 893 km, with another 49 km planned to build. The first HSR line from Seoul to Dongdaegu went into operation on 1 April 2004 [7]. The HSR network comprises two corridors, one from Seoul to Busan, an area where over 70% of the population lives, and another one from Seoul to Gwangju. The lines have a standard gauge of 1435 mm and are built on ballasted track with concrete sleepers. The estimated cost of 411 km HSR from Seoul to Busan was US\$17 billion [67]. It was an expensive project, as 190 km is in tunnels and 120 km on bridges and viaducts. The HSR network gives an opportunity for commuters to travel to most parts of the country in 1 h 30 min. Figure 15 shows the ratio of structure type by the length of HSR in the corridor Seoul-Busan.

With the opening in 2004 of HSR, the travel time on the Seoul-Busan route was reduced from 4 h and 10 min to 2 h and 40 min and after developing a new line between Daegu and Busan times was cut to 1 h 46 min [69]. Table 6 shows the HSR lines in South Korea.

In the first 10 years, from 2004 to 2014, KTX carried around 414 million passengers with an average of 150,000 per day [70].

**Figure 14.** HSR network in South Korea, in 2015 Reprinted from ref. [7].

**Figure 15.** Ratio of structure type by length of HSR Adapted from ref. [68].

**Table 6.** High-speed lines in South Korea on 1 October 2019 Adapted from ref. [1].

According to UIC HS Rolling Stock tables, Korea has in operation 117 KTX (Korea Train Express) trains, with a maximum operational speed of 300 km/h [7]. The train design was based on the French TGV Reseau. The first 46 trains went into operation in 2004. A total of 12 of them were supplied by Alstom and 34 manufactured in Korea by Hyundai Rotem. Korea was the fourth country in the world after Japan, France, and Germany, which uses its own technology to build trains with a maximum speed of 330 km. The first 46 trains have a set formula of 2 locomotives, 2 motorised trailers, and 16 trailer coaches. The trains are 388 m long with a maximum number of seats of 935 and with maximum traction power of 13.200 kW. The latest 71 trains manufactured by Hyundai Rotem comprise two locomotives and eight trailer coaches. They are 201 m long with 363 seats, 30 seats in first class and 333 seats in second class, and a maximum traction power of 8800 kW [7].

These trains a lighter as the car body is made from aluminium alloy, and the powered car body is made from mild steel, compared to the first 46 trains, in which the bodies were made from steel. The weight of 10 cars is 434 tonnes. The train is powered by an overhead electric line with a voltage of 25 kV 60 Hz, equipped with an Automatic Train Control (ATC) system, which is continuously checking the speed of the train [7].

#### **4. Conclusions and Prospects**

In the present chapter, different HSR systems worldwide have been discussed and what they have in common, and how they differ from each other. HSRs worldwide have some technical and organisational differences, such as a difference in operating voltages (Spain), different gauges (Spain, Japan), signalling systems (France, Germany), and languages (Eurostar). The most challenging of them is the Eurostar, which crosses three different railway networks. It has three different power collection systems and signalling systems for Belgium, France, and the U.K. The first HSR was built in 1964, but now 44 countries worldwide have constructed or are planning to construct HSR systems. Amongst them are countries such as India, Indonesia, South Africa, Iran, Brazil, and Russia, and the numbers are growing.

High-speed trains can be very competitive for trips between 150 and 700 km for links between urban centres with a high density of population. It can break the dependency of the transport system on fossil fuels. HSR is powered by electricity and can be almost zero carbon, but it must be powered by renewable energy, as the energy type can significantly influence the level of environmental impact. To support the decarbonisation of railways, there is a need to improve the operational strategies, optimise scheduling, and maximise the use of rolling stock and infrastructure.

The role of the HSR for moving people and goods will steadily increase. HSR must be cheap to build, cheap to maintain, and with high occupancy of seats. Sometimes, it would be more beneficial to upgrade the existing railway network than build new HSR lines. There is a need to ask, are the conventional lines profitable, and if not, why would the new HSR perform any different?

The HSR network in Europe was developed in different countries at different times and standards and to link all the railway networks into one system. There is a need to have compatible technical standards. The standardisation and harmonisation of track gauge, maximum load per axle, systems of electric traction, signalling systems, and line profiles is crucial to operating effectively across Europe. The differences in railway technical standards in Europe create an additional cost for all railway systems. By reducing the number of different HSR technologies and increasing the standardisation, it will improve safety, reduce the capital cost, and increase the compatibility of railways and, in the long term, will reduce the journey costs.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Entry Link on the Encyclopedia Platform:** https://encyclopedia.pub/13532.

#### **References**

## *Entry* **Geometric Design of Suburban Roundabouts**

**Saša Ahac \* and Vesna Dragˇcevi´c**

Faculty of Civil Engineering, University of Zagreb, 10000 Zagreb, Croatia<EMAIL_ADDRESS>**\*** Correspondence<EMAIL_ADDRESS>
**Definition:** A modern roundabout is an intersection with a circulatory roadway at which the vehicle speed is low, and the traffic is continuous and circulating in one direction around the central island towards the exits at the approach legs. Modern roundabout design is an iterative process that is composed of the following steps: (1) the identification of the roundabout as the optimal traffic solution; (2) the definition of the number of lanes at the intersection based on the required capacity and the level of service; (3) the initial design of the roundabout geometry; (4) design vehicle swept path, the fastest path analysis, and visibility performance checks; and (5) detailed roundabout design if the results of the performance checks are in line with the design recommendations. Initial roundabout geometry design elements are not independent of each other; therefore, care must be taken to provide compatibility between them. An overview and a comparative analysis of the initial geometric design elements for suburban single-lane roundabouts defined in roundabout design guidelines and norms used in Croatia, Austria, France, the Netherlands, Germany, Serbia, and Switzerland is given in this entry.

**Keywords:** approach alignment; outer radius; circulatory roadway; apron; splitter island; roundabout entry; roundabout exit; longitudinal slope

#### **1. Introduction**

The development of modern roundabouts began in the 1960s in the United Kingdom with the adoption of the yield-at-entry rule, which gave the circulating traffic priority over entering traffic [1]. Modern roundabouts spread to other parts of Europe in the 1980s [2]. Intensive construction of roundabouts has begun in Europe in the last 30 years. European countries that stand out in the total number of roundabouts are France (63,212), Spain (36,762), and Italy (30,917) [3], and states like the Netherlands, Sweden, Switzerland, Denmark, Finland, Germany, and Austria are also pursuing policies of mass roundabout construction.

Modern roundabout design is an iterative process, and it begins with the identification of the roundabout as the optimal traffic solution in the given conditions. The initial roundabout design refers to (1) defining the size of the intersection by selecting the outer radius, (2) laying the approach leg axes, and (3) defining the geometry of the design elements on roundabout entry and exit lanes, circulatory roadway, and central island. The initial design of the roundabout is usually followed by three performance checks: the examination of the design vehicle swept path, the fastest path analysis, and the visibility checks. If the results of these checks are not in line with the design recommendations, the geometry of the elements applied in the initial design phase is modified.

In this entry, guidelines for the initial design of individual geometric elements at a suburban single-lane roundabout, given in design guidelines and norms that are used in Croatia [4], Austria [5], France [6], the Netherlands [7,8], Germany [9], Serbia [10,11], and Switzerland [12–15], are presented. These documents are selected for the following reasons. Firstly, they all define the following geometric elements of suburban roundabouts: the outer radius, the circulatory roadway, the apron, the splitter islands, entry and exit design, and the longitudinal slopes of the approaches and/or the intersection plane. Secondly,

**Citation:** Ahac, S.; Dragˇcevi´c, V. Geometric Design of Suburban Roundabouts. *Encyclopedia* **2021**, *1*, 720–743. https://doi.org/10.3390/ encyclopedia1030056

Academic Editors: Raffaele Barretta, Ramesh Agarwal, Krzysztof Kamil Zur and Giuseppe Ruta ˙

Received: 17 June 2021 Accepted: 4 August 2021 Published: 5 August 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the geography and the terrain in the countries these documents originate from are different, which affects the intersection design elements' dimensions and shapes. For instance, predominantly flat terrain can be found in the Netherlands, whereas predominantly mountainous terrain is found in Switzerland and Austria. At the same time, all terrain types are represented in France, Germany, Croatia, and Serbia. The third reason for the selection of the documents is the year in which they were issued, ranging from 1991, when Swiss guidelines were published, to the year 2014, when the newest edition of Croatian guidelines for suburban roundabouts was issued. Namely, this 23-year range was marked by the mass construction of roundabouts in all the above-mentioned countries, so we believe it would be interesting to observe whether and how roundabout design approaches changed in that period. The Federal Highway Administration (FHWA) guidelines are not included in this entry because the American design vehicles used in the swept path analysis (and, consequently, the dimensions of roundabout design elements) are larger than the vehicles that can be found on European roads. The reason the United Kingdom and Australian guidelines are omitted from this entry is the fact that the UK and Australian traffic drives on the left.

#### **2. Suburban Roundabout Design Elements**

Modern roundabout design is a process of determining the optimal balance between safety provisions, operational performance, and accommodation of the design vehicle. The criteria for the acceptability of roundabouts are usually defined in national guidelines and are adapted to the local circumstances [16]. These criteria include a functional criterion, capacity criterion, spatial criterion, design and technical criteria, traffic safety criterion, and economic criterion [4,7,10]. According to [4], to determine whether a roundabout is the best solution at a particular location, it is necessary to examine what the primary function of the planned intersection is (source-destination or transit traffic), what its role in the traffic network is, and its position concerning settlements (urban or suburban), as well as in the wider traffic network. The capacity criterion should examine whether the roundabout is an acceptable solution for the existing and expected circumstances, given the traffic flows and the distribution of traffic. The spatial criterion examines the availability of space for the geometric elements of the intersection. Design and technical criteria examine the geometry of the intersection, the position of the approaches, the number of the approaches, and the angle between the axes of the consecutive approaches. Traffic safety criteria should examine whether the roundabout in the existing conditions is a solution that guarantees safety to all road users. To achieve the required level of traffic safety, roundabouts must be recognizable in the traffic network and their geometry must force traffic to enter and circulate at a slow speed. Therefore, we should strive for the application of (1) uniform design solutions and (2) standardized design elements that provide sufficient deflection around the central island, with the minimal adaptation of these elements to the limitations arising from the specific location of the planned intersection.

According to the above-mentioned criteria, relevant parameters of roundabout design are spatial requirements and limitations, traffic load, design speed, and traffic flow structure. Based on these parameters, the geometric elements of roundabouts, which are described below, are defined. As these geometric elements are not independent of each other, care must be taken to provide compatibility between them to meet the design vehicle swept path requirements, overall safety, and capacity objectives. Geometric elements of modern suburban single-lane roundabouts are presented in Figure 1 and described below.

The central island is a raised physical barrier with a (usually) circular ground plan, placed in the center of the roundabout around which traffic circulates.

The circulatory roadway is a lane at which vehicles circle the central island in a counterclockwise direction. Vehicles at a circulatory roadway have priority over vehicles entering a roundabout.

The outer radius is the radius of the outer edge of the circulatory roadway. It is the sum of the central island radius and the circulatory roadway width.

**Figure 1.** The geometric elements of single-lane roundabouts.

The apron is the traversable part of the central island that may be needed to allow long vehicles to negotiate a roundabout. It differs from the circulatory roadway by the cross slope, material used for the final layer, and/or color.

The entry line marks the point of entry into the circulatory roadway. This line is an extension of the circulatory roadway edge line, and it functions as a yield or give-way line: Entering vehicles must yield to any circulating traffic coming from the left before crossing this line into the circulatory roadway.

The splitter island is a raised or drawn element on the roundabout approach intended to channel traffic flows at the entrance and exit of the intersection.

The roundabout entry is bounded by a curb or edge of pavement consisting of a curve leading into the circulatory roadway. This curve is tangential to the outside edge of the circulatory roadway.

The roundabout exit is bounded by a curb or edge of pavement consisting of a curve leading away from the circulatory roadway. This curve is tangential to the outside edge of the circulatory roadway. The exit radii are usually larger than the entry radii to minimize the likelihood of congestion and crashes at the exits.

The entry width is the shortest distance between the intersection of the line connecting the vertices of the opposite dividing islands and the entrance line (point S) and the right curb or edge of the pavement at the entrance to the roundabout.

The deflection is the distance between a straight line joining the vertices of opposite dividing islands and its parallel that is tangent to a central island.

#### *2.1. The Approach Alignment*

Roundabout approach alignment affects the curvature of the vehicle's trajectory when passing through the intersection, the accommodation of the design vehicles, and the viewing angles on the adjacent approaches. The optimal design of a roundabout depends on the size and position of the roundabout concerning the approach alignment.

The standard approach alignment on suburban roundabouts is radial, where the approach axes intersect in the center of the outer radius of the intersection (Figure 2a). This alignment allows the appropriate design of the geometric elements of single-lane roundabouts, which ensures low vehicle velocity. The radial alignment of the approaches ensures the visibility of intersections in the traffic network and minimizes the need to modify the geometric elements on the approaches. An alternative to radial alignment is a displacement of the intersection approach axes, which can be carried out either by the displacement of this intersection to the left of the center of the outer radius (Figure 2b), or by laying the approach axes so they intersect to the right of the center of the outer radius of

the roundabout (Figure 2c). Displacement to the left increases the curvature of the entrance path of the vehicle, i.e., provides better speed control at the intersection. Displacement to the right (Figure 2c) should be avoided, as it does not provide sufficient curvature of the vehicle entry path, i.e., this alignment allows vehicles to enter the intersection too fast, which increases the risk of collision.

**Figure 2.** The approach alignment variants according to [8]: (**a**) radial alignment geometric elements of single-lane roundabouts, (**b**) displacement to the left, and (**c**) displacement to the right. The red "×" marks the center of the roundabout outer radius.

The optimal angle between the approach axes is 90◦. If the axes intersect at an angle significantly greater than 90◦, the permitted driving speed may be exceeded for right turns. If the approach axes intersect at an angle significantly less than 90◦, the required accommodation of long vehicles is disabled. Increasing the radius of curve at the right edge of the pavement at the entrance to such intersections (which must be done to accommodate long vehicles) increases the entrance width, which reduces the level of intersection safety, as it can result in higher vehicle velocity. Reducing the angle between the approach axis results in the need to increase the outer radius of the roundabout to meet the conditions of the design vehicle swept path and speed requirements at an intersection.

#### *2.2. The Outer Radius*

At single-lane roundabouts, the size of the outer radius depends on the spatial requirements and limitations, the number and alignment of the approaches, the traffic requirements of the design vehicle swept path (the outer radius must allow the passage of the design vehicle), and the design speed (the curvature of the vehicle's trajectory must limit driving speeds to ensure low vehicle speeds when passing through the intersection).

When examining the swept path of the design vehicles, it must be demonstrated that the design vehicle can pass through the intersection using the available lane width while ensuring minimum lateral clearance (usually between 25 and 50 cm wide) and respecting road markings. The design vehicle is a vehicle of a certain type and dimensions that characterize a certain group of vehicles and fully complies with the legal regulations on vehicle dimensions, i.e., international recommendations. Design vehicles are used to examine the possibility of vehicles of a certain category passing through an intersection. They are most often selected according to the position of the intersection in the road network, depending on the structure and category of vehicles that appear on the observed section of the road.

The recommended and limit values of the outer radii at suburban single-lane roundabouts listed in the analyzed documents are shown in Figure 3.

**Figure 3.** Recommended outer radii for single-lane roundabouts according to [4–12].

#### *2.3. The Circulatory Roadway*

At single-lane roundabouts, the width of the circulatory roadway depends on the swept path requirements of the design vehicle, whereas the cross slope of the circulatory roadway depends on surface drainage and topographic conditions. The cross slope of the circulatory roadway is usually directed towards the outside of the intersection. Such a direction is suitable for the following reasons: It increases the safety of intersections by raising and improving the visibility of the central island, it reduces vehicle speed on the path around the central island, it minimizes cross slope changes at the entry and exit, and it helps surface drainage.

According to [4], the minimal width of a circulatory roadway is determined based on the swept path of a selected two-axle design vehicle for driving in a full circle. Recommended values for the width of the circulatory roadway range from 4.5 to 6.0 m, and the recommended value of the cross slope of the circulatory roadway according to [5] is smaller than or equal to 2.5%.

According to [5], the width of the circulatory roadway ranges from 6.5 to 9.0 m. When determining the width of the circulatory roadway, it is necessary to ensure the deflection of the vehicle driving around the central island. If sufficient deflection is not achieved, it is necessary to plan the construction of a larger central island with an apron. The recommended value of the cross slope of the circulatory roadway according to [5] is 2.5%, where the slope is directed towards the outer edge of the intersection, as shown in Figure 4a. If, due to topographic conditions, it is necessary to construct an intersection with a longitudinal slope greater than 2.5%, the circulatory roadway is placed in one plane, with a slope less than or equal to 4% (Figure 4b). The minimum value of the cross slope is 1.5%, which ensures sufficient surface drainage of the intersection.

According to [6], a circulatory roadway should not look like a one-way multi-lane road, but a single lane of sufficient width to ensure the accommodation of the design vehicle. On single-lane roundabouts, the width of the circulatory roadway depends on the outer radius and the width of the widest entrance to the intersection. The width of the circulatory roadway is fixed and 20% larger than the widest entrance, with the minimum width of the circulatory roadway being 6 m and the usual width 7 m. The use of an 8-m-wide circulatory roadway is justified at roundabouts where the design vehicle is a tractor with a semi-trailer. The cross slope of the circulatory roadway is also constant along its entire length and its value can range from 1.5 to 2%. The cross slope is directed towards the outer edge of the intersection. These values do not apply to roundabouts on steep slopes, which should in any case be avoided. Additionally, the maximum cross slope of the roundabout is 3%.

**Figure 4.** Circulatory roadway cross slope according to [5]: (**a**) recommended cross slope direction and (**b**) circulatory roadway in one plane. Slope direction is represented by the arrows.

According to [7,8], the standard width of the circulatory roadway at single-lane suburban roundabouts is 5.25 m, regardless of the dimensions of the design vehicle. It is noted that these standard values must not be adopted as absolute: The width of the circulatory roadway must meet the requirements of the design vehicle, but also the condition of limiting the speed of the vehicle passing through the intersection. Circulatory roadway width must be fixed along the entire length of the lane. Figure 5 shows the values of the width of the circulatory roadway ("B") as a function of the outer and inner radius of the roundabout ("Rbu" and "Rbi") for single-lane roundabouts. The cross slope of the circulatory roadway according to [7] is 2.0 to 2.5% and is directed towards the outer edge of the roundabout.

**Figure 5.** Circulatory roadway width ("B") depending on the outer and inner radius ("Rbu" and "Rbi") [8].

According to [9], the circulatory roadway, together with the apron, which is not included in the width of the circulatory roadway, forms a circular ring of intersection (width "Bk"). The circulatory roadway is of constant width and a constant cross slope. The circulatory roadway width depends on the size of the outer radius—the widths of the circular ring of intersection ("Bk") for single-lane roundabouts depending on the size of the outer radius are given in Table 1. For suburban roundabouts, the widths of the circular ring of intersection ("Bk") shown in the table correspond to the widths of the circulatory roadway, since the aprons are not constructed at these intersections. At suburban intersections with increased truck traffic, it is possible to apply larger widths of the circulatory roadway than those given in the table. The cross slope of the circulatory roadway is directed towards the outer edge of the intersection and must be at least 2.5% to ensure proper surface drainage of the pavement.

**Table 1.** Circular ring width ("Bk") depending on the outer radius [9].

According to [10], the width of the circulatory roadway ("bk") arises from the swept path requirements of the design vehicle and the driving conditions. It can be standardized to some extent for typical suburban road network conditions for single-lane and two-lane roundabouts. The standard widths of the roundabout ("bk") for single-lane roundabouts (Figure 6) are defined depending on the diameter of the inscribed circle of a roundabout ("D") and include marginal strips that are 0.20 m wide.

**Figure 6.** Circulatory roadway width ("bk") for single-lane roundabouts [10].

According to [10], the standard cross slope of the circulatory roadway is 2.5%. The cross slope is directed towards the outer edge of the intersection. The maximum cross slopes of the circulatory roadway ("ip") as a function of the slope of the intersection plane ("iNkr") is shown in Table 2. At the same time, the largest total slope is 4% [10].

**Table 2.** Circulatory roadway slope [10].

According to [12], the total width of the circulatory roadway and apron depends on the outer diameter of the roundabout and the swept path conditions of the design vehicle. Figure 7 shows a diagram used to determine the minimum widths depending on the outer diameter of the intersection. These widths are determined by swept path analysis for the design vehicle defined according to [13]. These widths do not include lateral clearances. Larger circulatory roadway widths should be avoided for safety reasons. It is recommended that for a width of the circulatory roadway greater than 5.5 m, the apron be used, intended only for tractors with semi-trailers and trucks with trailers. The maximum cross slope of the circulatory roadway is 5% (exceptionally 7%, in demanding topographic conditions). The minimum cross slope of the circulatory roadway is limited to 3% due to the pavement surface drainage conditions.

**Figure 7.** Minimum circulatory roadway width (together with apron width) depending on the roundabout inscribed diameter [12].

Standard circulatory roadway widths for suburban single-lane roundabouts listed in the analyzed documents are shown in Figures 8 and 9. The values in Figure 8 refer to single-lane roundabouts with and without the apron, the application of which depends on the choice of design vehicle: The apron is intended exclusively to accommodate trucks (tractors with semi-trailers, trucks with trailers).

**Figure 8.** Recommended and limit circulatory roadway widths and cross slope according to [4–12].

According to the data shown in Figure 9, the highest values of the width of the circular lane are defined by the German guidelines and the lowest values by the Dutch guidelines. The width of the circular lane determined by the diagrams shown in Figure 9 is usually rounded up to a larger 0.25 m. According to [9–12], the width of a circular lane includes an additional element—an apron, the transit edge of the central island, described below.

#### *2.4. The Apron*

At roundabouts, to ensure the low driving speed of passenger cars while meeting the traffic conditions of long vehicles, it is sometimes necessary to provide an apron on the edge of the central island.

According to [4], the apron width is determined based on the swept path of a design vehicle for driving in a full circle. For suburban roundabouts, this design vehicle is a 16.5 m-long tractor with a semi-trailer.

According to [5], the apron is constructed only when the required deflection cannot be achieved due to the conditions of the swept path for the design vehicle. The widths and other design features of the apron are not described in the document.

**Figure 9.** Standard circulatory roadway widths for suburban single-lane roundabouts [8,11].

According to [6], the apron is constructed at roundabouts with an outer radius in the range of 12 to 15 m. The apron width is in the range of 1.5 to 2.0 m (Figure 10). The apron may also be constructed at roundabouts with an outer radius larger than 15 m if convoys of special cargo pass through the intersection. The cross slope of the apron is directed towards the outer edge of the intersection and ranges from 4 to 6%. It is recommended to raise the apron above the circulatory roadway by applying a low curb (3 cm high) and to make a final layer of stone cubes so that the edge is visible in both day and night driving conditions.

**Figure 10.** Standard circulatory roadway cross-section for roundabouts with an outer radius of 15 m, according to [6].

According to [7,8], the apron width depends on the design vehicle and the combination of dimensions of the remaining geometric elements of the roundabout. The standard cross slope of the apron is 1%. The apron is constructed with a low curb with a slight slope (the height difference compared to the circulatory roadway pavement is 5 cm at most) and with a paved surface (cubes) [8]. The standard apron width at suburban single-lane roundabouts is 1.5 m. Depending on the dimensions of the design vehicle, the apron width is 3 m (for a vehicle 22 m long) or 4 m (for a vehicle 27 m long) [7].

According to [9], aprons are not constructed on suburban roundabouts.

According to [10], the inner edges of the circulatory roadway are formed by applying elements of a different structure (e.g., paving elements, a small cube). The central island is shaped with one hog curve at the center of the island and two sag curves at the outer edge of the island (Figure 11), and the slope of the tangents between these elements is 4%. The document in question, which refers to suburban roundabouts, does not envisage the construction of the apron. According to [11], the apron width is determined based on swept path analysis for the design vehicle, with the minimum apron width being 1 m. This element is raised above the circulatory roadway by 3 cm, and its cross slope is 4% (3% in exceptional cases).

**Figure 11.** Central island design [10].

According to the standard [12], for circulatory roadway widths greater than 5.5 m, it is recommended to use the apron, intended only for trucks. The design of the apron is not defined in the specified standards. According to [15], the usual width of the apron (paved with stone cubes) at single-lane roundabouts is 1.5–2.0 m.

The standard widths and cross slopes of the apron at suburban single-lane roundabouts listed in the considered documents are shown in Table 3.

#### *2.5. The Splitter Island*

Splitter islands are mandatory elements of modern suburban roundabouts. They provide better control of the vehicle speed by channeling traffic flows and provide space for vertical signalization. When designing roundabouts, it is necessary to determine the space for the splitter island before defining the width of the entrance and exit lanes.

The basic functions of the splitter islands are as follows:

According to [4], the use of the triangular or funnel-shaped splitter island is mandatory on suburban roundabouts. These splitter islands can be raised above the pavement by 15 cm, or drawn, which is a more flexible solution at intersections with heavy truck traffic.

According to [5], the splitter islands must be provided on all approaches to the intersection. In addition, at roundabouts located in unfavorable topographic conditions (resulting in convex curves in vertical alignment or large turning angles on the approaches), it is possible to ensure the required recognizability of intersections by applying long splitter islands. The minimum width of the splitter island is 2 m, but this guideline recommends the use of a wider island to separate the entry and exit lanes. The rounding of the top of the splitter islands is formed by a radius of at least 0.75 m, and the distance of the island from the outer edge of the roundabout is at least 0.25 m [5].

According to [6], raised splitter islands are used for the physical separation of entry and exit lanes on the roundabout approaches. The splitter island on the suburban roundabout is usually funnel shaped. The standard length of the island ("H") is equal to the value of the outer radius of the roundabout ("Rg"), as shown in Table 4. The splitter island on suburban roundabouts (where the outer radius is equal to or larger than 15 m) is shifted to the left to ensure that the approach axis passes through the center point at the top of the island (point C, Figure 12). The minimum width of the splitter island is 2 m, and the usual width at the entrance to the intersection ("B") is equal to a quarter of the outer radius of the intersection.

**Table 4.** Splitter island elements according to [6].

**Figure 12.** Splitter island elements according to [6]. The center point at the top of the island (point C) is represented by the red circle.

> According to [7,8], splitter islands are placed on each approach to separate incoming and outgoing traffic flows. They can be raised or drawn, which is a more flexible solution at intersections with heavy truck traffic. The distance between the edge of the raised splitter island and the outer edge of the roundabout is approximately 1 m, to improve the traffic conditions at the intersection. In terms of their layout, the splitter islands used in the Netherlands are exclusive of the radial type (Figure 13), i.e., the edges of the islands are parallel to the axis of the approach, which passes through the center of the roundabout. The advantages of such a layout are as follows:

**Figure 13.** Radial splitter island elements for suburban roundabouts according to [7,8].

The recommended length of the raised part of the splitter island at single-lane suburban roundabouts ("Lm") is 10 to 15 m, and the standard width of the island ("Bm") is 3 m (Figure 13).

According to [9], the axis of the splitter island is perpendicular to the outer edge of the roundabout, with the smallest width of the island being 1.6 m. At suburban roundabouts, funnel-shaped splitter islands are used to achieve the required lane width at the entrance to the intersection and to ensure that the curvature of the splitter island drawn edge follows the curvature of the vehicle path (Figure 14).

**Figure 14.** Funnel-shaped splitter islands on suburban roundabouts according to [9].

According to [10], the shape of the splitter island is conditioned by the desired level of traffic-flow channeling. If at single-lane roundabouts the outer radius of the intersection is greater than 20 m and/or the maximum design speed is equal to or greater than 60 km/h, the highest level of channeling is applied (Figure 15). A medium level of channeling (Figure 16) can be applied at a roundabout with an outer radius between 14 and 20 m with a maximum design speed between 50 and 60 km/h. The lowest level of channeling (Figure 17) is applied at the intersections of access roads and intersections of collector and access roads when an outer radius is smaller than 15 m and the highest design speed is below 50 km/h.

**Figure 15.** Splitter islands for a high level of traffic-flow channeling [10].

**Figure 16.** Splitter islands for a medium level of traffic-flow channeling [10].

**Figure 17.** Splitter islands for a low level of traffic-flow channeling [10].

According to [12], the shape and dimensions of splitter islands depend on the widths of the entry and exit lanes, and the smallest dimensions are defined by the norm [14]. The minimum allowable width of splitter islands is 1.2 m, and the length of the island ranges from 30 to 50 m. A splitter island at a suburban roundabout defined according to the guidelines [15] is shown in Figure 18—the use of funnel-shaped islands with a minimum width of 5 m is recommended.

**Figure 18.** Splitter island according to [15].

An overview of the recommended dimensions of splitter islands listed in the considered documents is given in Table 5.

**Table 5.** Recommended design shape and dimensions of splitter islands according to [4–12].

#### *2.6. The Entry and Exit Design*

The design of the roundabout entry and exit refers to the definition of the widths of lanes and the radius of curvature of the right edge of the pavement at the roundabout entry and exit. The widths of the entry and exit lanes are determined based on the swept path requirements of the design vehicle. Additional widening of the entry and/or exit lane, necessary to accommodate the design vehicle, is performed by rounding the right edge of the pavement or curb using one or two radii of the appropriate size.

According to [4], a prerequisite for unobstructed vehicle movement on roundabout entry and exit is a proper design of the right pavement edge and selection of the following design elements: entry and exit radii ("Rul" and "Riz"), entry and exit widths ("e" and "e'"), and circulatory roadway width ("u"). The right pavement edge can be designed in two different ways:

**Figure 19.** Entry design according to [4]: (**a**) shorter effective widening length and (**b**) longer effective widening length; "v" is the approach lane width, "m" is the splitter island length, "Rv" is the roundabout outer radius, and "Rul" is the entry radius.

The designer must choose the way that will ensure unobstructed vehicle movement, and the decision must be based on the design vehicle swept path analysis. Additionally, entry design should ensure that the entrance angle ("Φ"), which is the tangent angle between the vehicle paths at the roundabout entry, is around 30◦. In terms of the ratio between entry and exit radius, Croatian guidelines [4] recommend that the exit radius should be greater than or equal to the entry radius. The roundabout exit width ("e ") depends on the swept path width made by the design vehicle (Figure 20).

**Figure 20.** Exit design according to [4]: "v'" is the approach lane width, "m" is the splitter island length, "Rv" is the roundabout outer radius, and "Rul" is the entry radius.

According to [5], on single-lane entrances to roundabouts, the width of the entry lane along the splitter island is at least 3.75 m. At the same time, the width of the exit lane along the splitter island is at least 4 m. The right edge of the pavement at suburban roundabouts is rounded with a radius varying from 12 to 16 m at the entrance (entry radius "RE"), whereas a radius varying from 15 to 25 m is applied at the exit (exit radius "RA"), as shown in Figure 21. The radii at the entry are smaller than the radii at the exit to reduce the speed of vehicles at the entrance while facilitating the exit of long vehicles and buses from the intersection. The radii applied must meet the design vehicle swept path requirement.

According to [6], the recommended entry width ("le") at single-lane entrances, measured between the boundary lines, is 4 m, and the entrance radius ("Re") must always be less than or equal to the outer radius of the roundabout ("Rg"). The standard values of the entry radius range from 10 to 15 m, depending on the position of the approaches. The entry lane is bounded by boundary lines. The standard configuration of the single-lane entrance is shown in Figure 22 (for an outer radius of 20 m). The width of the exit lane ("ls") ranges

from 4 to 5 m for single-lane approaches and depends on the size of the outer radius of the roundabout ("Rg"). The exit radius ("Rs") must be larger than the inner radius of the intersection ("Ri"), with a minimum recommended value of 15 m and a maximum of 30 m. The values of these parameters depend on the size of the outer radius of the intersection ("Rg") and are shown in Table 6.

**Figure 21.** Right edge of the pavement at the roundabout entry and exit according to [5].

**Figure 22.** Single-lane approach for an outer radius of 20 m according to [6].

**Table 6.** Design elements for roundabout entry and exit according to [6].

According to [8], the widths of entry and exit lanes do not have a large impact on the speed at the roundabout but affect the accommodation of design vehicles and visibility at the roundabout. The width of the entry lanes ("Bt") should be in the range of 3 to 4 m—wider lanes encourage drivers to drive around the roundabout at higher speeds, which reduces safety. The width of the exit lanes ("Ba") depends on the design vehicle swept path requirements, and ranges from 3.75 to 4.50 m. The standard values of the entry radii ("Rt") at single-lane suburban roundabouts range from 8 to 12 m, and the exit radii ("Ra") from 12 to 15 m. Design elements of entry and exit at suburban single-lane roundabouts are given in Figure 23.

**Figure 23.** Design elements of entry and exit at suburban single-lane roundabouts according to [8].

According to [9], the approaches should be perpendicular to the roundabout, which is achieved by radially laying the approach axis to the outer radius of the intersection. The center of the roundabout must be as close as possible to the intersection of the approach axis. At suburban roundabouts, the recommended entry lane width in the vicinity of the splitter island ("BZ") ranges from 3.50 to 4.00 m, whereas the recommended exit lane width ("BA") is in the range of 3.75 to 4.50 m (Figure 23). The rounding of the right edge of the pavement at the roundabout entry and exit lane is formed by applying a circular arc, the size of which depends on the desired speed limit and the design vehicle swept path requirements (Figure 24). The standard radius of the circular arc at the entry lane ("RZ") ranges from 14 to 16 m, whereas the radius of the circular arc at the exit lane ("RA") is in the range of 16 to 18 m. The stated values of the exit radii can be increased by 30% at suburban roundabouts (i.e., "RA" is in the range of 21 to 23 m).

**Figure 24.** Design elements of roundabout entry and exit according to [9].

According to [10], the geometric elements of entry and exit are the starting point for the design vehicle swept path analysis and the definition of the roundabout design speed, thus directly affecting the capacity and safety of the roundabout. The standard width of the entry lane ("bu") at single-lane roundabouts is 3.5 to 4.0 m, whereas the standard width of the exit lane ("bi") is 3.75 to 4.50 m. Regardless of the required level of traffic-flow channeling, the initial condition for the entry radius ("Ru") is as follows: The radius ("Ru + bu") can ultimately tangent (but not intersect) the edge of the roundabout, as shown in Figure 25. The standard values of the entry radius ("Ru") are in the range of 12 to 16 m. The condition of the ratio of entry and exit speeds requires that the exit radius ("Ri") be larger than the entry radius by 2 m. The standard values of the exit radius are in the range of 14 to 18 m.

**Figure 25.** Initial design parameters for single-lane roundabout entry according to [10].

The number and size of the radius of curvature at the right edge of the pavement at the roundabout entry and exit and the design of the transition from the width of the approach lane to the width of the entry and exit lane depend on the desired level of traffic-flow channeling traffic.

According to [12], a value of 3.0 to 3.5 m has been defined as a suitable entry lane width ("be") for single-lane roundabouts in terms of safety, whereas a value of 3.5 to 4.5 m is defined as a suitable width of the lane at the exit ("ba"). According to [12], the right edge of the pavement at the roundabout entry and exit is formed by applying two radii, as shown in Figure 26. At suburban roundabouts, the recommended size of the inner entry radius ("Re2") is 12 m, whereas the outer entry radius ("Re1") is five times larger. The recommended size of the inner exit radius ("Ra2") is 14 m, whereas the outer exit radius ("Ra1") is four times larger.

#### *2.7. The Longitudinal Slopes at Roundabouts*

Longitudinal slopes at a roundabout (longitudinal slopes of the approaches and the circulatory roadway) reflect the topography of the area.

According to [5], the longitudinal slopes of the circulatory roadway and the approaches at 20 m from the outer edge of the roundabout must not exceed 4% (2.5% if a large proportion of heavy vehicles at the intersection is foreseen). The difference between the longitudinal slope of the approaches and the cross slope of the circulatory roadway must not exceed 5%.

**Figure 26.** Design elements for single-lane roundabout entry and exit according to [12].

The recommended values for the design elements on entry and exit at suburban single-lane roundabouts listed in the analyzed documents are shown in Table 7.

**Table 7.** Design elements for roundabout entry and exit according to [4–12].

According to [6], the construction of roundabouts on sections of roads with longitudinal slopes of up to 3% is not considered an issue. At longitudinal slopes between 3% and 6%, instability of trucks passing through an intersection can occur. For longitudinal slopes greater than 6%, it is necessary to relocate the intersection or modify the longitudinal profile of the road. Therefore, the longitudinal slopes of the roundabout approaches must not exceed 3%.

According to [9], the longitudinal slope of the roundabout approach should not exceed 6%. If the terrain slopes are higher than 6%, it is necessary to place the entire surface of the intersection in one plane.

According to [10], roundabouts should be in locations where terrain slopes are less than or equal to 2.5%. Longitudinal slope changes (sag or hog curves) of the approaches should be located at approximately half of the distance "Lo" from the outer edge of the roundabout (Figure 27, "Vras" is the design speed). If the longitudinal slopes of the approaches are greater than 2.5%, it is necessary to mitigate them, as shown in Figure 28.

The minimum values of the vertical hog and sag radii are determined according to the relevant speed of entry or exit. The tangent of the vertical curve should end at the edge of the circular pavement along the splitter island. Mitigation of longitudinal slopes to a value equal to or below 2.5% (Figure 28) is mandatory at all one-lane roundabouts with an outer radius equal to or greater than 20 m and/or when the maximum design speed is equal to or larger than 60 km/h [10]. Mitigation of longitudinal slopes to a value between 2.5% and 4.0% (Figure 28) can be applied in conditions of spatial constraints at single-lane roundabouts with a design speed below 50 km/h [10]. The highest longitudinal slopes in the roundabout zone (Figure 28) are applied only under strict spatial restrictions at intersections with an intersection design speed below 50 km/h and low probability of occurrence of trucks and buses in the roundabout (the share of trucks and buses at the roundabout is equal to or lower than 2%) [10].

**Figure 28.** Mitigation of longitudinal slopes on roundabout approaches (Rv\*—hog vertical curve radius; Rv\*\*—sag vertical curve radius) [10].