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Speed from Skids: A Modern Approach
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English
Abstract
An automobile equipped with a conventional brake system often will produce four skid marks on a roadway surface during maximum braking. This condition often occurs immediately prior to a collision in a traffic accident. Knowing the length of the skid marks, SS, and using the dynamic coefficient of friction for the roadway surface, μ, a reconstructing engineer can determine the amount of kinetic energy converted to work during the skidding process on a level roadway. The equation used in this process states that, the portion of the kinetic energy of the vehicle that was used to perform the work of slowing the vehicle is equal to the braking force applied to the vehicle through the skidding distance. Solving the equation for the speed of a vehicle that skids to a stop yields, 
the traditional speed from skids equation.
the traditional speed from skids equation.Problems exist with the traditional speed equation that limit its practical use in traffic accident reconstruction. A major problem is that the equation does not account for the energy converted to work during the transient portion of the braking process. The transient portion of braking occurs prior to the onset of visible skid marks on the roadway surface.
A new speed from skids equation is set forth in this paper that will allow the reconstructing engineer to determine the speed of a vehicle at the onset of maximum braking.
In addition, new equations are presented that determine the total braking distance and total braking time. An empirical investigation was used to gather data on a test vehicle. This data was used in a comparison of the accuracy of the new method and the traditional method in estimating the speed, braking distance and braking time of the test vehicle.
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Authors
Citation
Neptune, J., Flynn, J., Chavez, P., and Underwood, H., "Speed from Skids: A Modern Approach," SAE Technical Paper 950354, 1995, https://doi.org/10.4271/950354.Also In
References
- Garrott, W.R. Guenther D. Houk R. Lin J. Martin M. “Improvement of Methods for Determining Pre-Crash Parameters from Skid Marks,” National Highway Traffic Safety Administration DOT HS 806 063 May 1981
- Baker Stannard J. Fricke Lynn B. “The Traffic Accident Investigation Manual, At Scene Investigation and Technical Follow-up,” Northwestern University Traffic Institute Evanstan, IL 1986
- Fricke, Lynn B. “The Traffic Accident Investigation Manual, Volume 2,” Northwestern University Traffic Institute Evanstan, IL. 1990
- Reed, Walter S. Keskin A. Taner “Vehicular Response to Emergency Braking,” SAE Paper 870501
- Reed, Walter S. Keskin A. Taner “A Comparison of Emergency Braking Characteristics of Passenger Cars,” SAE Paper 880231
- Robinson, Edward L. “Analysis of Accelerometer Data for Use in Skid-Stop Calculations,” SAE Paper 940918