A Method of Estimating the Free Speed Distribution for a Road

The distribution of speeds which drivers would adopt if they were not influenced by the presence of other vehicles is an essential input for both simulation studies and theoretical models of traffic movement on a road. A simple procedure for estimating the free speed distribution, which can be used even when interference between vehicles is substantial, is developed. The procedure requires vehicles traveling at speeds less than their free speed to be identified. Criteria for defining such impeded vehicles are described and compared using data from the M4 motorway. Free speed distributions are obtained for each lane and the estimates of mean free speed are compared with those obtained from a linear regression of average speed against flow.