Accident Investigation Rules

Topher

I was curious to what rules Police accident investigation people use to work out the speed of cars before accidents. Also an explanation of how these work would be useful as well.

I have also herd but i don't know how true it is that the coefficient of friction from any make of tyre is about 0.7. What does this mean? and why do more expensive tyres seem to work better even know that they still have coefficient of 0.7.

Any other information about how the accident investigation people work things and how things happened would be useful as well

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russ_watters

Mentor
.7 is probably dynamic friction - its what they use when they look at skid marks. The damage to the cars also makes a difference - they probably have to estimate impact speed based on damage.

LURCH

I do not know what "rules" are applied, but I think it likely that those who investigate crsh sites have experimental data to compare to the on-site evidence.

Investigation seems to be rather "ad-hoc" when it comes to car crashes. A friend of mine had a 17-year-old son get into an accident, and the police just issued him a ticket on the spot. My friend had to go to the site of the accident himself, drive on the street at the posted speed limit, and slam on his breaks where the skid-marks began, to discover that his vehicle came to a stop in less than half the ditance as the skid-marks. With this information he was then able to go to court and show that the car that hit his son must have been going more than 70mph in a 35mph zone, and the accident was entirely their fault.

EDIT:

I found this site, which looks like it contains just about everything a person would want to know about investigating a crash site. Heven;t had a chancs to read through it yet, but it looks fascinating.

Last edited:

cops also use force and momentum when examining a car crash. The cars after a crash will me in different zones, and lanes. Cop examinors wan estimate speeds by looking at this displacement and angles from impact. Then using linear momentum, they calulate at what speeds the cars were going.

Topher

Thank you for that site, it had alot of information but didnt have any rules. I think you have to attened those semanars and i just cant do that since I am in Australia. But thank you anyway if anymore information can be provide i would be apprechitive

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