Traffic assignment procedures and the impacts on mobile source emission estimates

The assignment procedure is one of the major components of the urban transportation planning model. This research reviewed different commonly used assignment procedures, and examined the impacts of these distinctive procedures on vehicular emission estimates through a series of carefully designed experiments by using the Denver travel model as a case study. Three aspects of the traffic assignment procedures were studied: the algorithms, the volume-speed functions, and the time-of-day procedures. Several related issues, such as the definition of volume-speed functions at volume-to-capacity (v/c) ratio greater than 1.0 area, the assignment termination condition, the possibility of improvement of multi-period off peak period assignment, were also studied. It was concluded that different assignment procedures have relatively small differences on the Vehicle Miles Traveled (VMT) estimates and large differences on speed estimates. For the vehicle emission estimates, large variations (20%) were observed when the assignment procedures were altered. In addition to the speed, the research found that the VMT distribution among highway facilities varied considerably, and this was a major cause of the vehicle emission estimate differences. Recommendations were made for the application of different assignment procedures and future assignment procedural improvements.