how effective is 'skid testing' by cops?

[bugjuice]

just watching the news now about an unfortunate accident in Ponsonby, Auckland. A pedestrian was hit by a Skyline. The news showed a cop driving down the road and skidding to try to figure how fast the Skyline was going.

I have often wondered how effective that is, bearing in mind that there's so many varibles to consider, is it even worth bothering?

The varibles I can think of off hand would be of the suspect vs cop car:

Tyres - different grip between cars, pressure, brand of tyre, wear on the tyre

The car - ok, so a Skyline and Holden could be similar in weight, but some cars are way heavier or lighter which would effect the stopping distance

Was the driver braking before the wheels started to lock up causing the skid?

Brakes - are they defective on the back, causing the front load more..

Just so many things to think about, I can't see the point in trying.. I even once saw (this is what got me on about it years and years ago) on TV, a huge old Rover 3200 trying to find the speed of a mini by its' skid marks. Gotta be kidin me..

not bashin cops here, just trying to understand how it could even be thought to be done. I can (at a realistic guess) imagine that it'd give a very rough idea which could be +/- 15kph or so of the impact/skid speed, but again, what about braking before the skid?

/rant

[scumdog]

Can't quote precise figures but was really surprised at how close the braking distances were in a variety of unrelated car, the results were put on a graph and it was easy to follow the speed/distance thing..

[Ixion]

But surely that is completely meaningless ? A (very) good driver - or one with ABS brakes - will stop without *any* skid marks. Once you are leaving skid marks you have less than optimal retardation.

So does that mean that a driver who is driving very fast but is very skilled , holds the brakes just under lock up point until just before impact, then locks up and skids, would be assumed to be going very slowly ?

[Motu]

After the Boi Racer ''accident awhile ago outside our place...the Cops blocked off the road and did some skid testing to try and get an idea of his speed...I hear they backed of at 120kph,so they know he was going way faster than that.I don't see they wasted their time...but they coulda asked me..I reckon 160kph.

[Sniper]

Hmmm, Im with Buggy on this one, way too many variables to consider

[tl_tub]

A device they use sometimes is a piece of tyre (from memory around 1/4-1/3), with a certain mass of concrete in it, tyre is pulled (with tread side down ofcourse) with a gauge attached to measure how much force is required.
Ofcourse the more scientific way is to test the surface using a british pendulum.
:msn-wink:

[madboy]

The data I've seen in various car mags over the years showed huge variances between cars. Falcons outbraking MX-5s, even though 500kg heavier (almost 50%) Falcons had ABS, the MX-5 tested didn't. Who woulda thought? And I know from personal experience that a performance tyre is not even remotely comparable to a budget tyre.

As BJ alluded to, some poorly maintained piece of junk with Jap import tyres (harder compound) vs a well maintained more modern vehicle with quality tyres... not a particularly accurate nor scientifically comparable result.

I'm assuming the serious crash units know a bit more of the science behind this and can actually do a decent job of allowing for the variables in their scientifically sound calculations. But it is always a big assumption to make that the cops are as on to it as they make out (not bashing, just commenting on my experience).

[marmel]

Next time you see a cop doing skid testing have a look to see if there is a very expensive looking piece of kit stuck to the inside of the windscreen. It is used to find out just what the friction rating of the surface is, they don't start laying down rubber and measuring it, that is just a by-product of testing the surface friction, it's all calculated electronically. I have been involved in measuring a few scenes and speed was normally calculated down to 93.73 etc by measuring the length of the original skidmarks and using a formula including the friction rating of the surface. ABS on vehicles does negate this however, you can't measure speed of a vehicle accident scene without skid marks....

[Skunk]

Next time you see a cop doing skid testing have a look to see if there is a very expensive looking piece of kit stuck to the inside of the windscreen. It is used to find out just what the friction rating of the surface is, they don't start laying down rubber and measuring it, that is just a by-product of testing the surface friction, it's all calculated electronically. I have been involved in measuring a few scenes and speed was normally calculated down to 93.73 etc by measuring the length of the original skidmarks and using a formula including the friction rating of the surface. ABS on vehicles does negate this however, you can't measure speed of a vehicle accident scene without skid marks....
I was going to guess that they would be measuring the surface friction and apply a formula to that to work out the other cars speeds. Sort of geting a baseline to measure from to plug the variables into.
:unsure:

[marty]

but it's so much more fun slagging off the cops for doing their job than it is actually doing some research before opening one's un-informed gob

[madboy]

but it's so much more fun slagging off the cops for doing their job than it is actually doing some research before opening one's un-informed gobDamn right - never let an opportunity to slag off the 5-0 go wasted!! (PT)

[ricksta]

i saw the fotage on the news... the witness said that he said that he saw the woman was hit and landed about 17-18 metres away... thats insane!

[StoneChucker]

i saw the fotage on the news... the witness said that he said that he saw the woman was hit and landed about 17-18 metres away... thats insane!
Didn't the witness (the guy) say 80 meteres? (He said it may have been 70).
When you think about it, 20 meters is not far really...

I'm sitting here thinking it must of been 18, not 80. But then, why would someone give two figures, so close together. I'm sure he said 80. Either way, thats a witness account, and people always exaggerate. (Like the fish I caught the other day, it was THIS big...)

I do know, that at motorway speeds, people can be catapulted MANY meters, possibly 100? SD, SC, Marty? Is this correct?

[bugjuice]

Next time you see a cop doing skid testing have a look to see if there is a very expensive looking piece of kit stuck to the inside of the windscreen. It is used to find out just what the friction rating of the surface is, they don't start laying down rubber and measuring it, that is just a by-product of testing the surface friction, it's all calculated electronically. I have been involved in measuring a few scenes and speed was normally calculated down to 93.73 etc by measuring the length of the original skidmarks and using a formula including the friction rating of the surface. ABS on vehicles does negate this however, you can't measure speed of a vehicle accident scene without skid marks....
good call, but then that only takes off a few of the varible factors. There's still the effectiveness and condition of the tyres on the suspects car, the weight of the car, the braking before locking up etc...
I can see how it'd work, but I can still see how it'd be flawed..

SC, 80m is a fuk of a way to be bounced.. you'd be mush at that speed, surely? I can't remember what I heard myself, but even -20m would be more than enough to kill, and this is in a very busy road.. I'm thinking the 17-18m..

[Skunk]

The witness said 70-80 metres and the guy they interviewed said the car was going faster than 70khp. Something sounds off to me. (It was Ponsonby...)

[marty]

i've seen bikers go over 100m after hitting something, but they are usually going a bit quicker, and they were all dead. had a head inside a helmet go 150m once. that took some finding too!

also had a kid asleep in the back of a car that hit a power pole down sh27 many years ago - he was ejected out of the car (through the back window) and found with a broken leg about 60m away.

i could see how the car would stop 70-80m away from the person they hit, but most pedestrians go over the bonnet, windscreen, and into the air, landing reasonably close to the point of impact. unless it was a Hummer that hit her...

[Lou Girardin]

If she was thrown 80 metres, she would've landed in pieces. Throwing a pedestrian that far is totally different from a body thrown from an already moving vehicle.
As for stopping distances, I have strong suspicions about the accuracy of testing. It may work for an average car on average tyres, but when you start considering sticky tyres and hi performance brakes (even ABS) it starts to seem less than accurate. After all, how many bike/car tests show differences of up to 7 metres stoping from 100 km/h? But we never did skid testing so I stand to be corrected.

[Drunken Monkey]

...After all, how many bike/car tests show differences of up to 7 metres stoping from 100 km/h? ...

some examples, FYI:
Tested with an electronic G-timer, braking from 60mph (97km/h) to 0mph (damn imperial devices. Assumes 3feet per metre (close enough, but not exact)
All vehicles tested with 1 driver and 1 passenger unless otherwise stated.

2004 2L Mazda 6, auto, ABS:
42.0m, 2.86s
42.3m, 2.95s
40.3m, 2.77s
variation over 3 tests over the same surface = 2.3m

2004 2.3L Mazda 6 auto, ABS:
41.6m, 2.78s
34.0m, 2.01s ! <= braking peaked at 1.09Gs
41.3m, 2.68s
variation over 3 tests over the same surface = 7.6m

2005 Holden SV6, auto, ABS:
41.6m, 2.88s
46.6m, 2.99s
variation over 2 tests over the same surface = 5.0m

2005 Mitsubishi Lancer, manual, NO ABS:
57.6m, 4.34s
57.0m, 3.83s
variation over 2 tests over the same surface = 0.6m
(In this test the driver simply 'put the foot in it', there was no serious attempt at seeing if this was the fastest way to stop the car)

Some others (best braking acheived over 2 - 3 runs):
2005 Suzuki Swift, auto, ABS: 38.6m, 2.68s
2005 Ford Courier, auto, (braking type not recorded), driver only: 48.0m, 3.46s
2005 Mercedes C180K, auto, ABS: 39.3m, 2.65s

We have lots more data, I can't be arsed typing any more out.

We haven't done any bikes yet, still working on a easily transportable power source (probably going to make a cigarette lighter + fuse that can connect directly to the bike's battery).

[bugjuice]

so is your point that they are accurate or inaccurate?

[Drunken Monkey]

so is your point that they are accurate or inaccurate?

No point, just displaying some of the raw data. You need to draw your own conclusions.

Although one item does re-inforcce Lou's comment, there was 1 vehicle that had over 7m variation in stopping distance. This was also the shortest braking distance vehicle we have tested = 34m from 60mph. How much distance do the LTNZ say to allow for? (Add approx 30m travelled for your '1 second reaction' time)

[Skyryder]

I would have thought that tyre pressure would have an 'impact' excuse the pun on skid distance. Surely softer pressure would enhance stoppage as against harder pressure. Is that factured into the equation??

Skyryder

[bikerboy]

Not sure it really matters what the speed was. Fact is the person was killed.

From the police perspective the driver had to be speeding, otherwise at 50kpm the driver would have been able to stop, correct. Speed kills.

From the driver's point of view, no matter what fine, sentence, etc. they get, they still must live with the fact that they caused someone's death. No matter how cynical one is, still a hard ask in my book.

From the family's point of view, a tragic loss.

Will any of that change with the speed, or is it explotation of a someone's death to peddle the status quo?

[bugjuice]

Not sure it really matters what the speed was. Fact is the person was killed.

From the police perspective the driver had to be speeding, otherwise at 50kpm the driver would have been able to stop, correct. Speed kills.

From the driver's point of view, no matter what fine, sentence, etc. they get, they still must live with the fact that they caused someone's death. No matter how cynical one is, still a hard ask in my book.

From the family's point of view, a tragic loss.

Will any of that change with the speed, or is it explotation of a someone's death to peddle the status quo?
no, not at all.. This isn't what I'm asking, and it is very unfortunate that someone has died.
This isn't the focus of this thread.

The point is aside from the fact that this was a tragic accident, how can the police put a figure on the speed of the car in question? The end result of the skid isn't the point, it could have been into another car, or a lampost etc. It is unfortunate that it ended with the death of someone, but it's not the question/discussion. Could the car have been going faster than what the police say? This would make him even worse off.. or was he telling the truth and the police car looks to be going faster.. that's the point I'm getting at. There is no question about his guilt, or even the remotest possibilty that he might be innocent. By no means. He's guilty.
This is just a hypothetical thread that was brought about by a tragic accident. We're not that shallow!

At times like this, everyones thoughts are with the family. without question.

[Lou Girardin]

Not sure it really matters what the speed was. Fact is the person was killed.

From the police perspective the driver had to be speeding, otherwise at 50kpm the driver would have been able to stop, correct. Speed kills.

From the driver's point of view, no matter what fine, sentence, etc. they get, they still must live with the fact that they caused someone's death. No matter how cynical one is, still a hard ask in my book.

From the family's point of view, a tragic loss.

Will any of that change with the speed, or is it explotation of a someone's death to peddle the status quo?

Come on now, you don't know the circumstances at all. Do you think that at 50 k's a car can stop instantly? Step out in front of a car doing 50 k's and if there isn't room for the car to stop you're toast. Most pedestrian fatalities are actually from drunks walking out in front of cars