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How math, science and forensics come together to unlock collision reconstruction cases

Click to play video 'A closer look at how collision reconstruction investigations work' A closer look at how collision reconstruction investigations work
WATCH ABOVE: Nick Westoll speaks with members of the York Regional Police major collision investigations unit to get a fuller understanding of how math and science propel reconstruction cases forward.

As first responders respond to tens of thousands of crashes that stem from impaired, stunt and distracted driving annually, it falls to specialized collision reconstruction investigators and units to piece together the aftermath in order to find out what happened.

“There’s a lot of physics, a lot of math that we do,” York Regional Police Det. Const. John Paterson, a qualified engineer-turned-investigator, told Global News.

“Looking at how different things interact and the dynamics of a crash all add to an understanding of what happens in a collision.”

Read more: Distracted driving is No. 1 cause of deaths on Ontario roads

In York Region alone, statistics showed there were more than 13,000 collisions in 2019 leaving 28 people dead and 3,743 injured. So far in 2020, 17 people have died in collisions and the major collision investigations unit was probing 24 different incidents.

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“Most of the collisions that we’re investigating involve speed, drug or alcohol use — so impaired driving — or distracted driving. Most of the injuries come from lack of seatbelts,” Paterson said.

“We’re seeing a lot of crashes related to distracted driving, especially related to pedestrian collisions — people just really aren’t paying attention to what’s going on.”

Click to play video 'What do police investigators look for after a collision?' What do police investigators look for after a collision?
What do police investigators look for after a collision?

Global News met up with Paterson and unit investigators at a closed-off road in East Gwillimbury to learn more about the role reconstructionists. During this visit, they were conducting a study of how police vehicles respond at high speeds to a newer standard of pavement being used in the region.

The work wasn’t just about how fast the vehicle could go, they were looking at things like the temperature of the wheels after a hard brake.

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What do investigators look for at a scene?

While interior dashcams have become a major tool to help figure illustrate the moments leading up to a crash, investigators still rely on visual clues and calculations.

“Skidmarks on the roadway, so the actual marks that tires are leaving (whether it’s rubber or the oil that’s coming out of the pavement during the heat), we’re looking at where the vehicles have contacted the ground (if there’s a tire that’s come off a rim, where the rim gouges are, where an engine block might have made an impact with the ground when it hits another car) … transmission (fluid) and oil and the directionality of that, where the glass is,” Paterson said.

If a vehicle gets airborne off of a hill or a ditch, he said the distance from the jump point, along with a potential skid line, can help produce a calculation showing how fast it was travelling when it took off.

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Airbag control modules, which became actively used in vehicles made after 2012, can shed light on how the vehicle operated before a collision.

Paterson said it collects data for about five seconds before the crash, noting it can report how many people were in the vehicle, the vehicle’s speed and direction of impact.

However, the indicators aren’t just restricted to the engine or the outside of the vehicle either.

READ MORE: Toronto sees nearly 600% increase in stunt driving during coronavirus pandemic, police say

“If (a vehicle occupant) hits the front windshield, we know that obviously the car stopped really quickly — especially if they’re not wearing their seatbelt. If they’ve hit the side window, we get an idea looking at the inside the car where the occupants were and where the direction of force happened,” he said.

In making their calculations, the laws of conservation of momentum and conservation of energy. Darren Singh, an astrophysics major and a computer science minor student at York University, is working as an intern with the unit, tracking the vehicle stats for the service’s study.

He said understanding conservation of momentum is important for investigations, citing a cannonball being fired out of a cannon. Both are at rest before firing.

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“To compensate, just because the cannonball goes forward to cancel it out the cannon must go backward,” he said.

Click to play video 'Explaining how physics relates to collisions' Explaining how physics relates to collisions
Explaining how physics relates to collisions

“So you think about applying that to a collision, you know the two cars are going in one direction before so you know the total momentum at that one second and then you know when they collided you know what has to happen because you know the total momentum.”

Although he isn’t a collision reconstruction investigator, Singh said he likes to look at collisions from a mathematical perspective — and knowing the direction each of the involved vehicles was travelling is essential.

“I’d want to know who is going northbound, who is going southbound, who is going northbound who is going eastbound, because once you know their initial directions, from there the next step is to figure out the initial velocities, from there you look for video,” he said.

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“Use conservation of momentum, put in a direction because it’s useless unless you know where everyone was going, you can’t put an initial velocity without a direction.”

Processing the evidence

Paterson said officers will need to shut down the road surrounding a scene in order to look for as much information as possible. Investigators can often be seen physically measuring tread marks and other roadway evidence. However, the unit now uses drones (for scenes over a large area) and 3D laser scanners.

The scanners, he said, can capture every detail to within a millimetre of accuracy. He said a typical investigation will require 20 scan points at a scene with each scan taking two to three minutes.

With those captured 3D scans, it allows investigators to go back in time if need be. Unit members also said that data can be used in court in different forms — such as creating videos moving the viewer through a scene, recreating the collision on a 3D printer to show as a diorama.

“So a lot of the magic happens back at the office with physics, math and textbooks,” Paterson said.