Abstract
A microscopic simulation model for traffic flows on two-lane, two-way highways was developed to include all important factors known to affect these flows. The simulation which provides results in agreement with field data was applied to flows in level terrain, and on sustained grades. Results from the model indicated that the truck factor, which currently has a linear form, should be nonlinear. A nonlinear form was derived and successfully applied to summarize results for a variety of terrains and vehicle populations.