This paper describes a human factors simulation study of the decision making behavior of drivers attempting to avoid nonrecurring congestion by diverting to alternate routes with the aid of in-vehicle navigation systems. This study is the first phase of a two part project in which the second phase will apply the driver behavior data to a simulation model analysis of traffic flow. The object of the driver behavior experiment was to compare the effect of various experimental navigation systems on driver route diversion and alternate route selection. The experimental navigation system configurations included three map based systems with varying amounts of situation information and a non map based route guidance system.The overall study results indicated that navigation system characteristics can have a significant effect on driver diversion behavior, with better systems allowing more anticipation of traffic congestion. Subject route familiarity, commercial driving experience and gender did not significantly affect the results. Alternate route analysis tended to confirm the main route diversion results, and also showed that a majority of drivers were willing to accept alternate routes suggested by advanced navigation systems. These results were consistent over three significantly different congestion conditions. Driver age was also a factor, with old drivers being more reluctant to divert from the main freeway route. The paper describes the simulation approach and summarize results on diversion decision behavior and alternate route selection.