A fatal traffic crash can happen in the blink of an eye.

Witnesses aren't always reliable, if there are witnesses, notes Sgt. Ross Milward, an RCMP traffic reconstructionist and the head of the Yukon's Traffic Services.

That's when RCMP have to rely on skid marks and vehicle damage to tell the real story of what happened.

Measurements plugged into physics formulas give police numbers that enable them to make interpretations of the events leading up to the crash.

Accident investigation makes up 20 per cent of the Yukon Traffic Services workload.

Milward estimates that RCMP reconstructionists investigate about 30 serious injury or fatal vehicle accidents a year.

The RCMP investigate these crashes to determine if criminal charges need to be laid or if there are any civil liabilities.

'We go to a lot of crashes where there will never be any criminal charges, because the people have been killed, but there are still always civil repercussions,' Milward said in an interview last week.

'We're there to serve the public and it's in the public interest that we collect the data from a crash so somebody can be compensated for their injuries.'

Accident reconstructionists regularly deal with destruction, so they have to go for yearly psychiatric evaluations.

'Part of it is the death and the carnage you see on the highways, although most often the reconstructionists arrive after the fact,' Milward said.

By the time reconstructionists get there, victims are usually gone.

'We're primarily interested in the vehicle itself,' said Milward, adding that sometimes they might want to look at injury patterns on a body if they don't know who was driving in a roll over for instance.

They might also look at seat belt marks on a body, or the lack there of, said Milward.

Webbing on a seat belt can also show whether it was worn, said Milward.

When RCMP reconstructionists attend an accident they try to figure out what happens.

In a two-vehicle collision, they will look for points of impact.

If RCMP notice a gouge or scrape mark at the point of impact caused by one of the vehicles they want to be able to determine what part of that vehicle caused the mark.

They'll look at where on the road an accident happened, if a person crossed the centre-line and by how much, said Milward.

'The precision has to be there. It can be just a matter of centimetres and sometimes it's metres,' said Milward.

Police also look at speed and the time-distance relationship between vehicles and who might have seen who first or who could have seen who first, said Milward.

'The faster you go, the more metres per second you cover, so your reaction distance speed is less,' said Milward.

Reconstructionists can calculate speed from skid marks.

'That only gives you the minimum speed, because most often in a crash a vehicle stops prematurely so there is energy that is unaccounted for,' said Milward.

Yaw marks, are curved marks that demonstrate a vehicle was sideslipping. A vehicle in law would rotate around the centre of mass. With the radius of the yaw marks, Milward can calculate an exact speed.

Average speeds can also be a vehicle, if police know what time a vehicle left a certain point.

'But that doesn't give you the speed at the time of the crash,' said Milward.

If it is an accident involving a pedestrian, police will look a whether the person is walking or running.

'The faster they're going, the less time the driver would have to react,' he said.

RCMP will also look for view obstructions, like trees, bushes and other vehicles.

Accidents at controlled intersections can be interesting if it becomes a question of the red light.

It can be difficult, if not impossible, to determine the status of a red light without witnesses, said Milward.

In an ideal situation, there would be red-light cameras that would record the action through the intersection.

Many cities down south have them, but Whitehorse does not.

A red-light camera would be particularly useful at the Hamilton Boulevard-Alaska Highway intersection, said Milward, who has worked for the RCMP Traffic Services for most of his career.

'Of the 27 1/2 years in the organization, probably 25 of those have been in traffic,' he says matter of factly.

Twenty of those years have been spent in the Yukon.

When Milward first started, Traffic Services used to be known as the Highway Patrol.

Getting into Traffic Services is a staffing decision.

'Most guys spend three or four years on the section and then they are transferred off to make room for others,' said Milward, adding he's unique in having spent most of his career with Traffic Services.

Milward specialized in collision reconstruction early in his career and came to the Yukon specifically to do such work.

'That's basically kept me in the Whitehorse area,' he said. 'But that's not the norm.'

There are three traffic reconstructionists in the territory. Aside from Milward, there is Const. Scott Wessell and Const. Paul Thalhofer.

Wessell is currently working up in Old Crow, but he and Thalhofer, who is based in Whitehorse, will swap places this spring.

Milward, as the supervisor of Traffic Services, spends less time out in the field then Thalhofer and Wessell would.

'I do the odd case, anything that is high-profile, out of the ordinary,' said Milward.

Tools of a reconstructionist's trade involve measuring tapes and the coefficients of friction.

Such coefficients are measured by a computer in the police car that can determine the level of friction as an officer applies the brakes to his car on the accident surface.

Coefficients of friction can also be determined by dragging a piece of tire behind a police vehicle or by putting the vehicle that was involved in the crash through some skid testing.

'The interesting part is collecting the data and going to the scene, and trying to put the puzzle together,' said Milward about his work.

He said the more difficult part lies in writing a report that means something to a lay person, like those that work in the insurance industry.

'That's where I find the stress is more involved preparing the report and the precision that has to be put into the report and then presenting it in a court case or a civil case or something like that,' said Milward.

'You have to be accurate. If you go into court and give a speed estimate that isn't right, if you calculate a speed that is 130 and the vehicle is actually going 80, and another engineer testifies that and the court believes that then you're credibility doesn't look very good.'

That's what happened in the Const. Jeff Monkman case, where a Whitehorse RCMP officer, then stationed in Carcross, was found guilty of careless driving after his police vehicle rolled near Carcross in 2003. Heather Benson, a prisoner who was in his care, died in the crash.

Milward miscalculated the speed because he didn't take into account the slope that the rear tires where seated on. An independent engineer pointed out this mistake after Milward testified in court.

'We're not rocket scientists. We're all humans and people do make mistakes, but it's nice to recognize these mistakes before you get to court and somebody could be convicted in a very serious offence.'

RCMP plug measurements into the computer to determine speed and other factors. They also calculate everything longhand to double-check the numbers.

'There's whole number of formulas that you are trying to use, depending on what you're trying to find,' said Milward. From time-distance formulas, 'airborne' equations and the conservation of linear momentum, RCMP use a number of complex sciences to determine what happened.

Milward himself had to undergo a lot of special training from advanced accident investigation courses to collision reconstruction courses and applied physics courses.

'You have to keep current in this field, he said.

To be a part of Traffic Services, one must demonstrate the aptitude and the desire to work in that department.