Tire marks 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.
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 traveling 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 contibute 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.
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.
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 a mark. Some braking is actually done before the
beginning of the visible skidmark, 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.
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.
Constable Tim Schewe (Retired)
DriveSmartBC: Where better than average road users satisfy their curiosity.
