Rather than put this in the emotive '4 police cars crashing' thread I thought it would merit a separate thread.
Midlifecrisis wrote:
"However that 'faffing' about with coloured sticks and theodolytes, along with skid tests, revealed that the other driver involved in the collision was travelling at 52mph (30mph limit.) They also revealed that if he had been travelling at 30, he would have stopped 9 meters before the point of impact."
There was a programme on TV recently where measured skid marks(of the motoring variety) were the basis of calculating the EXACT speed of the offending vehicle.
This raises two questions:
Firstly a high percentage of cars now have ABS. Is there any police method of calculating speed before application of brakes?
Secondly there are so many factors that can determine the length of skid marks. Type of tyre, its pressure and temperature. Type, condition(ie how worn) and temperature of road surface. Car weight, suspension etc.
In the latter case is it really possible to give such an exact estimate that would stand up in court to rigorous challenge by the defending lawyer backed by an 'independent' expert?
I use the term "independent" advisably as I was once heavily involved in the defence of someone accused of a very serious motoring offence. Because of the insurance implications, the high powered defence team brought along a highly qualified motoring expert. Without going into detail this expert's sole employment was to give evidence for the defence and challenge police evidence.
I think he would have field day challenging such an exact estimation of speed.
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While I do not have any experience of this field of work, I would be surprised if any responsible engineer or scientist would claim any calculation of this type to be exact. There is virtually always some uncertainty in the input data. The best engineering calcs give an error estimate, or bounds, as well as an 'answer'.
As for calculating the speed before a skid, this would be my approach...
The imapct site must be found - both vehicles may be found away from the impact site.
Given the amount of damage evident at an accident, the impact speed must be estimated. The final location of the vehicles post impact may help in this estimation.
The upper and lower bounds of dynamic friction may be estimated by skidding instrumented cars near the site under the same atmospheric conditions.
The weights of cars are known from tabulated data, or the crashed vehicles may be weighed if carrying cargo or non standard adaptions.
A set of calculations can be done, feeding in each combination of input data. The output of this will be a range of speeds, between a lower and upper bound.
If the spread between the upper and lower bound speeds was large, I would keep looking for other data or experiments which could reduce the 'spread'.
As a first thought, I would spend most of my effort determining the impact speed.
If the lower bound calculated speed is above the speed limit, then it is very likely that that vehicle was speeding prior to the skid.
Would that type of analysis be easy to throw out of court?
number_cruncher
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Whether they can determine the speed accurately or not, does it really matter when they ignore it? I've posted before about my friend who was hit by a car travelling at over 60 in a 30 who lost control when she pulled out a junction as he came around the bend in to view. They got the figure of 60+ from the skidmarks, proved she had started pulling out before he came in to view but she still she got full blame and no recompense for fairly serious injuries suffered.
(Indeed, the response of one of the officers later on was that she'd be lucky not to be prosecuted for dangerous driving)
They might as well have just cleared away the bits and not bothered investigating.
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Secondly there are so many factors that can determine the length of skid marks. Type of tyre, its pressure and temperature. Type, condition(ie how worn) and temperature of road surface. Car weight, suspension etc.
Other variations that you've missed out:-
Road surface variations, both in surface texture and undulations.
Road and tyre contamination, oil, diesel, rain, loose gravel &c.
Slight inclines will cause significant variations in stopping distance. Gravity will will either help or hinder.
Why does it take so long to measure these, surely a few photos from a helicopter with some known length markers and the jobs done.
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I read often, only post occasionally
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Hasn't criticism of police inaction at the scene of the accident been done to death in the '4 Police cars' thread?
Please lets not continue it in here.
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Agreed. Anything further will be removed.
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I'm not criticising what they did at the scene of the accident - they got some good evidence that the other driver was breaking the law and driving in a dangerous manner. My point was that they refused to use this evidence. Indeed, I don't recall a case where it was used.
I'd echo the question as to how accurate it is, but also ask if it can actually be used as evidence or is it just data collected purely because they can, rather than for any purpose? (Thus the question as to why they actually bother - do they gain anything from it)
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A few years back, there was a situation here involving a high-speed collision, where a car entering a rural road was hit by somebody travelling at great speeds. A few people were seriously injured.
The case, IIRC, was about the high-speed fellow's speed as he approached the junction, and the fact that the driver whose vehicle was hit could not have seen him.
The court heard evidence from rally driver (and occasional le mans driver) Sarel van der Merwe, who had been given a similar car, and had been allowed to drive on the portion of road involved, after it had been blocked off to normal traffic.
He testified that from his view of things, and from purposely re-creating the 'skidmarks' on the road, that the car would have been travelling at over 210km/h!!
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Is there not an even larger anomaly here?
Presumably the length of the skid is taken to be indicate of the deceleration incurred by the car?
Surely this is then invalid in the consequence of:
1) The driver braking hard, but not hard enough to have skidded before impact
2) The driver braking hard enough to skid, and then, realising that skidding does not slow a car as quickly as braking with the wheels still turning, lifts off slightly some time before impact
I can't help feeling that i must be writing a stupid post with an obvious answer here, but i can't think of it at the moment!
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You still get 'skid marks' when braking heavily but without the wheels locking (otherwise tyres would last forever).
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So skid marks of the almost invisible kind that police equipment can detect, and detect the amount of rubber residue and therefore braking force applied?
Righteo, thanks for that martin
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Martin,
So how do you determine if those marks were caused by light, moderate, heavy braking? - and from what speed? The same argument as in the initial post applies.
I would have thought the skid marks that police measure are those produced by a locked wheel.
C
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Oh really. It's quite simple. Skid marks give a rough and ready idea, not a perfect calculation. But that is all you need.
If you have a skid mark a mile long then you know he was going seriously fast. If you have one an inch long then he wasn't.
If you know that the best deceleration a car can achieve is x and the skid mark is y long, then x and y tell you a minimum speed. If that minimum is welll over the limit, bingo.
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usualllll qualityyy off typingg, I seeeeeee....
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Keyboard fault Patently - switch to Linux:-)
Happy Motoring Phil I
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If you know that the best deceleration a car can achieve is x and the skid mark is y long, then x and y tell you a minimum speed. If that minimum is welll over the limit, bingo.
Patently,
OK we know its Friday!
C
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"..a high percentage of cars now have ABS"
ABS only comes into effect when the braking effort is enough to slow one wheel more than the others, or to lock it. We have some interesting marks outside our front gate, being patches of rubber about three feet long, with slightly shorter gaps, all over a distance of a good hundred feet.
The ABS seems to have done its job, as the hedge bounding the curve in the road remains intact...
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If ABS has done its job. There wont be any skid marks. Idea of ABS is to prevent wheel lock.Full stop. If faulty maybe.If ABS worked ok. Their wont be any skid marks.
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Was mech1
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Another factor is the quality of the tarmac. I actualy twist my shoe and try to get a feel for it when walking the hill before a Speed Hill Climb.
ABS will leave a residue of rubber, as will anyone who is breaking heavily but not locking their wheels. No where near as obvious though and I guess you would have to rate their tyres to have any idea of the speed at start of the braking.
Extreme contrast, but the rubber on my tyres on a Landrover is as hard as nails, 60000 miles and they are just over half worn, wheras in 6000 miles I have nearly finished a set of road legal race tyres. I would therefore expect the latter to leave a lot of rubber under heavy braking compared to the land rover, but I would expect to stop much quicker. (And I can).
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Those truncated skid marks, are usually ABS lorry brakes, these are a lot slower at releasing than the hydraulic disc brakes on cars, as all but the latest ones are air-operated drum brakes, where the air holds the brake off, instead of applying the brakes - at least on the ones I've seen..
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Steve,
There is just a little bit more to it than that.
A free rolling wheel has 0% slip.
A fully locked wheel has 100% slip.
Tyres generate maximum traction or braking forces at about 20% slip. This optimum slip is what ABS aims to acheive.
So, under a heavy stop, with ABS active, you would find rubber on the road - although not as much as a fully locked wheel.
number_cruncher
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