The purpose of this paper is to develop an experimentally-based rear end collision warning algorithm for the situation where two vehicles are initially traveling at the same speed in the same direction when the lead vehicle begins to brake. The full variety of initial conditions of vehicular motion are analyzed to determine the proper collision warning algorithm. The analysis shows that knowledge of traveling speed, headway, and leadvehicle deceleration is sufficient to determine the type of relative motion. This can be coupled with warning logic and principles of vehicle dynamics to produce warning algorithms. An approach to warning implementation is suggested that avoids the difficult problem of estimating the lead-vehicle deceleration;​ using instead other measurable quantities such as range and range-rate.