Mathematically we can use mechanics to model this event in terms of $s$ , the length of the skid, $u$ , the speed of the vehicle, $g$ , the acceleration due to gravity and $\mu$ , the coefficient of friction times braking efficiency. The model links the cause (the speed of the car) to the effect (th e distance of the skid):

$s=\frac{u^2}{2\mu g}$

This can be rearranged so that given the skid distance we can mathematically determine the speed of the car, usually giving a lower bound:

$u=\sqrt {2\mu gs}$

But for this to work, we need to have an accurate estimate of $\mu$ , the value describing friction and braking efficiency!