# Skid Mark (Length) Calculator

In many automobile accidents, it is important to determine who was at “fault,” or who was “negligent,” in causing the accident. It is also sometimes important to determine the speed of vehicles for other purposes. The drivers to the accident and eyewitnesses will often have differing versions of the accident. For this reason, the physical evidence from the scene is often very important, and it is critical in some cases to conduct an investigation of the accident before too much time elapses. As time elapses, physical evidence will begin to deteriorate. For example, glass (or plastic) from the lights (or light covers) at the accident scene are often a good clue when determining where the impact occurred.

Another important piece of information is the skid marks (or “tire impressions”) left from the vehicles involved in the accident. The investigating police officer often measures these. Further, they will remain on the roadway for some period of time after the accident; the length of time depends on several variables, incuding the weather.

From the skid marks, an expert can often determine the path taken by a given vehicle. Further, an expert can often determine the speed of the vehicle which left the skid marks. In particular, the skid marks can be used to determine the loss of speed in that particular vehicle prior to impact. This loss in speed must be added to the speed at impact to determine the original traveling speed. (The precise method for doing this is more complex than simply adding the two speeds.)

The effectiveness depends on several variables, including the treads on the tire.

A precise measure of the speed of a given vehicle typically requires the use of an expert in accident reconstruction. Further, in North Carolina there is a particular statute addressing those circumstances under which an expert (i.e. a person who did not witness the accident, with training or expertise in computing speed) can testify about the speed of a vehicle. In most states, such an “accident reconstruction” expert typically has wide latitude in giving such testimony; but in North Carolina, due to a quirk in our case law rendered such opinions inadmissible, such opinions are admissible now only if they meet the criteria of North Carolina Rule of Evidence 702(i), which states, “A witness qualified as an expert in accident reconstruction who has performed a reconstruction of a crash, or has reviewed the report of investigation, with proper foundation may give an opinion as to the speed of a vehicle even if the witness did not observe the vehicle moving.”

Determining the speed of a vehicle based on skid marks depends on several variables. These include the “coefficient of friction” of the roadway. This can be estimated based on general road conditions, and an expert can also perform a “drag test” to determine this variable. The presence of water, ice, or snow on the road will also affect the computation, as will the condition of the tires and the grade (slope) of the roadway. In general, however, one can make a general estimate of the loss of speed of a vehicle while skidding. The form below will provide such an approximate calculation.

Select surface type:

Input the slope in percent (i.e. “rise over run * 100”; e.g. a 45 degree angle would be “100”):
Use 0% for level.
Use negative number if traveling downhill.

%

Enter the length of the skid:

feet

The reduction in speed will be shown in a new window.