Samenvatting
A simple procedure for estimating the free speed distribution, which can be used when interference between vehicles is substantial, is developed. The procedure requires vehicles travelling 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.