STATISTICAL PROPERTIES OF VEHICLE TIME HEADWAYS

The properties of vehicle time headways are fundamental in many traffic engineering applications. The shape of the empirical headway distributions is described by density estimates, coefficients of variation, skewness, and kurtosis. The hypothesis of exponential tail is tested by Monte Carlo methods. The independence of consecutive headways is tested using autocorrelation analysis, runs tests, and goodness of fit tests for geometric bunch size distribution. The power of these tests is enhanced by calculating combined significance The variation of significance over flow rates is described by moving probabilities. It is shown that speed limit and road category have a considerable effect on the statistical properties of vehicle headways. The results also suggest that the renewal hypothesis should not be accepted under all traffic conditions.