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. For heavy trucks, a speed surge is seen immediately before shifting from 2nd to 3rd gear, but due to the low acceleration values, little impact is seen in the time/distance profile. The constant acceleration assumption for heavy trucks appears valid for situations where the driver is shifting. In these tests the approximate constant acceleration was 0.07 g's. When the driver of a heavy truck does not shift, the transmission gearing as well as the weight of the load is important in determining the acceleration of the vehicle. When a heavy truck is not shifted, using constant acceleration 0.05 g's usually under-estimates the distance traveled in the 4-8 second range and over-estimates the distance traveled after 8 seconds. Based on the time and distance measurements for the 219 trucks, calculated average accelerations were 0.085, 0.106, and 0.138 g's over the first 50 ft for the flatbed, box, and bobtail configurations, respectively. Over a distance of 100 ft, the average accelerations were somewhat lower: 0.064, 0.073, and 0.118 g's for the flatbed, box, and bobtail configurations, respectively.