Post-calibration and quality management of weigh-in-motion traffic data

[摘要] ENGLISH ABSTRACT: Weigh-in-motion (WIM) scales are installed on various higher order South African roads toprovide traffic loading information for pavement design, strategic planning and lawenforcement using a scientific approach. The two most respected international guidelinedocuments for WIM systems are the American ASTM E1318 Standard and the COST 323European Specification, yet neither are fully suited to be applied to local WIM systems.The author developed a post-calibration method for WIM data, called the Truck Tractor (TT)Method, to correct the magnitude of recorded axle loads in retrospect. It incorporates a seriesof powerful data quality checks. The TT Method is robust, accurate and adequately simple tobe used on a routine basis.The TT Method uses the truck tractor loads of articulated 6- and 7-axle trucks with singlesteering- and double driving axles – these vehicle are called Eligible Trucks. Only EligibleTrucks with average axle loads between 6.5 t and 8.5 t are used in the calibration process –these vehicles are called Selected Trucks. A calibration factor, kTT, is determined using a fullyautomated iterative procedure, and multiplied with all axle load measurements to producedata for which the average truck tractor load of Selected Trucks, TTT, is equal to 21.8 t. TheTT Method can be used for WIMs in various operating environments and is not sensitive tothe extent of miss-calibration of a WIM, clipping of sensors owing to poor lane discipline ordifferent extents of loading on different routes.The TT Method includes a series of data quality checks that can be used on a routine basis.They are summarised as follows:- The standard deviation of truck tractor loads for Selected Trucks, STTT, should alwaysbe below 2.0 t, but preferably below 1.9 t.- The standard deviation of front axle loads for Selected Trucks, SFTT, should always bebelow 0.9 t, but preferably below 0.8 t.- The post-calibration factor from the TT Method, kTT, should be between 0.9 and 1.1.The factor for any month should not deviate by more than 3% from the movingaverage of the previous five months.- The average of front axle loads of Selected Trucks, FTT, should be between 5.6 t and6.6 t; the exact values are influenced by load transfer between the steering and drivingaxles.- A procedure was formulated using the Front axle / Truck tractor Ratio, FTR, toidentify the percentage of Eligible Trucks that in all probability clipped the sensor.The percentage of these records must be below 10 %, but preferably below 6 %.The TT Method has the potential to significantly improve WIM data collection in SouthAfrica. The calibration module of the TT Method, i.e. the procedure to calculate kTT, hasalready been accepted by SANRAL. Most of the data quality checking concepts associatedwith the TT Method were also accepted, although their threshold values are still being refined.

[发布机构] Stellenbosch University