It was a dark, gloomy night. Joe was driving west down the street next to the university, listening to his favorite Squirrel Nut Zippers CD. Suddenly, he saw someone step from behind a bush into the pedestrian crossing 50 feet ahead. He immediately slammed on the brakes, skidding on the concrete street into the pedestrian. The car came to a stop five feet beyond the crosswalk.
The pedestrian was shaken up, but fortunately sustained only bruises to his left leg, hip, and arm. However, he immediately reported to the police that Joe was speeding. Joe stated that he was driving within the speed limit (20 mph).
How do we determine if Joe was speeding? Fortunately, there is usually a telltale trail left by tire marks that, like trail signs, tell many tales. When a tire is rolling without slipping, it may leave little evidence. However, if the surface is snow, soft dirt, or other soft material it will leave an impression. Imprints sometimes leave a track as distinctive as a horseshoe that can be used to identify the vehicle. Moderate braking may leave evidence as partial slipping and stretching of the tire occurs, and tire material is pressed into irregularities in the road surface. When brakes lock there is 100% slipping and the abrasive effects of the surface remove material from the tires, leaving skid marks on the road surface.
Skidding on a smooth surface (like ice) or hydroplaning may leave no skid marks. When skidding on gravel or other loose materials, tires will leave skid tracks. Sideways slipping of a rolling tire will also leave yaw marks or tracks. All of these situations provide evidence that can be used in reconstruction.
Skid marks may indicate the speed of any vehicles involved, the motion of the vehicles prior to and following a collision, inflation status of a tire, etc. The length of the skid marks can be related to the speed. During 100% skidding straight and level, the friction force is equal to the coefficient of friction (mu) times the weight (W). This force times the skid distance (d) gives the work done by friction, which is equal to the change in kinetic energy. Since the kinetic energy is equal to one half the mass (W/g) times the square of the velocity (V), the distance "d" can be used to determine the change in velocity during the skid. If the vehicle skids to a stop:
V2 = 2 g mu d
This assumes a constant coefficient of friction.
Using mu = 0.75, the above equation indicates a speed of 35 mph, ignoring the energy loss at impact. Joe was speeding by at least 15 mph. Work/energy relations show a speed at impact of about 11 mph.