It is important to determine the number and types of wheels/axle loads that the pavement will be subject to over its design life when designing pavement for a roadway. The primary concern is the damage to the pavement caused by the wheel loads. Given the types of wheels and axles in mixed traffic, a common approach is to convert the damage from wheel loads of various magnitudes and repetitions to damage from an equivalent number of “standard” loads. The most commonly used equivalent load is the 18,000 lbs (80KN) single axle load, or the 18 KIP Equivalent Single Axle Load (ESAL).

The ESAL Forecasting Process is necessary for pavement design for new construction, reconstruction, lane addition, or resurfacing projects. While the total volume of traffic influences the geometric requirements of the highway, the percentage of commercial traffic and frequency of heavy load applications have major impacts on the structural design of the roadway. Truck traffic and damage factors are needed to calculate axle loads expressed as ESALs.