Models however, also have limitations. The model used for electric circuits by scientists today makes use of the idea that all substances contain electrically charged particles (see the focus idea Macroscopic versus microscopic properties).
Depending on the materials used, a driveway can be utilitarian, like gravel, asphalt, or concrete, or decorative, like stamped asphalt, stamped concrete, or cobblestone. Typical costs: A gravel driveway starts at roughly $0.75-$3 or more a square foot, or $300-$1,000 for a 12'x25' single-car driveway. Costs can double for a roadway-quality driveway. Loose gravel will need replenishing periodically, but a correctly designed, installed and maintained gravel driveway can last 50-100 years or more.
(A cubic yard of gravel is 3'x3'x3' and weighs roughly 1.2 to 1.7 tons or more). A 12'x25' single-car driveway 9 thick requires an estimated 10-14 tons, which can cost $100-$850. A 14'x100' driveway 6 thick requires about 26 tons of gravel, or $260-$1,560 for just the stones.