Since cars need friction to travel on highways, hows does Caltrans figure out what the right amount of friction would be for a road?

Caltrans conducts pavement skid testing to determine how rough the pavement should be.  As stated previously, generally the minimum roadway coefficient of friction is 0.30.  Caltrans would consider increasing the pavement roughness at locations where increased pavement traction can help reduce the number of accidents.  #caltransanswers