An ex-father-in-law was retired California HP, and he use to do accident investigation. I recall him commenting that once a vehicle started skidding it didn't matter too much about weight or typical roads, they all tended to skid the same distance from the same speed. Anti-lcok brakes will probably make 'skid analysis' more difficult these days though.
http://www.harristechnical.com/articles/errors.pdf
Drag factors
Much of the time, assigning a precise value for drag factor isn’t necessary. In the overall scheme of things, there isn’t a lot of difference between the speeds generated from a 0.65 drag factor as with a 0.75 drag factor. For one reason, you end up taking the square root of the value. As an example, if a car skids to a stop in 100 feet on a 0.65 surface, the slide-to-stop speed is around 44 mph. On a 0.75 surface, everything else being equal, the speed equates to about 47 mph. What’s three miles an hour among friends?
http://www.harristechnical.com/articles/errors.pdf
Drag factors
Much of the time, assigning a precise value for drag factor isn’t necessary. In the overall scheme of things, there isn’t a lot of difference between the speeds generated from a 0.65 drag factor as with a 0.75 drag factor. For one reason, you end up taking the square root of the value. As an example, if a car skids to a stop in 100 feet on a 0.65 surface, the slide-to-stop speed is around 44 mph. On a 0.75 surface, everything else being equal, the speed equates to about 47 mph. What’s three miles an hour among friends?