Transportation Engineering

Equivalent Single Axle Load (ESAL)

The number of 18,000 lb single-axle passes that would do the same pavement damage as a mixed traffic stream.

Pavements are designed for traffic expressed as a count of 18 kip single-axle loads. Each real axle is converted by a load equivalency factor, which rises steeply with load — roughly the fourth power — so heavy trucks account for almost all of the damage and cars almost none.

Design ESALs are built from daily truck counts, the average ESALs per truck, the design life, growth and the share of trucks in the design lane. The total sets the structural number a flexible pavement needs or the slab thickness of a rigid one.

How much of the FE Civil exam is Transportation Engineering?