Traffic Loading Characteristics of South African Heavy Vehicles

Roads and railways are in a sense the arteries that keep the lifeblood of the economy flowing. Approximately 70 percent of freight moves by road in South Africa and it is growing by approximately four percent per annum. Data and information on heavy vehicle (HV) volumes and axle masses are therefore indispensable to road authorities for design and maintenance purposes. Heavy vehicle data from three sources, namely 11 weigh-in-motion (WIM) scales at Traffic Control Centres (TCCs) on national roads N3 and N4, 34 WIM scales on some of the main national roads and 35 continuous traffic counting stations, was analysed. Heavy vehicles were classified into 22 classes and average axle masses and spacings were calculated for these based on the N3 TCC data. Typical E80 values for the 22 classes were calculated using the average axle masses as well as the measured mass of each individual axle. It was found that the E80-values from the latter method correspond well with the average values recommended in the TRH 16 (1991). The composition of the HV population, i.e. percentage of 2-, 3-, 4-, 5-, 6and 7+-axle HVs, was used to classify the main road network in SA into Low, Medium and High HV roads. On Low HV roads more than 55 percent of HVs are 2-axle HVs whereas on High HV roads less than 35 percent are 2-axle HVs. This classification can be used to transfer measured HV characteristics, such as E80s/HV, from say one Low HV road to another Low HV road where such characteristics are unknown. A methodology is also proposed on how to estimate traffic load spectra from the WIM data. Such load spectra can be used in the mechanistic design of road pavements. Suggestions are also made on how the HV information can be used at weighbridges where HVs are visually directed to the weighbridge. Further analyses on HV data from other sources are recommended in order to confirm the conclusions in the paper or to modify them.