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When analyzing the time/distance performance of vehicles accelerating from a stopped position, a constant acceleration rate is often assumed. Acceleration profiles as a function of time are examined in this paper in order to identify errors associated with the constant acceleration assumption for a passenger car and a large truck. The paper also includes acceleration data collected from 219 large trucks measured over distances of 50 and 100 feet. For passenger cars, the assumption of constant acceleration is appropriate when evaluating velocity/distance scenarios with displacements of interest greater than 10 ft. For 5 ft or less, variable acceleration is recommended. When time factors are of special interest, attention must be given to the lag times associated with variable acceleration. The lag time does little to affect the velocity/distance relationship; however, it alters time/distance/velocity relationship by as much as 2 seconds.