Planning agencies design roadways to have a specific lifespan based on the expected volume and weight of vehicle traffic . Since traffic streams are composed of a variety of vehicles of different weights and axle configurations, an equivalent unit of pavement damage is used to calculate the wear caused by different types of vehicles. An equivalent single-axle load (ESAL) is a standard unit of pavement damage and is based on the amount of force applied to pavement by an 18,000-pound axle, which is roughly equivalent to a standard truck axle. This unit may be used to calculate the cumulative damage caused to a roadway by an expected traffic stream.
1. American Association of State Highway and Transportation Officials, Guide for Design of Pavement Structures (Washington, DC: 1993), p. I-10 and appendix D.