Editor's note... As of September 2007, we are told that this calculator has been used in 17 court cases. If you are an attorney looking to use this in a case, please do not request assistance from us. We don't have the time. Do the research to see if this has been used in a jurisdiction that is similar to yours. Then and only then, we will assist you, at our normal technical time rates, if you wish to retain us. Information gathered is from public files and public information released from the NTSB, General Motors, Ford Motors, Chrysler Corporation, various highway patrols and state police, and the Insurance Institute for Highway Safety. To the best of our knowledge, it is a "one of a kind" calculator that is extremely accurate; we neither have the manpower nor financial resources to authenticate the accuracy or validity. It is offered for use on the Internet as a public service "as is".
Note that this calculation implies a stopping distance independent of vehicle mass and of driver reaction time. It also implies a quadrupling of stopping distance with a doubling of vehicle speed. You may wish to evaluate our Vehicle Stopping Distance And Time Information for a slightly different view point.
One experiment with trained drivers asked the drivers to stop a vehicle on signal by 1) locking the wheels and 2) stopping as fast as possible without locking the wheels. On dry flat concrete, the stopping distances were very nearly the same. Both tests yielded coefficients of friction near 0.8 for tires with new tread on the surface. The coefficient of kinetic friction is considerably less on a wet surface, or similarly less than optimum surfaces or in less than optimum conditions where the water can act as a lubricant. It is also noted (above) that worn tires have a considerably smaller coefficient of kinetic friction than static friction.
Stopping Distance For A Vehicle
Assuming proper operation of the brakes on the vehicle, the minimum stopping distance for a vehicle is determined by the effective coefficient of friction between the tires and the road, and the driver's reaction time in a braking situation. The friction force of the road must do enough work on the car to reduce its kinetic energy to zero. If the wheels of the car continue to turn while braking, then static friction is operating, while if the wheels are locked and sliding over the road surface, the braking force is a kinetic friction force only.
To reduce the kinetic energy to zero:
so the stopping distance is
Note that this implies a stopping distance independent of vehicle mass, and in this case, driver reaction time. It also implies a quadrupling of stopping distance with a doubling of vehicle speed.
Stopping Distance Calculation
For calculating minimum stopping distance, a value of 0.8 is a nominal value for the coefficient of static friction between good tires and a good road surface. Almost always, coefficients of kinetic friction are less, and are dramatically less for wet, icy, slick, sandy, dirty very smooth or oily surfaces. For many newer high performance tires with good tread, the coefficient of kinetic friction on a dry road surface may approach 0.8 if the braking is not so prolonged as to cause tire melting. You may wish to plug in a smaller value such as .7 or .6 for a vehicle with normally driven and worn tires. Poor condition tires might yield .5 or .4 for a closer representation of friction.