Axle Spacing and Load Equivalency Factors

Weigh-in-motion (WIM) systems measure the axle spacings and axle loads for every vehicle in the traffic stream. There is a great need to estimate load equivalency factors (LEFs) from these WIM data. LEFs are approximately a fourth-power relation relative to a standard axle load. However, this ignores axle spacings, which have a significant effect. The American Association of State Highway and Transportation Officials derived LEFs for single and tandem axles, and these were extended to triple axles. Some have extended LEFs to quadruple axles, though there is no common agreement about this. However, larger axle groups also occur. Moreover, WIM data from the United States reveal a great variety of irregular and unusual axle spacings, and procedures to estimate LEFs for them are completely lacking. An attempt is made to rectify this and to estimate LEFs for vehicles with any axle spacing. An LEF adjustment method is developed, and examples are given of its use.