Research in Intelligent Transportation Systems (ITS) has been increasingly focussed on the development of Collision Avoidance Systems (CAS). A CAS would reduce the incidence of collisions by providing warnings to the driver to take evasive action. Because single vehicle roadway departures, also known as Run-off-Road (ROR) events, are a cause of a significant portion of vehicle accidents and fatalities, an effective CAS for ROR can potentially improve highway safety dramatically.The development of performance specifications for CAS for ROR events is a part of an ongoing three-phase program for NHTSA (National Highway Traffic Safety Administration). This paper focusses on the development and application of a powerful simulation tool, RORSIM, for CAS assessments over a wide range of environmental, roadway, driver, vehicle and CAS operating conditions. The results of CAS effectiveness studies are presented. These results indicate that such systems can improve highway safety significantly by providing consistent and timely warnings to the driver.