Speed from Skidmarks

skidmarksTire marks left on the pavement at a collision scene tell the investigator many things about the events involved in a motor vehicle collision. One of the more interesting involves the calculation of pre-collision speed. Even more interesting was the opportunity to teach it to a class of physics students at a Qualicum Beach high school.

My supervisor and I started the class by deriving the slide to stop formula from the basic equations the students were learning. Simply put, the speed of the vehicle is equal to 15.9 times the square root of the skid distance multiplied by the coefficient of friction for the road surface. This applies to a level surface and will work for both ABS and non-ABS braking systems.

Next we went to the parking lot where I readied the shot marker on my police vehicle and had one student sit in the passenger seat to verify the speed by watching the radar display. After reaching 50 km/h I braked to create the skid and the shot marker fired a piece of blackboard chalk onto the ground when the brakes were applied. By measuring the distance from the chalk mark to the shot marker at the other end, the exact skid distance was known.

My supervisor led the others through the use of a drag sled, which is essentially a section of tire weighted with lead or concrete inside. Weighing it and then measuring the force required to slide it over the pavement allowed the students to calculate the co-efficient of friction for the road surface.

Back in the classroom we used the formula, the skid distance and the co-efficient of friction to calculate the police vehicle's initial speed when the brakes were applied. The answer was exactly the speed shown on the radar! While real world collisions are often much more complicated, this was a great opportunity to show the students an application of what they were studying in a manner that they had not considered.



That is cool to teach the kids about real life applications! What else can you tell about skid marks, aside from the speed of the vehicle? 


The width of the mark itself and the distance between marks can be used to relate them to the vehicle that made them. Four distinct marks would indicate that all 4 wheels had brakes that were locked. Skip skids can indicate unevenness in the road surface and gap skids indicate that the driver stopped braking for a moment. Light at the beginning and dark at the end can indicate direction of travel. That's all that comes to mind right now...

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