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A typical speed limit sign in the United States showing a fifty mph restriction. A road speed limit is the maximum speed allowed by law for road vehicles. Speed limits are commonly set and enforced by the legislative bodies of
From Wikipedia, the free encyclopedia
A typical speed limit sign in the United States showing a fifty mph restriction. A road speed limit is the maximum speed allowed by law for road vehicles. Speed limits are commonly set and enforced by the legislative bodies of nations or provincial governments, such as countries within the world. The first maximum speed limit was the 10 miles per hour (16 km/h) limit introduced in the United Kingdom in 1861. The Isle of Man is the only place in the world that does not have a general speed limit. In Germany, 57% of the autobahn system remains free from speed limits. Currently, the highest posted speed limit in the world is 140 kilometres per hour (87 mph) on Polish motorways, although a variable speed limit up to 160 kilometres per hour (99 mph) was permitted experimentally on a stretch of Austrian motorway in June 2006. 
A typical 60km/h speed limit sign used in Australia Locomotive Act of 1861 (or "Red Flag Act") in the United Kingdom (automobiles were in those days termed “light locomotives”). In 1865, the revised Locomotive Act reduced the speed limit to 4 mph (6 km/h) in the country and 2 mph (3 km/h) in towns. The 1865 Act required a man with a red flag or lantern to walk 60 yards (50 m) ahead of each vehicle, enforce a walking pace, and warn horse riders and horse drawn traffic of the approach of a self-propelled machine. The replacement of the "Red Flag Act" by the Locomotive Act of 1896, and the increase of the speed limit to 14 mph (23 km/h) has been commemorated each year since 1927 by the London to Brighton Veteran Car Run.
Safety and efficacy
The kinetic energy involved in a motor vehicle collision is proportional to the square
The first speed limit was the 10mph (16.1 km/h) limit introduced by the
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of the speed at impact. The probability of a fatality is, for typical collision speeds, empirically correlated to the fourth power of the speed difference (depending on the type of collision, not necessarily the same as travel speed) at impact, rising much faster than kinetic energy.
A 1994 study by Jeremy Jackson and Roger Blackman showed, consistent with the risk homeostasis theory, that although increased speed limits and reduced speeding fines significantly increased driving speed, there was no effect on accident frequency, with the 24 participants maintaining the same level of risk and risky behaviour. It also showed that an increased accident cost caused large and significant reductions in accident frequency but no change in speed choice. The abstract states that the results suggest that regulation of specific risky behaviors such as speed choice may have little influence on accident rates.
Speed limits, actual speeds, and aggregate safety
The 1998 report Synthesis of Safety Research Related to Speed and Speed Limits sponsored by the Federal Highway Administration found that changing speed limits on low and moderate speed roads appeared to have no significant effect on traffic speed or the number of crashes, whilst on high-speed roads such as freeways, increased speed limits generally resulted in higher traffic speeds and more crashes. The report commented that on high-speed roads traffic speeds would change by about one-quarter of any speed limit change, and that international studies suggested that a 1 mile per hour (1.6 km/h) speed change would result in a 5% change in the number of injury accidents. The report noted that traffic calming significantly reduced speeds and injuries in treated areas but that the decrease may be due to reduced traffic volumes. The report also suggests that "variable speed limits that adjust with traffic and environmental conditions could provide potential benefits" as most of the speed related crashes involve speed too fast for conditions. The report noted the landmark study (D. Solomon, "Accidents on Main Rural Highways Related to Speed, Driver, and Vehicle", Federal Highway Administration, Washington, DC, July 1964) that observed a "Ushaped curve" -- subsequently referred to a