How are Equivalent Single Axle Loads (ESALs) calculated for pavement design?

ESALs convert mixed traffic to equivalent 18-kip (80 kN) single axle passes using load equivalency factors. For flexible pavement: LEF approximately equals (axle load/18)^4 (fourth power law). Tandem and tridem axles have different factors. Design ESALs = sum of (daily trucks * growth factor * LEF * design period * directional distribution * lane distribution). Example: one pass of 36-kip axle equals 16 passes of 18-kip axle. Heavy trucks dominate pavement damage despite being small portion of traffic.