You can calculate the speed of a vehicle by the length of the skid marks with an equation. Measure the skid mark, and if there's more than one, measure each one. Get the average skid mark length by adding the number of skid marks together, then divide by how many skid marks there are. Drag factors are also used in the calculation. Drag is how much the type of road surface has slowed the car down.
. Once all the variables are set, you can determine speed by using math, The complete formula can be found at http://docs.google.com/gview?a=v&q=cache:WjctV9BE_18J:www.harristechnical.com/articles/skidmarks.pdf+calculate+skid+marks+speed&hl=en&gl=us.
