The accuracy of traffic noise predictions obtained using the TRRL computer model of traffic noise depends to a considerable extent on the degree of simplification adopted in categorising vehicles according to their sound output and speed in the traffic stream. This report examines and summarises the available data on the acoustic classification of vehicles in traffic streams for predicting traffic noise. Measurements of speed, noise level and vehicle type have been made in road conditions ranging from fairly congested urban situations with speeds around 20 km/h to free flow on motorways with speeds over 100 km/h. At speeds greater than 50 km/h vehicle noise increases with speed at approximately 9 db(a) per doubling of speed and there is a clear distinction between the noise from heavy vehicles, i.e. those exceeding 1525 kg unladen weight, and light vehicles. At low speeds, below 30 km/h, vehicle noise levels are independent of vehicle speed. The vehicle population can be conveniently divided into three distinct groups; light vehicles less than 3000 kg unladen weight, medium heavy commercial vehicles with 2 axles including buses and coaches and heavy vehicles which include all commercial vehicles with 3 or more axles. The measurements have been used to construct approximate vehicle noise levels and speed characteristics over the speed range 20-100 km/h for up to 6 vehicle categories, and used as input in the TRRL computer model of traffic noise. By varying the number of vehicle categories in the model it was found that the least number of categories that could be used without causing a significant change in the accuracy of prediction for urban roads was three. A comparison between measured and predicted values of l10 using the computer model with three vehicle categories at 11 urban sites showed fair agreement, indicating the accurate computer predictions of traffic noise for urban traffic conditions can be achieved by extending vehicle noise level and speed characteristics to the lower speeds typical of urban traffic conditions. (Author/publisher)
Samenvatting