Conventional traffic assignment methods assume that for any given link, all vehicles on that link travel at the same speed. This basic assumption does not represent actual traffic conditions where travel speeds can vary quite widely, depending on the time of day, sometimes over even relatively short periods of time. The process of assigning traffic requires reasonably accurate estimates of the final travel speeds on each link as input. Where traffic can choose between equally acceptable routes, it is very difficult to forecast the correct split of traffic between the routes as, essentially, only one speed is used for each link. The method of incremental assignment described here has been designed to model the variation in travel speed throughout the day as flow rates alter. Separate programs to the actual assignment process use a profile of traffic flow throughout the day, with hourly and daily capacities, traffic composition, passenger car unit (PCU) equivalents and hourly rate speed flow curves, to construct all day average time flow curves. By differentiation, these curves are turned into the time of the marginal vehicle being added to the system for use during assignment. On completion, two speeds are saved in the final output file, the all day average speed, which is used in trip distribution, and the speed of travel in the peak period, which is used in the mode choice model. Time delays at all intersections are estimated separately and added to the time to travel the length of the link. Finally, an interesting application of the program is described. (a)
Abstract