Representing stochastic transit dwell times in traffic signal optimization

It has been demonstrated that while the TRANSYT traffic model simulates transit vehicles in mixed traffic operation, it does not adequately consider the effects of bus or streetcar stops on the travelled roadway near signalized intersections. Its assumption that the transit vehicles do not hold up other vehicles while they are loading and unloading passengers is also invalid when midblock stops occur in the travelled lanes. To account for the effects of transit stops, an alternative type of network formulation which uses dummy nodes and dummy links with appropriate link costs is proposed for modelling the effects of transit stops. It approximates transit stop dwells by discrete distributions, requiring 1 dummy node and 4 dummy links for each nonzero value used in the approximating distribution. Realism for such operation can be improved significantly, usually with the use of only 1 or 2 dummy nodes per transit stop. Parameters for the dummy links have been tested over a wide range, and a set of operational values is recommended. Flow profiles illustrating the need for and the effects of the recommended formulation are presented in the paper.