WEIGHING TRUCKS ON AXLE-LOAD AND WEIGH-IN-MOTION SCALES

Truck weight information is used for (a) commerce, (b) statistical data, and (c) law enforcement. Single-draft weighing on a vehicle scale that meets the required basic maintenance tolerance of 0.2 percent of applied load is the only acceptable way to determine gross vehicle weight for commercial purposes. Axle-load scales and wheel-load weighers that meet basic maintenance tolerances of 0.2 and 2.0 percent of applied load, respectively, may be used to weigh trucks for the other two purposes. Tolerances and use requirements for weigh-in-motion (WIM) equipment have not yet been established by the National Bureau of Standards. Analysis of a data set from a carefully controlled field experiment in which 100 trucks were weighed statically on two different axle-load scales and at a slow speed on a WIM scale indicates that the weight of a truck is redistributed among the axles and wheels as the truck moves along the road surface and as it accelerates and stops for weighing on static scales. The variability in gross vehicle weights calculated from successive weighings of the axles or wheels of a truck on the axle-load and WIM scales that performed within small tolerances is discussed. Observed variations in axle-group weights, axle weights, and wheel weights under these conditions are also discussed. Implications of this analysis are that in establishing weight enforcement tolerances and interpreting statistical weight data consideration must be given to the fact that even though truck weight is virtually constant, it is not always distributed among the axles and wheels of the truck in the same proportions.