Speed Equation

This is an example of a common speed equation often used in reconstruction. In the following equation, we will use distance variables (in feet), drag factor, and braking efficiency to determine how fast a vehicle was going at the time of an accident.

S = (30 x D x F x N)

S = speed

D = distance

F = drag factor

N = braking efficiency

At this particular car accident, the vehicle in question had a stopping distance of 60 feet, a drag factor of 0.75” and a braking efficiency of 100 percent, or 1.00. Speed will equal the square root of 30 multiplied by the distance, drag factor and braking efficiency. The variables that will be used in the equation are defined below:

S = (30 x D x F x N)

In this equation, the value for “D” (distance) is 60 feet, the value for “F” (drag factor) is 0.75” and “N” (braking efficiency) will be 1.00.

D = 60 feet

F = 0.75”

N = 1.00

When these variables are inserted into the formula, the following solution develops:

S = (30 x D x F x N)

30 x 60 x 0.75 x 1.00 = 1350

1350 = 36.7423

Thus, speed equals 36.7 miles per hour.

Source : James Harris, Harris Technical Services

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