Headway model For the purposes of traffic analysis and simulation, the headway model is an important approach used in the analysis. A number of headway models have been developed over the last decade and their application is dependent on the nature of the roadway. Basically, vehicle headway is a measure of the temporal space between any two vehicles which can be described as the time that is elapsed between the arrival of the leading vehicle and the following vehicle at a designated point of testing. The value of the minimum acceptable mean headway normally usually determines the capacity of the roadway. In addition, vehicle headway distribution is closely linked to the vehicle merging, permitted left turn movements at roundabouts and junctions. Proper headway models can be applied to generate vehicle events in microscopic simulation models and for traffic safety analysis.
For data collection, the Advanced Loop Event Analyzer (ALEDA) system can be used for collecting accurate headway data. This data analyzer is portable and analyzes data in real time. The system has the capability to poll and store high resolution traffic data. The accurate data collected using the analyzer enables further investigation and robust estimation on the characteristics of the headway at various flows levels especially the short headways. Before applying the model, the parameters for the headway distribution must be determined.
The accuracy of the parameters determines the goodness of fit of the headway model. The headway model can be either single or mixed and a construction of both models can be done in order to determine the goodness of fit of the data collected using the analyzer. For the purposes of fitting the headway data, the most recommended method of distribution is the lognormal distribution method. This method best fits the data onto the lanes and headways depending on the prevailing conditions. Headway method provides a very good analysis for High Occupancy Vehicles (HOV) with mixed composition of vehicles and hence gives a much better analysis of the distribution of the vehicles.
Cowan, R. J. Useful headway models. Transportation Research Vol. 9, No. 6, 1975, pp. 371-375.
Tolle, J. E. The lognormal headway distribution model. Traffic Engineering and Control, Vol. 13, No. 1, 1971, pp 22-24.