Drivesmart column: Calculating speed from skidmarks

The taxi driver had slammed on the brakes but was unable to avoid hitting the left turning vehicle.

Drivesmart column: Calculating speed from skidmarks

By Tim Schewe

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 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.

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.

Tim Schewe is a retired constable with many years of traffic law enforcement. To comment or learn more, please visit DriveSmartBC.ca

Get local stories you won't find anywhere else right to your inbox.
Sign up here

Just Posted

Duncan’s Knights of Columbus hand out cheques to a slew of deserving organizations in an online event Nov. 8, 2020. (Submitted)
Duncan Knights of Columbus hand cheques to lucky 13 in virtual event

Another historic first for the Knights was to have two area mayors join the presentation

Changes to the holidays due to COVID-19 will be a challenge for people with dementia. (Shalu Mehta/News Staff)
Webinars aim to help those with dementia approach holidays

“The holidays can present difficulties for people living with dementia for a multitude of reasons”

Theresa Bodger at the Duncan Curling Club, displaying the championship trophy and a photo of her mom, Georgina Falt. (Photo by Don Bodger)
Cowichan Alzheimer’s fundraiser surpasses $65,000 since inception

Courier editor and his wife overwhelmed by continued support

“Jeff Abbott sinks in a mass of mud where a tributary to Coon Creek earlier ran. The creek supplies 60 families in Youbou with drinking water.” (Lake News, Nov. 19, 1980)
Flashback: Winter tires, winter weather, watershed watch

Remember these stories from Lake Cowichan

Duncan’s Callum Davison is up for a bursary from the Golf Journalists Association of Canada. (Submitted)
Duncan golfer Callum Davison up for national bursary

20-year-old finished fifth in Canada Life Series this summer

Kyle Charles poses for a photo in Edmonton on Friday, Nov. 20, 2020. Marvel Entertainment, the biggest comic book publisher in the world, hired the 34-year-old First Nations illustrator as one of the artists involved in Marvel Voice: Indigenous Voices #1 in August. THE CANADIAN PRESS/Jason Franson
VIDEO: Indigenous illustrator of new Marvel comic hopes Aboriginal women feel inspired

Kyle Charles says Indigenous women around the world have reached out

Bank of Montreal, located on Burrard Street in downtown Vancouver. (Google Maps)
Heiltsuk man files human rights complaint against Vancouver police, BMO after bank arrest

Pair remains distraught after employee falsely reports fraud in progress leading to their arrest

RCMP Cpl. Cory Lepine pictured at BC Livestock Producers Co. in Kamloops, Nov. 16. (Phil McLachlan - Black Press Media)
Meet B.C.’s only cowboy cop; a voice for the livestock industry

Cpl. Cory Lepine serves as a bridge between the law and those who make a living off the land

BCHL
BCHL pushes back season start due to provincial health orders

The delay is minimal, just six days, for now. But the league is open to starting up after Christmas

Prime Minister Justin Trudeau speak to the media about the COVID-19 virus outside Rideau Cottage in Ottawa, Friday, Nov. 20, 2020. THE CANADIAN PRESS/Adrian Wyld
Long-awaited federal rent subsidy program for businesses hurt by COVID-19 opens today

The new program will cover up to 65 per cent of rent or commercial mortgage interest

Join Black Press Media and Do Some Good
Join Black Press Media and Do Some Good

Pay it Forward program supports local businesses in their community giving

An Oceana Canada audit of Canadian fish stocks reveals a growing number with critical populations, calling on Fisheries and Oceans Canada to enact existing commitments. (File photo)
B.C.’s declining fisheries the result of poor DFO management: audit

Oceana Canada calls for follow through on government commitments

Most Read