Effects of speed distributions on the Harmonoise model predictions

It is known that speed variation affects pass-by noise levels and the well developed speed level functions based on statistical and controlled pass-by methods have been produced for different road surfaces and categories of vehicle. Therefore the accurate prediction of vehicle noise from passing vehicles of known speed presents few difficulties. However, the pressing practical problem is how to assess the traffic noise produced by traffic streams over an extended period of time e.g. for the calculation of Lden . The problem is often more complicated in urban areas where the traffic flow is congested for a significant proportion of the day. It has been established that when traffic is freely moving the speed distribution of a given category of vehicle approximates to a normal distribution. It is possible from transport statistics to derive relationships between the width of the distribution (standard deviation) and the mean speed for different classes on different roads. However, under congested conditions the distribution is far from normally distributed. This paper examines the errors in noise prediction which would result if the mean speed was used for prediction purposes rather than the actual speed distribution. Examples are taken from real traffic data both for freely flowing and congested traffic.