Extensive modeling and simulation studies have been carried out to evaluate the performance of systems for avoiding run-off-road crashes. Results show that the effectiveness of in-vehicle crash avoidance systems depends on how well they can be tailored to specific vehicle, driver, and roadway characteristics. To this end, a major focus of these studies is the development of improved driver lane-keeping models based on statistical analyses of data collected in driving experiments conducted on highways, rural roads, and test tracks. In recent simulation studies using improved driver models, the performance of crash avoidance systems in tractor-trailers and passenger cars has been compared over a wide range of incipient run-off-road crash conditions. Heavy trucks present a greater challenge for run-off-road crash avoidance systems, because they slightly but frequently leave the lane even under controlled driving, and because they are less stable during recovery maneuvers.