Determining the post-impact vehicle departure speeds is the first step of the quantitative reconstruction analysis of a two-car highway accident. This should always include the essential check on the correlation of these speeds. In a rear-end collision, for example, the departure speed of the struck vehicle must be equal to or greater than the departure speed of the striking vehicle. If this is a relatively high-speed plastic impact, however, the departure speeds will be close to the same value.

Speed Correlation DrawingIn an angled intersection collision, as shown
here, the impact will not only produce a speed change for each vehicle, but will also cause both vehicles to rotate. Let’s assume we have the front of a northbound vehicle (V1) strike the right front of an eastbound vehicle (V2), causing a post-impact clockwise rotation of V1. In this impact configuration the eastbound component of V2’s departure speed must be greater than the eastbound component of V1’s departure speed. Similarly, the northbound component of V1’s post-impact speed must be less than or equal to the northbound speed component of V2.

If these speed correlation requirements are not met, there is an obvious error in the computations; or the assumptions and/or the estimates used in the calculation of one or both of the departure speeds are not valid. Prior to proceeding with the impact and pre-crash phases of the quantitative reconstruction process, it is then necessary to review the entire post-impact analysis and make the appropriate adjustments. This required speed correlation is an essential post-impact analysis tool.

More information on related topics can be found in Highway Accidents: Investigation, Reconstruction and Causation available at: Information about the book and author is available at: