More than 30% of all vehicle crashes in the United States occur at intersections; these crashes result in nearly 9,000 annual fatalities, or approximately 25% of all traffic fatalities. Moreover, these crashes lead to approximately 1.5 million injuries per year, accounting for approximately 50% of all traffic injuries. In rural Minnesota, approximately one-third of all crashes occur at intersections. AASHTO recognized the significance of rural intersection crashes in its 1998 Strategic Highway Safety Plan and identified the development and use of new technologies as a key initiative to address the problem of intersection crashes. A study of 3,700 rural Minnesota intersections showed that right-angle crashes account for 36% of all rural intersection crashes. Approximately 50% of crashes at intersections that have higher than expected crash rates are right-angle crashes. Further investigation also found that poor gap selection is the predominant causal factor in these crashes. To address the problem of poor gap selection, the University of Minnesota and the Minnesota Department of Transportation have a rural intersection decision support (IDS) system under development. When deployed, it will provide a driver with the additional information needed to make correct decisions concerning the available gap. The surveillance component of a rural IDS system is described. The surveillance system uses sensors, processors, and a communication system to determine the intersection "state," including location, speed, acceleration, lane of travel, and vehicle classification (where necessary) of each vehicle in the surveillance zone. This state information will determine when to activate alerts and warnings.