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A police transit van (riot type) made an emergency stop on our street last night and left skid marks 25m long - the van was not chasing anything and had no blue lights or sirens on. I suspect he was doing in excess of the 30mph limit and had to pull up sharply due to a car emerging from a side street in to the road. There was another transit van parked outside a house opposite so cars exiting the junction couldn't see without edging out. Is it possible to work out an estimate of the speed of the van from the skid mark evidence alone?
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Why?
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"Ah...beer - my only weakness - my achilles heel if you will"
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Yes. Just ask if you can borrow the van and do a few emergency stops yourself at varying speeds.
But seriously, it depends on several factors such as the friction of the road surface, the balance of the brakes on the vehicle, the state of the tyres, the weather.
If the skid marks are 25 metres long then the vehicle has braked for at least that distance. The highway code stopping distance for a dry road at 40mph is 24 metres, so its likely he was doing around 40.
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He could have been on a shout-often police vehicles need to make a silent and stealthy approach so as not to scare the bad person stealing things from your house.
I think there are too many variables in this case to warrant an immediate letter to the chief. However repeated situations would bring cause for concern.
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Skidding for 25 metres. Is that silent and stealthy?
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Straight forward brakes no ABS or Limiters:
To obtain speed in mph from skid marks when D = (length of mark in feet) and F (Drag Factor - friction factor from road surface) then:
Speed = 5.5 \/(Square root of D x F)
(Accident Investigation by Mathematics - J Standard-Baker)
tinyurl.com/35p2x may give a better idea of the system.
DVD
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V squared = U squared + 2 times F times S is the general formula for things accelerating uniformly ~ or decelerating if you use a minus sign for F.
V = final speed, which I will assume was zero at the end of the skid mark, i.e. that the vehicle came to a standstill
U = initial speed ~ i.e. at the start of the skid mark
F = acceleration which I will assume was minus g (i.e. minus 32 feet/second squared) ~ and it was probably (numerically) less than this
Using the above assumptions, the initial speed was 50 mph. If the vehicle was still moving after the end of the skid mark, or if the deceleration was less than the assumed figure then the initial speed of the vehicle was more than 50 mph.
Be very wary of making a complaint ~ the allegedly offending officer may wish to retaliate, and very few of us are completely squeaky clean! Also, it may be construed as wasting police time. Best thing is to see all, hear all, and say nothing ~ in my not-so-humble opinion.
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L'escargot by name, but not by nature.
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Thanks for the replies. I've no intention of contacting the Chief Constable! I raised it because the police actually got out and measured the length of the skid marks and looked concerned at how long they were i.e. the car driver could have a legitimate case against them for driving too quickly for the conditions. Not much chance of that now as I think they may have been after the car, or the driver was in the wrong place at the wrong time! Driver was arrested for drink driving (saw the breath test - lots of red lights!) and possibly for possession of drugs - police found a bag of white powder in the car.
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"if the deceleration was less than the assumed figure then the initial speed of the vehicle was more than 50 mph."
Escargot - I agree with your original maths, but it would seem to me that the lower the acceleration, the LOWER the original speed. Halve the acceleration, reduce the inital speed by a factor of 1.414 (root2) giving around 35mph.
At a coefficient of friction of 0.8 giving a/f of 25.6ftpsps (experimental data in cars (sic) on dry road, see below), the initial speed would be 43mph.
hyperphysics.phy-astr.gsu.edu/hbase/crstp.html
We'll get there eventually. Now all we need is experimental data on the tryes on the vehicle in question against the road in question.
V
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Straight forward brakes no ABS or Limiters: To obtain speed in mph from skid marks when D = (length of mark in feet) and F (Drag Factor - friction factor from road surface) then: Speed = 5.5 /(Square root of D x F) (Accident Investigation by Mathematics - J Standard-Baker) tinyurl.com/35p2x may give a better idea of the system. DVD
This formula cannot be correct. Using this calculation the longer the skid marks the less the speed would be.
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The highway code stopping distance for a dry road at 40mph is 24 metres
I think you'll find that this includes the "thinking distance", i.e. the distance travelled from the point at which the driver first decided to stop to the point at which he actually applied the brakes.
--
L'escargot by name, but not by nature.
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And maximum breaking efficiency is achieved just before the wheels lock (which is how ABS/cadence braking work). As the wheels had locked, clearly breaking was less efficient than it could have been.
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Showing your age, L'escargot. We younger scholars use 'a' instead of 'f'. Intrigued you used 'g' for 'f'.
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Showing your age, L'escargot.
I can smile to myself in the certain knowledge that not too far into the future some cocky young whipper-snapper will be pointing that you are older than them!
<< We younger scholars
You're still at the learning stage then!
<
"g" is the maximum acceleration/deceleration that you can get on a wheel-powered/braked vehicle where there is purely sliding motion of the tyres on the road surface with no shearing of the rubber made possible by the road and tyre surfaces keying into one another. "Slicks" benefit from this latter phenomenon by having maximum contact area (i.e. no tread) and by being made from very soft rubber.
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L'escargot by name, but not by nature.
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They got out and measured thier own skid marks? Sounds more like the police officer was concerend at how long it took him to stop and wanted to get evidence of this so that the garage could fix his brakes.
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Singer-G
Well they would have been before another vehicle pulled out on them.
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This post is going to sound rude but it's not meant to be. (I'm in an exam this afternoon so am in a bad mood)
Am I missing something? The police van stopped a drugged up drunken driver who could have killed someone and obviously was forced to brake harder? I have to say - I'd be glad they've got someone like this off the roads even if they did have to skid!
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"Ah...beer - my only weakness - my achilles heel if you will"
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Always still learning. Now very intrigued as to the proof that 'g' is the maximum acceleration under those circumstances?
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I have seen on one of those fly-on-the-wall type documentaries about traffic cops that when officers are investigating an accident they will run one of their vehicles at a set speed, skid it to a halt then measure the length of the skid marks to determine the resistance given by the tarmac at that section of road.
Presumably this information is used to calculate the speed of the vehicle involved in the accident.
In the episode I saw they had to disconnect the ABS on the Galaxy they were using.
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"Now very intrigued as to the proof that 'g' is the maximum acceleration under those circumstances?"
Experimental details at site shown in my answer above. Where they talk about the coefficient of friction, it is true to say that that is the proportion of g that they have achieved. They managed 0.8, so their maximum deceleration was 0.8g. The limit is generally the tyres rather than the brakes.
Racing cars can significantly exceed this figure. I can't find accurate figures, but anecdotally, > 3g. I have heard that braking pulls the tears out of Schuey's eyes onto the inside of his visor. The Merc SLR that we saw on Top Gear this week manages 1.3g (must have good tyres!).
V
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Adski, hope the exam is a greater success than your interpretation of this thread.....
I suspect Daveyjp is curious rather than vengeful. As posted above, he has no intention of complaining or causing a fuss with this info. Just good old-fashioned curiousity*.
(*History doesn't record if a cat was killed in the skidding incident)
ND
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No Dosh - yes it's curiosity. It appears the driver was a scroat so I'm not concerned they skidded - better off the road as far as I'm concerned. Like I said they could have been looking for the car anyway, seen it at the junction sped up and skidded to prevent him getting away.
The police did move the car following the arrest and parked it opposite an already parked car, facing against the flow of traffic and half on the pavement! The car has now gone, the guy they arrested probably picked it up!
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I can only apologise ND and Davey. I was probably fraught with nerves over the exam.
Sorry again.
Adam
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"Ah...beer - my only weakness - my achilles heel if you will"
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Don't worry about it mate.
Hope it went well
ND
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>> The highway code >> stopping distance for a dry road at 40mph is 24 metres I think you'll find that this includes the "thinking distance", i.e. the distance travelled from the point at which the driver first decided to stop to the point at which he actually applied the brakes. -- L'escargot by name, but not by nature.
I think you'll find it doesn't.
Thinking distance 12 metres. Braking distance 24 metres, Total stopping distance 36 metres.
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Will the highway code ever get updated with revised stopping distances? Or is that some way away when they can be assured that the vast majority of cars on the road have decent (and possibly ABS) brakes. Another 5-10 years?
I know all new cars launched soon will have to have ABS regardless of model/spec.
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Common (and quite scary when you think about it) mistake.
ABS does not reduce your braking distance, it just means you can steer while full on the brakes.
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Common (and quite scary when you think about it) mistake. ABS does not reduce your braking distance, it just means you can steer while full on the brakes.
ABS itself does not reduce braking distances. However, before ABS most cars had limiting valves in the rear brake circuits to reduce the possibility of rear brake lock-up. Therefore the rear brakes rarely worked to their full potential. Now that cars have ABS more braking force can be applied at the rear, to give better overall braking.
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I think you'll find it doesn't. Thinking distance 12 metres. Braking distance 24 metres, Total stopping distance 36 metres.
Wow!
When I worked in the Engineering Dept of the company that invented disc brakes, our test drivers regularly achieved 0.95g and above in a car with a well-balanced braking system and vehicle loading (and with good tyres) on dry tarmac. And that was in the 1960s.
24 metres from 40mph equates to a mean deceleration of under 0.7g.
Is the average driver/car/road combination really that bad nowadays?
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L'escargot by name, but not by nature.
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I know a lot of police vehicles are being fitted with data recorders of various types these days. Some will are set to 'go off' so to speak in the event of a major deceleration and have to be reset after investigation as to why the vehicle was being driven so fast.
Maybe in this case a data recorder needed to have the event checked to be signed off by a supervisor.
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I agree that the highway code stopping distances are a lot longer than most cars can achieve. I threw them into this thread originally to illustrate that if the Transit took 25 metres to stop it was probably doing AT LEAST 40mph.
When were the stopping distances last reviewed? I passed my test in the 70s and I'm fairley sure they haven't changed since then.
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> the highway code stopping distances are a lot longer than most cars can achieve.
A common (and popular) misconception.
There rarely is such a thing as perfect conditions and as always these things come down to generalisations.
The table in HC is based on a co-efficient of friction of about 0.66g.
A poor road surface in an urban situation, heavily contaminated with oil and tyre rubber will perform a lot less than this and could give a .mu (the sliding co-efficient of friction) in the region of 0.4. - I have known less.
Equally a good road surface like those orange coloured rubber strips that can be seen at the entrance to some roundabouts will regularly give better than 0.8 though I have never achieved the 0.9 that they are supposed to generate.
Putting some figures to it:- 30 mph slide to stop @ .mu 0.4 = 22.9 metres, .mu 0.67 = 13.7 metres, .mu 0.8 = 11.5 metres.
So the road is a fairly variable factor.
Brakes are an improvement as they can arrest a spinning wheel much more effectively than when I was a child but of course that is not a lot of use if the tyre cannot grip the road and utilise that force.
Suspensions have also improved but only in as much as they allow the tyre to stay in contact with the road better.
The pneumatic tyre is quite good at doing that anyway at urban speeds so not much help there.
So, modern technology does not make much difference in 30 mph or similar areas. There will be differences at motorway speeds that are measurable and probably savings of several metres.
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