In the last few decades several car-following models have been proposed and tested using mainly vehicle location data. The use of high-precision Global Positioning System (GPS) data to test several car-following and collision constraint models is reported, with a critical evaluation of these models and proposal of a modified collision constraint formulation. GPS receivers typically report time-stamped location or position fixes and velocity. For a pair of leading and following vehicles, location and velocity data were used to examine estimates of acceleration, velocity, and headway by Pipes’s; modified Pitt’s, or FRESIM; CARSIM; and INTELSIM car-following models. The important aspects of collecting accurate GPS data are also highlighted.