Skid Speed Calculator

This skid speed calculator can be used to estimate the speed at which a car was travelling when it left skid marks based on the grade, vehicle braking efficiency, road conditions, and skid length.

How to use the skid speed calculator:

• 1. Select the surface conditions of the road from the options provided in the drop-down menu.
• 2. Enter the slope of the road in percent (If the road is flat, use 0%; if the road has a downhill slop, input a negative number [-]).
• 3. Enter the braking efficiency of the vehicle (If the vehicle has left four skid marks, the breaking efficiency is 100 percent. Subtract 20% for each front wheel that did not leave a skid mark and 30% for each back wheel that did not leave a skid mark).
• 4. Enter the length of the skid in feet or meters (If there is more than one skid mark, measure them all and use the average).
• 5. Click on the "Calculate" button to determine the estimated speed of the vehicle at the time the skid marks were left.

This calculator determines the speed a car was travelling at according to the general conditions of the road and the vehicle.

The output is an estimated speed. The road conditions vary according to several factors including overall surface condition, depth of snow or ice, level of water on the road, etc. This calculator does not take into consideration any loss of speed that may have occurred as a result of a collision with a different object, any hydroplaning, etc. It is also not suitable for estimating the speed of cars that were traveling over more than one type of road surface; for example, over gravel and asphalt within a single skid.

Furthermore, this calculator is not suitable for some types of vehicles; for instance, trucks, motorcycles, or cars that are pulling trailers.

Due to the limitations listed above, this calculator should not be used during any legal proceedings or within insurance claims. It does not represent a valid substitute for thorough investigations.

Formulas & Example

This calculator utilizes the following equation:

V = √(2g * F * D * n)

Where:

V is the speed of vehicle (ft/sec),

g is the acceleration due to gravity (32.174 ft/sec²),

F is the adjusted friction, F = (μ ± G);

μ is the coefficient of friction between the tires and the roadway, G is the roadway grade (for 3% use 0.03),

D is the distance of skid mark (ft),

n is the braking efficiency as a decimal (for 100% use 1.0).

To convert between miles per hour (mph) and feet per second (ft/sec):

V (mph) = V (ft/sec) * 0.681818

An example:

A car skids across a flat asphalt surface, leaving four clear skid marks that have an average of 50 feet. The skid tests indicate there was a friction coefficient of 0.68. As all four wheels were braking, the braking efficiency (n) was 100% or 1.00.

The value of "D" is 50, while the value for "F" is 0.68.

When the values outlined above are inserted into the formula, a speed of 31.89 miles per hour is determined (V = 31.89).

This gives us an indication of the MINIMUM speed at which the vehicle was traveling when it started to skid.