The National Highway Traffic Safety Administration (NHTSA), supported by The Johns Hopkins University Applied Physics Laboratory (JHU/APL), has developed an algorithm for use with rear-end collision avoidance systems that alerts drivers to potentially dangerous driving situations and the need to take evasive action. This algorithm is to be integrated into a General Motors (GM) developed collision warning system for use during the Automotive Collision Avoidance System (ACAS) Field Operational Test (FOT). The NHTSA algorithm uses the host vehicle velocity and acceleration along with the collision warning systemsupplied values for range, range rate, and relative acceleration of the lead vehicle to calculate a missdistance between the host and lead vehicles at 0.1- second intervals. The miss-distance is the closest distance that occurs between the two vehicles if the driver of the host vehicle were to initiate braking after a delay time at a designated host vehicle maximum braking capability. This calculated distance is compared to a miss-distance threshold and if it is less, a warning is provided to the driver. The algorithm accounts for a driver sensitivity setting and includes a look-ahead calculation to determine if the threshold would be passed before the next time interval. The performance of the algorithm has been examined against designated operational scenarios. These scenarios include cases of a constant speed host vehicle encountering a stopped lead vehicle, a constant speed host vehicle encountering a constant but slower speed lead vehicle, and a constant speed host vehicle encountering a lead vehicle braking from the same initial speed.