Skid Mark and Yaw Mark Analysis

Below is a calculator for analyzing the minimum speed of a vehicle. Simply enter the blanks and the computer will show the minimum speed of the vehicle.

Compute Slope
Vertical distance (rise):
Enter in inches (use minus sign if negative).
Horizontal distance (run):
Feet: Inches:

Compute Drag Factor with a Test Skid
Enter the length of the longest test skid in feet:
Longest Skid:
Speed of test vehicle
Enter slope of test roadway

Skid Marks Coefficient of friction:
Slope (enter in decimal, e.g., -.04)
Vehicle Type (Braking Percentage):

Enter skid mark Length in feet of each tire:
Left Front: (.30) Right Front: (.30)
Left Rear: (.20) Right Rear: (.20)
Prior computed speed or C.D.R. if applicable.

Yaw Marks Coefficient of Friction (mu):
Chord in feet
Middle Ordinate in feet
Track Width in feet

Skid Marks Analyzer Instructions
Enter the Coefficient of Friction.
Enter the Drag Factor or the Use Test Skids button to computer a drag factor from test skids.
Common level drag factors from the Crash Reconstruction Unit (C.R.U.) website are as follows:
Asphault is between 0.5 and 0.9.
Gravel is between 0.4 and 0.5.
Ice is between 0.1 and 0.25.
Snow is between 0.1 and 0.55.
Enter the Slope.
Enter the grade of the roadway if the roadway is not level. If the car skidded down a grade, enter the slope as a negative number. The formula used to compute speeds is designed for slopes less than 10%.
Choose the Type of Vehicle.
Click on the type of vehicle that crashed. Either front-wheel drive, rear-wheel drive, a truck, or a motorcycle. The type of vehicle will effect the braking efficiency, but you can also enter you own breaking efficiency by choosing the 'user-defined' option.
Enter the Skidmark Length, in feet, for Each Tire in Decimal Format
Enter skid length: 20 feet 6 inches equals 20.5, be sure the skid length is in 10ths, not inches.
1"=.08, 2"=.16, 3"=.25, 4"=.33, 5"=.41, 6"=.5, 7"=.58, 8".66, 9"=.75, 10"=.83, 11"=.91
If you did not find a skidmark for a tire(s) then leave it blank or enter 0. If you are computing a motorcycle, the top blank is the front tire and the bottom blank is the rear tire.
Prior computed speed or C.D.R. if applicable.
Enter the speed from a Crash Data Recorder (C.D.R. at impact, or enter the speed computed from skid marks working back from the impact, i.e., if a vehicle leaves skid marks on asphault and then goes into the grass before coming to a stop, calculate the speed from the grass and enter it here before attempting to compute the speed from asphault skid marks.
Click on Compute
The Minimum Speed at the start of the skid will be computed and will show the average length of the skidmarks and the total percentage of braking.

Rudy Degger & Assoc.
National Crash Analysis Center
Macinnis Engineering
Institute of Traffic Crash Investigators
Technical Services

The formulas used for this website are based on information presented in the Basic and Intermediate Traffic Crash Investigation courses presented at the Ohio State Patrol Academy with funding from the National Highway Traffic Safety Administration

The formula used to compute the drag factor is as follows: the drag factor equals the known speed of the test vehicle squared, divided by the result of the constant of thirty times the distance of the longest skid in feet. The vehicle used as the test vehicle should be a car of equal size and weight as to the car that crashed or if possible the crash vehicle itself, more often it is likely to be your police cruiser.

The formula to compute the minimum speed of the crash vehicle is as follows: The minimum speed of the test vehicle equals thirty times the distance of the average of the skid lengths, times the drag factor, times the percentage of braking efficiency, i.e., which tires left a skid. The square root of the above product is then taken to determine the mimimum speed of the crash vehicle in miles per hour when it entered the skid. The braking efficiency used is based on the Society of Automotive Engineers Technical Paper #830612 which was provided by the Ohio State Highway Patrol.