The computer program “IMPAC” (impact Momentum of a Planar Angled Collision) was developed for use in accident reconstruction to study the impact phase of a collision. It may be used for vehicle to vehicle or vehicle to fixed object impacts. Collisions are modeled as a vector impulse in two-dimensions taking place at a specified point in each two-dimensional vehicle. A common velocity is reached at this point for the inelastic collision with complete lockup. An optional condition is available to study sideswipe type collisions in which the colliding points are permitted to slide relative to each other at a prescribed speed along a defined plane of slip. The program has been validated by comparison with the data from 16 staged crash tests conducted by the NHTSA.