The Skid Mark / Crush Factor Method
When investigating an accident a common question that arises is, “How fast were the vehicles going?” If the skid distance of the striking vehicle and the maximum crush depth of the target vehicle are known, a simple estimate of the speed of the striking vehicle can be made. This is done by determining the impact speed of the striking vehicle by measuring the maximum crush depth of the target vehicle and inputting the distance in the Crush Factor Formula.
The Minimum Speed Formula uses the pre-impact skid distance of the striking vehicle and the roadway drag factor.
Both the Crush Factor Formula and the Minimum Speed Formula are then combined in the following way to determine the Striking Vehicle Start of Skid Speed.
30 - Mathematical Constant in the formula
d1 - Striking Vehicle’s Pre-Impact Skid Distance (measured in feet). Note: Measure the skid marks from the start to the point of impact (offset in the mark) and then subtract the wheelbase (front to rear axle distance of the skidding vehicle) from the skid distance.
f - The adjusted drag factor of the vehicle leaving the skid marks on the roadway surface. Note: if the vehicle is a passenger car, van, SUV or pickup truck and all four wheels left skid marks and the roadway was level, the roadway coefficient of friction is the vehicles drag factor. For a dry traveled asphalt surface the coefficient of friction is usually within the range .6 to .8 g’s. If the roadway surface is wet or has gravel on it, the coefficient of friction can be significantly less.
d2 - The target vehicle’s maximum crush depth (measured in feet from the normal undamaged position to the maximum permanent crushed position) of either the side or rear surface. Note: this calculation can not be used for head on collisions. This may only be used for t-bone or rear end collisions.
cf - The crush factor of the target vehicle is vehicle specific, but the average crush factor for the side and rear surface is 27. The crush factor values are based upon statistical analysis of 1000 vehicles involved in accidents where the speeds of the vehicles were verified by independent means.
The following example illustrates how the combined speed formula works:
The striking vehicle left 59.5 feet of pre-impact skid marks and had a wheel base distance of 9.5 feet. Subtracting the wheelbase distance from the total skid mark distance gives a pre-impact skid distance of 50 feet. The skidding occurred on a roadway that was level, dry asphalt. The drag factor was measured to be .7 g’s. The vehicle impacted into the side of another car and left 18 inches (1.5 feet) of permanent crush damage.
The combined formula was used to determine the start of skid speed.
This formula works as an approximation of the start of skid speed for situations where one car, van, SUV or pickup truck impacts into the side or rear of another car, van, SUV or pickup truck. This is a relatively easy way to determine if the vehicle was traveling in excess of the speed limit and to decide whether or not a more detailed accident reconstruction would be helpful. This simple estimate of the striking vehicle speed and the speed of the target vehicle can be confirmed by the conservation of linear momentum method. In collisions that involve vehicles that impacted either head-on or head-on at an angle, this skid mark / crush factor method can not be used. In those cases either conservation of linear momentum or some other method needs to be used to determine the speed of the vehicles.