You're quietly sitting by a street one day, enjoying the sun, when you notice a speeding car traveling at 20 m/s down the street. The driver notices that there's a stop sign in the street and hits the brakes 40 m before the stop sign, coming to a stop right at the sign itself. What is the coefficient of friction between the car's tires and the road?

Details and assumptions

Assume that once the driver hits the brakes the wheels no longer turn but instead slide against the road.