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Accident Reconstruction Calculators

Below are some basic accident reconstruction tools. They should be used to estimate ranges of answers for common reconstruction calculations. Most problems involve more complicated factors so make sure to consult an Automotive Engineer. 

Speed from Skids (Pre-braking speed)

This equation is used to estimate the minimum speed a vehicle was travelling before skidding to a stop. The pre-braking speed can be estimated using the length of the skid marks and the Coefficient of Friction* of the road surface. 


Speed = √(30 x Coefficient of Friction x Distance)


S = √(30 x f x D)


S = Pre-Braking Speed (mph)

30 = Conversion factor (converts feet per second to mph)

F = Coefficient of Friction of Roadway Surface

D = Distance (length) of longest skid mark (feet)

* The coefficient of Friction quantifies the amount of adhesion between the road surface and the vehicle's tires. Slippery roads (wet, icy, etc.) have lower Coefficients of Friction that dry road surfaces. A range of Coefficient of Friction should be used to achieve a realistic range of answers. 

Example Calculation:

A car left 150 feet of skid marks on new Portland cement before coming to a stop in an intersection. How fast was it travelling before braking?

Low : S = √(30 x f x D) = S = √(30 x 0.7 x 150) = 56 mph

High: S = √(30 x f x D) = S = √(30 x 0.95 x 150) = 65 mph

Answer: 56-65 mph

COEFFICIENTS OF FRICTION OF VARIOUS ROADWAY SURFACES                            DRY                               WET                      

ASPHALT OR CONCRETE                                                                                        From 0.70 to 0.95                From 0.40 to 0.75

GRAVEL                                                                                                                      From 0.40 to 0.80                From 0.40 to 0.75

SNOW                                                                                                                         From 0.10 to 0.35                From 0.30 to 0.60

ICE                                                                                                                               From 0.07 to 0.20                From 0.05 to 0.10

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