To a police investigator, skid marks evidence is extremely important in determining how vehicles moved in a collision. Skid marks are the equivalent of fingerprints in a criminal investigation, and just as important. The speed a car was traveling can be calculated using the skid marks made as the car came to a stop. The formula for finding the speed of the vehicle is:

S equals square root of 30 times D times f times n end root

Where:

S = speed in miles per hour
30 = a constant value used in the equation
D = length of the skid marks in feet
f = drag factor for the road surface
n = braking efficiency as a percent

A car skids to a stop, leaving four skid marks with an average length (D) of 60 feet. The road is asphalt. Skid tests reveal a drag factor(f) of 0.75. Since all four wheels were braking, the braking efficiency (n) is 100% or 1.00. To the nearest mile per hour, what was the car’s speed at the time of the accident.