Speed from Skids: A Modern Approach

Abstract

This paper describes how an automobile equipped with a conventional brake system will often produce four skid marks on a roadway surface during maximum braking. This condition often occurs immediately prior to a collision in a traffic accident. Knowing the length of the skid marks, S\ds, and using the dynamic coefficient of friction for the roadway surface, \gm, a reconstructing engineer can determine the amount of kinetic energy converted to work during the skidding process on a level roadway. The equation used in this process states that, the portion of the kinetic energy of the vehicle that was used to perform the work of slowing the vehicle is equal to the braking force applied to the vehicle through the skidding distance. Solving the equation for the speed of a vehicle that skids to a stop yields, V\ds = 2 \2% g \2% \gm \2% S\ds, the traditional speed from skids equation. Problems exist with the traditional speed equation that limit its practical use in traffic accident reconstruction. A major problem is that the equation does not account for the energy converted to work during the transient portion of the braking process. The transient portion of braking occurs prior to the onset of visible skid marks on the roadway surface. A new speed from skids equation is set forth in this paper that will allow the reconstructing engineer to determine the speed of a vehicle at the onset of maximum braking. In addition, new equations are presented that determine the total braking distance and total braking time. An empirical investigation was used to gather data on a test vehicle. This data was used in a comparison of the accuracy of the new method and the traditional method in estimating the speed, braking distance and braking time of the test vehicle.