Tractor semitrailer accidents are often caused by impact loading to some part of the vehicle and this leads to a loss of steering control and jackknifing. Impact loading may come from contact with another vehicle, or may occur on an isolated tractor semitrailer from maneuvers such as braking with subsequent brake lock-up, from suddenly steering into a turn where the cornering may lead to lateral wheel skidding, or from some combination of these factors. Without regard to the location, magnitude, or cause of impact loading, the accident reconstruction analyses herein rely on a knowledge of the tractor semitrailer's pre- and post-jackknife trajectories leading to a rest state. The tractor semitrailer is modeled as two rigid bodies hinged together and in-plane motion, to which is applied a system energy balance and Gauss's principle of least constraints. Explicit results are derived for the pre- and post-impact vehicle speeds as a function of the system parameters. Results of a case study for a tractor semitrailer initially in straight line motion show the sensitivity of the velocities to a range of trajectory surface-tyre friction conditions. Further, for constant friction conditions, the maximum post-impact linear velocities are from 1–2% less than the velocities just prior to impact; and for impending jackknife, the angular velocities of the tractor are about three times that of the trailer.