Speed from Skidmarks

image of skidmarks behind a carSkidmarks left on the pavement at a collision scene tell the investigator many things about the events involved in a motor vehicle collision. One common example involves using the length of the mark to calculate pre-collision speed.

It was often an interesting exercise that could either verify or call into question information given by witnesses and drivers.

Using Evidence From Skidmarks

Following my training course the first opportunity that I had to try this out involved an intersection collision. A driver had turned left in front of a taxi. The taxi driver had slammed on the brakes but was unable to avoid hitting the left turning vehicle.

The taxi driver assured me that he had been travelling at the speed limit pre-crash.

I got out my calculator and measuring tape. The calculated speed was over the speed limit and as there was a crash, some of the speed of the taxi did not contribute to the length of the skidmarks.

When I told the taxi driver what I had just done and asked again how fast he was going, he hung his head and said that it was a bit over the limit.

It's Basic Physics

Even more interesting was the opportunity to teach the calculation of speed from skidmarks 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.

formula to determine speed from skidmarks

Real Life Demonstration

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 skidmarks to measure.

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.

The shot marker is important for accurate distance measurement as the tires take a bit of time to generate enough heat between themselves and the pavement to leave skidmarks. Some braking is actually done before the beginning of the visible skid, so these speed calculations always underestimate the initial speed slightly.

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.

Testing is Important

This is the process that was used to teach that section of my collision investigation course and one that is repeated time and again with varying circumstances on testing days. Testing days allow the investigator to gain experience with known data and satisfy the courts of the accuracy of speed calculations undertaken.

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RCMP Integrated Collision Analysis and Reconstruction Service

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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...

With an anti-lock braking system (a universal system these days), under hard braking wheel lock-up will be prevented or reduced considerably; that's why they came up with it.

This will surely produce a different skid mark, if any, no? Call it a rhetorical question if you like.

I wondered if anyone would pick up on that.

What we would do was to skid the police vehicle through the scene or on a similar nearby surface using a shot marker. That covered off calculating the coefficient of friction for ABS.

Yes, an ABS tire mark looked different from a mark left by locked conventional brakes. Conventional was uniformly black and ABS alternated between darker and lighter.

My question is now that the majority of on road vehicles have anti lock brake technology does this affect how you calculate distance and speed? The reason I ask is because I have had to ‘panic’ stop on both my 2016 Honda CRV and my 2011 Ram 3500. On both vehicles I cannot ‘lock’ my brakes up and make them skid thus leaving no marks on the asphalt that I can find.

The one occasion on the truck I was towing my 30 ft. 5th wheel trailer at highway speed when a vehicle pulled out of a side road right in front of me. The truck left no skid marks but the trailer sure did.

I found your article on the Physics class presentations very interesting. I have been doing a similar presentation with the Grade 11 / 12 Physics classes around the north Island for the past few years, with assistance from the RCMP North Vancouver Island Traffic Services members. It's really well received by the students, and the teachers seem to enjoy it.

I start off talking about my role as a Collision Reconstructionist with the RCMP. I show the students what equipment I have at my disposal, and how I use that to identify, record, and measure the physical evidence at a collision scene. I show the students how we measure the drag factor of the roadway with the drag sled, then we skid the police vehicle and calculate the minimum speed based on the length of the visible tire marks. I use the accelerometer in the vehicle as well, and we compare our calculated speed and drag factor to the actual speed and drag factor. We discuss how we are always calculating a minimum speed for the vehicles in a collision event, and why. We also talk about some of the challenges we face at collision scenes (short lived evidence, the influence of ABS & stability control, etc.).

We talk about perception / reaction times, and measure out approximate distances traveled at 50 km/h in the average 1.5 second perception / reaction phase. This gives us a visual of how far the vehicle actually travels from the point where the driver sees a hazard, to when it comes to a stop (approximately 35 m at 50 km/h). Then we look at adding 3 or 4 seconds to that, such as might be the case in a distracted driving situation, and measure out how far the vehicle has now traveled before it comes to a stop (approximately 69 meters at the same speed with a 4 second perception / reaction time). That one really seems to hit home with the students, and with the teachers. If we can keep them from being on their phones while driving, it's a win!

The presentation also makes for a great ice-breaker and we get into some really interesting discussions with the students, who rarely get to have an up-close relaxed conversation with RCMP members. We go off on tangents regarding impaired driving, distracted driving, speed limits and how they apply in different weather / road conditions, seat belts and airbags, and whatever other topics they bring up. It's one of the best parts of my job as a Reconstructionist!