Equivalent Single Axle Loads (ESAL) Traffic Loading

Pavement damage goes as roughly the FOURTH POWER of axle load.

Run the calculator

Example

You enter

You get

Details, formula, and sources

Which means one loaded truck does the damage of thousands of cars. Equivalent single axle loads convert a mixed traffic stream into equivalent 18,000 lb axles, and that conversion is why a road's truck percentage matters far more than its total volume. The fourth power is the fact that reorders every intuition about pavement. A 2,000 lb car axle has a load equivalency factor near 0.00015, so roughly six and a half thousand car axles equal one standard truck axle: a road carrying twenty thousand cars and two hundred trucks a day gets essentially all of its damage from the one percent that are trucks, and widening it for cars does nothing at all for its pavement life. That comparison is computed here rather than asserted, because it is the number that settles arguments about who is wearing out a road. OVERLOAD IS THE SAME FACT POINTED AT ENFORCEMENT. An axle at 22,000 lb rather than 18,000 does about 2.23 times the damage -- more than double for a twenty-two percent overload -- which is the arithmetic behind weight enforcement and why a few overloaded vehicles consume a pavement's design life quickly. THE GROWTH TERM COMPOUNDS AND IS EASY TO UNDERSTATE. Two percent annual growth over a twenty year design life is a factor of only 1.22 on the final year's traffic, but a factor of about 24.3 on the cumulative loading against one year's -- and using first-year traffic without the growth series badly undersizes a pavement. At zero growth that factor is simply the number of years, which is a useful check on the arithmetic. An estimate using factors the reader supplies. The fourth-power rule is a simplification: the published AASHTO load equivalency factors depend on axle configuration -- single, tandem, tridem -- on the pavement's own structural number, and on the terminal serviceability, and they give different values than a plain fourth power. Truck classification and axle load distributions should come from weigh-in-motion or classification counts rather than from assumed averages, because a site's actual loading spectrum drives the answer more than anything else here. Growth rates projected over twenty years are uncertain by their nature. This does not design a pavement, and mechanistic-empirical design does not use these at all -- it takes the load spectrum directly, which is one reason agencies are moving away from the method. The agency's pavement design manual, the traffic data, and the pavement engineer govern.

load equivalency factor = (axle load / 18,000 lb) to the fourth power; design lane ESALs = AADT x truck fraction x directional factor x lane factor x ESALs per truck x 365 x the compounded growth series ((1+g)^n - 1)/g, which becomes simply n years at zero growth.

The AASHTO load equivalency concept and the fourth-power approximation by name. The published AASHTO factors depend on axle configuration -- single, tandem, tridem -- on the pavement's structural number and on the terminal serviceability, and give different values than a plain fourth power. ESALs per truck, the distribution factors and the growth rate are ENTERED, ideally from weigh-in-motion or classification counts. The agency's pavement design manual, the traffic data, and the pavement engineer govern.

A fourth power and a geometric series on factors the reader supplies; no AASHTO equivalency table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: aadt, truck_percent, directional_factor, lane_factor, esals_per_truck, growth_percent, design_life_years, car_axle_lb, overload_axle_lb, trucks_per_day_design_lane, first_year_esals, growth_factor, design_esals, car_esals, car_share_pct, overload_lef

Related tools