Headway Distributions Based on Empirical Erlang and Pearson Type III Time Methods Compared

The study is aimed at exploring empirical headway derived from Erlang and Pearson type III distribution and comparing the resultant distributions. The Erlang distribution is a simplified form of the Pearson type III. In circumstance where vehicle overtaking is very limited and the ensuing headway values are small, Pearson type IIIor the Erlang distribution may be used to represent the headway frequency distribution. Based on the hypothesis that headways derived from Erlang and Pearson type III distribution will not differ significantly, headway study was carried out in Kulai, Malaysia under dry and daylight conditions. Twenty four hour volume, speed and headways and vehicle type data were collected continuously for eight weeks. The survey data were supplemented with highway design information culled from the Malaysian Design Manual. Empirical headway data were computed using Pearson type III and Erlang distribution methods. Results show that the difference between estimated headways using both techniques is very marginal. The study concluded that in cases where overtaking induces minimal headways, the Pearson type III or the Erlang distribution may be used to represent the headway frequency distribution. However, Pearson type III distribution has to be modified in circumstances where the probability of observing headway increases.