Planning agencies design roadways to have a specific lifespan based on the expected volume and weight of vehicle traffic. Traffic streams are composed of a variety of vehicles of different weights and axle configurations. Therefore, loading measurements are based on equivalent single-axle loads that are the damage to a pavement caused by a vehicle axle relative to an 18,000 pound force, which represents a standard axle. This unit may be used to calculate the cumulative damage caused to a roadway by an expected traffic stream.
American Association of State Highway and Transportation Officials, Guide for Design of Pavement Structures (Washington, DC: 1993), p. I-10 and appendix D.