calculating speed from skid marks

[VX2VESS]

who knows how to work that out ?

my son was in a car this arvo that crashed into another one. he's ok, sore should from the seat belt is all.

anyway i went there. the skid marks of the car he was in were 35 mtrs long. it was a nissan pulsar. it hit a commodore wagon in the side that was turning right coming from the opposite direction. the impact spun the commodore right around and 4 mtrs away up onto the sidewalk.

the commy side air bag went off and the opposite side was also buckled a bit chassis is bent.


From the length the side marks from the nissan just that was at least 100kph, but then it hit the other car, so would have skidding even futher if it didn't. I estimate maybe 120/130 plus and an impact speed of over 50kph.

anyone know how to calculate more accuratly ?

bloody idiots, was a 50 kph area. could have been a person they hit.

[HSVREDSLED]

try this...

http://www.e-z.net/~ts/speedch.htm

the problem is trying to accurately estimate the speed at impact then factor this into your calculations.

[Mozz]

Steve

Try these few links:

http://faculty.ncwc.edu/TOConnor/425/425lect11.htm
http://www.pimall.com/nais/n.skidm.html

This one is an online calculator:

http://www.harristechnical.com/skid33.htm

This stuff can get pretty scientific these days so it does help to understand a bit about skid analysis in terms of forensic traffic accident investigation before you start to make predictions about speed at time of the incident.

[HSVREDSLED]

Using the calculator and a very rough estimate, I would guess no less than 85km/h. Very rough guess and many variables. Again, estimated velocity at impact is the key. (and im being extremely conservative in my estimate)

[SSBarney]

Definetely was going fast enough for u to give him a swift kick i think:teach:

[CarlFST60L]

I was in a Dawoo travelling at 80km/h, our skid marks were 4 meters long, bent the commodore we hit about the same as u discribed, the commo was pushed 4 meters or so...

I would think that ur appoximation of 120 is about right

[clixanup]

----------

[Lofty]

Will a car with ABS still lock up the brakes under heavy emergency braking?

[Mozz]

Will a car with ABS still lock up the brakes under heavy emergency braking?

As far as I understand not if ABS is working properly but with regard to skid marks in the context of forensic traffic accident investigation - skid marks comprise a lot more than the traditional concept of a double or single black unbroken trail of destroyed rubber.

[Y2kGoofball]

I might be wrong but I think you still see some marks from where the pressure is being forced out the tyres, especially front because their copping an absolute pounding as the car rumbles to a stop ... but I could be wrong.

Pulsars are easy to lock up the fronts and they dont like to stop even at 60kph. I had a 2003 N16 and tried it once at Eastern Creek down the main straight, just on 60kph and hit the brakes ... front locks too easy but she dont stop which is when I realised that I dont ever want to have to try and push its breaking too the limit because it'll make an awesome burning rubber smell but thats about it.

Sounds like its fair to say though definately more then 50 kph. Hopefully the drivers learnt his lesson because I'm sure the poor Commy driver didnt deserve it, and I'm sure he'll wake up tomorrow feeling sore too.

A rant I know, but Ive got to live with a stuffed back thanks to a guy who ran into the back of me, and of a winter when its cooler i really feel it, and every morning when I wake up in pain or when I throw my back out at work, I still wonder why the hell I offered to drop my sister off at school that day, why I didnt turn right and take the short way home, why I decided to check the post office box and grab some bread instead of grabbing petrol in the opposite direction ... unfortunately though the fact is my back wont recover from it and Ive just got to put up with it during the cooler months. Thankfully it goes away once summer hits.

[HRT 8]

There is a equation that can be applied which is very accurate but the measurements that need to be taken, require some real accuracy.
It reveloves areound hte yaw marks on the road way, not so much the skid marks.
The coefficient of friction for certain types of road surfaces changes from road to road which makes a gernalised satement of "skid marks were 65m long so he must have been doing 105km/h" incorrect and innacurate.
Not that easy.

Quite esy to find skid marks on bitumen surfaces caused by cars fitted with ABS braking systems too.
We often get the smokies to lightly mist water onto the surface of the road. This causes the slight amount of rubber where the abs activates to bee seen. Its a bit hard to explain in theory but easy to se in practice.

[nudenut]

It all depends on the person. Personally I never find skidmarks to be a probably from road use, but I'm sure my mum suffers from them every time I give the SS more than half throttle when she's a passenger.

[VSSII]

For an accurate measurement you need to measure the grip of the roadway. I don't know how it's done these days but the AIS used to have an old tyre that was attached to the back of a car and dragged, various measurements were then obtained. There really may be a simple measurement to calculate the speed but you need all the information.

[GHZ28]

I have done the courses, have the software etc. but without examining the actual scene and vehicles would not be able to estimate the speed.

With the software I have, you actually enter in the depression and measurements of the collision damage, and relate that to a data base of the vehicles model's crash data. This gives an estimate of energy at impact, going back from there a relatively accurate estimate of pre-incident speed can be guaged.

That said, blind Freddy could safely summise that the travel speed prior to this incident was in excess of 50 km/h and that excessive speed for the conditions was a major contributing factor.

Glad your son is OK. At least he and the driver both get an opportunity to learn from this event.

Cheers,

Garry

[VX2VESS]

thanks all, so far can roughly say more than 100kph anyway.

Damage, the commy was hit smack bang in the center of the 2 doors, and dead straight, not at an angle, T bone! it was depressed at least 40cm and over a mtr wide or so for the size of the front of the pulsar. the front window had cracks, the roof was bent, the bottom sill well pushed in, panels on the other side were deformed from the chassis bending, door not lined up all around and adjar a bit, basically a bit of bannana-ised. side air bag went off.

his car squarely hit and was pushed back to both front wheels flat as a pancake, prob stopped somewhat by the motor.

from that i would say impact was at least 50 kph, plus the length of the skids marks to get the initial speed, 35 mtrs = about 80 kph so 130 total...

anyway pointless it was way to fast for the area. if it was wet they would be dead now

[nickh]

im unsure of the skid mark part but from the point of impact im lead to believe every 1 cm = 1km eg if the vehicles impact (eg damage was 40cm into the vehicle stucture) the point of impact was at 40kmph which then you can work backwards if you know what i mean.

[VX2VESS]

im unsure of the skid mark part but from the point of impact im lead to believe every 1 cm = 1km eg if the vehicles impact (eg damage was 40cm into the vehicle stucture) the point of impact was at 40kmph which then you can work backwards if you know what i mean.

i didn't know that, i worked the 50kph out myself so was close...

40cm crush = 40kph, plus the skids marks speed of 80kph = 120kph.

[vh-holden]

this is all a bit confusing, which is probably why i'm doing primary school teaching and not rocket science.

there just seems to be way too many variables involved.

speed
tyre width - effecting grip
tyre compound - effecting grip
road grip
road temperature - affecting grip
tyre temperature - affecting grip
weight shift of the car skidding
weight of the car skidding

oh well, back to planning a lesson on how to teach the internet to 10 year olds...

[HSVREDSLED]

oh well, back to planning a lesson on how to teach the internet to 10 year olds...


Too late for most 10 year old Im afraid. My 5 year old teaches me stuff all the time.

[vh-holden]

Too late for most 10 year old Im afraid. My 5 year old teaches me stuff all the time.


ergh, time to rethink that assignment.

[HRT 8]

A bit off side, but heres another general rule of thumb for measuring impact speeds with pedestrians.

If the head makes contact with the lower edge of the windscreen, speeds are generally 60'sk and below.

Contact in the middle of the screen indicates speeds around 80k's.

Contact with the top leading edge of the windscreen indicates speeds of 100k's.

This is a general guide only for an average sized adult.

[Speedy Gonzales]

Theres too many variables to get an accurate speed, it makes me wonder how they do come to the conclusion of what speed people are doing when such accidents occur.

No doubt they were driving inappropriately. Chalk it up as a lucky break, make them pay for the repairs to learn from it.

The only way to find out for certain would be to download the data log of the throttle and speed sensors leading up to the time of collision or........ get the same car in exactly the same condition in terms of wear and tear and repeat the accident on a day with same ambient temps......very unlikely.

[Mozz]

From what I understand, the skidmark analysis only really comes into play with respect to fatal or near fatal road incidents, and then it is but one of the "tools in the evidence/investigation toolbox" trying to sort out what happened and who is responsible/culpable for the incident. It might not be an exact science but can give pretty good approximations for use in the investigation/prosecution.

I once investigated a single vehicle accident in a 60kph zone (driver completely unhurt), semi rural area occurring just after a reasonable kink/bend in the road. 100metres of traditional skidmarks starting on the very edge of the wrong side of the double white lines then crossing over across the double white lines to the correct side of the road - car ended up - upside down on its roof in the front yard of a house not touching the 4 foot high front fence, after catapulting up a culvert.

Drivers first response to the "so what speed were you travelling?"

"about 60kph"

At which point I suggested the driver accompany me to the beginning of the skidmarks (five minute walk later) - had to stop and open the front gate of the house, negotiate the culvert, walk up the road and distinctly point out the black destroyed tire rubber burnt into the asphalt.

Once at the beginning the response then upped to something quite higher than the original response.

[BlueVZSS]

Very hard to work out with any degree of accuracy without more data but:

A 35 metre skidmark on asphalt (assuming a coefficient of friction of about .75. It could be anywhere betweeen .50 and .90) would yield a speed of about 85 kilometres per hour had the vehicle stopped at the point of collision.

[gaz vyss]

All i can say is that Mr Plod will look very closely at those skid marks... I except both drivers of said vehicles will receive a present in the mail. One for speeding and one for failing to give way. That's providing Mr Plod was advised, which going by your statement, is a requirement for insurance purposes:rolleyes:

[mmciau]

I was asked to give 'expert" evidence in a court case in Alice Springs in 1968 where a person doing a "U" Turn on a Holden was "T" Boned in the RHF door of a Kingswood by a youngster on a motorbike.

The police suggested that because the "skid mark' was umpteen feet long, the youngster was therefore "speeding" and was the 100% at fault for hitting the Kingswood bloke doing the U turn.

I gave "considered opinion" that the motorbike was only stopping because of the coefficient of friction of the rubber tyre and its reaction in relation to the road surface!

I boldly said that the motor bike's rear brake has ceased to function once it had "locked the wheel" and that the only thing retarding the bike was the reaction between rubber and surface. it was not possible to determine what speed the motor bike was travelling.

The magistrate threw out the speeding charge and pinged the Kingswood driver for driving without due care. The youngster got off!!

Mike

[macca33]

Speed from skidmark, yaw and crush equations are validated and are also an EXACT science. It is physics, plain and simple. One way by which a skidmark is produced, is by the tyre sliding along the carriageway, when the brakes are locked. Variables, such as co-efficient of friction, can be measured at the scene quite easily, using a drag sled, or by performing test skids.

Once the coefficient is known, the speed can be accurately determined.

Other methods are crush distance and, of course, vehicle movement(displacement as a result of a collision) in relation to one another.

If the pulsar was crushed in the frontal area to the point of around 1 metre, then I reckon that it was moving faster than 100kph, especially when looked upon in the context of combined crush of the side panels of the other car that was struck.

All I can say is that all occupants are extremely lucky.

Cheers,

Macca

[keepleft]

I concur Macca.

Remember that at 100km/h your are moving at 28 metres per second 'unlocked'.

A skid mark will also tell you if one (or all) of the tyres is under or over inflated, and of alignment generally.

Hard to tell without seeing the scene.

[VX2VESS]

heres something interesting on this crash

neither driver was charged with this :doh:

what up with these cops, apparently they can't decide whos at fault, can you belive that. so speedy gets to keep his licence :doh:

yet if you go 5 k's over your done for. but it ok to do three times the speed limit smash into someone and get nothing :doh:

i really find it hard to follow cops thinking on this

[SSidewayz]

In my personal experience was travelling at 70+ ks T boned Volvo, skidmarks 15mtrs, stopped moving 18 mtrs later. Wrote off Volvo in a big way and had massive crease in Excel plus flattened to the windscreen. Ended up accused of 100 Kms+ but it didnt go down like that. How come I couldnt prove the speed I was going off the report if it was an EXACT science?

[keepleft]

heres something interesting on this crash

neither driver was charged with this :doh:

what up with these cops, apparently they can't decide whos at fault, can you belive that. so speedy gets to keep his licence :doh:

yet if you go 5 k's over your done for. but it ok to do three times the speed limit smash into someone and get nothing :doh:

i really find it hard to follow cops thinking on this


There might well have been other contributory factors in the crash and legal issues, certainly detail not reported or known by the media.

[epping_clubie]

well i hope ur kid learns from his mistake mate

[kart_racer]

who knows how to work that out ?

my son was in a car this arvo that crashed into another one. he's ok, sore should from the seat belt is all.

anyway i went there. the skid marks of the car he was in were 35 mtrs long. it was a nissan pulsar. it hit a commodore wagon in the side that was turning right coming from the opposite direction. the impact spun the commodore right around and 4 mtrs away up onto the sidewalk.

the commy side air bag went off and the opposite side was also buckled a bit chassis is bent.


From the length the side marks from the nissan just that was at least 100kph, but then it hit the other car, so would have skidding even futher if it didn't. I estimate maybe 120/130 plus and an impact speed of over 50kph.

anyone know how to calculate more accuratly ?

bloody idiots, was a 50 kph area. could have been a person they hit.


This wasn't in Warrawong by any chance was it?

[BIG Shaun]

im unsure of the skid mark part but from the point of impact im lead to believe every 1 cm = 1km eg if the vehicles impact (eg damage was 40cm into the vehicle stucture) the point of impact was at 40kmph which then you can work backwards if you know what i mean.

wouldnt it just be easier to check your undies to evaluate the damage of impact after the SKID MARK!!!:lol: :lol:

[HRT 8]

In my personal experience was travelling at 70+ ks T boned Volvo, skidmarks 15mtrs, stopped moving 18 mtrs later. Wrote off Volvo in a big way and had massive crease in Excel plus flattened to the windscreen. Ended up accused of 100 Kms+ but it didnt go down like that. How come I couldnt prove the speed I was going off the report if it was an EXACT science?

The time involved and the equipment used to use this science is not available to the general duties cop or even most Traffic guys. Here in Vic we have a specialist unit, the Major Collision Investigation Unit.
They are used almost soley in the investigation of collisions involving the following.
Collisions involving 3 or more fatalities.
Collisions involving person/s with life threatening injuries where there is a suggesting of negligence by a surviving driver.
Where a police vehicle is involved.
Where a police member on of off duty was the driver of a vehicel involved.
The collision is the result of a police pursuit.
Note, I did say almost soley. They are avaliable, generally over the pohone with other investigations and are always, in my experience, very willing to give some advice or some expert opinion if asked.


As you can see there is no provision (in Vic) for the "experts" to examine each and every single collision that occurs. Its costly and time prohibitive.

I've attended countless numbers of collisions and there are quite a few that cannot be attributed to one driver or the other. The general investigation of collisions isnt rocket science until something is a little out of the ordinary. Then, well it is almost rocket science. Ive seen the equations used and they are very complex.

[Brendan]

For a bit more info about how crash investigations work, the Kiwi show Serious Crash Unit is back on CH7 2230 on Wed nights (in Vic anyway).

http://tvnz.co.nz/view/page/410965/597060

[SSidewayz]

Good explanation HRT 8. Cheers for that. I saw the report and the conditions, skidmarks, distances were all noted but it didnt look very thorough. Im in WA and not sure if we have a special unit for crashes, but this accident while major wasnt a fatal. In this case it came down to 3 peoples stories and from that they worked out that speed wasnt a factor. It was just frustrating having to prove it when I just assumed the maths would clear the issue.

[VicCop]

I've noticed lately when the firies turn up they're very careful to not drive over the marks or even worst, park on top of them.

One little trick I heard to calculate speed upon impact was to measure from the outside of the car body (where it should be) and into the dent. 1cm of impact for every km/h. So a car that's got a 50cm deep ding in it after smashing into a poll would have been roughly travelling at 50km/h when it hit.

I've always aspired to join MCIU but I'm thinking it's too much like homicide - dealing with death all the time.

[VRIIClubby]

It was just frustrating having to prove it when I just assumed the maths would clear the issue.

Money always proves ones innoncence:diddy:

[chesh2]

Hello All,
I'm a firefighter in melbourne and have seen some very horendous accident scenes. Impact speeds between cars is very scientific and the Victorian Police have a accident investigation squad that estimate these speeds. They spend Hours investigating scenes and punch alot of info into a computer to get an estimate, but that is all it is an estimate. All accidents scenes are different they are never the same. Road type, Cars driven, Weather conditions, Tyre conditions, Driver Alertness and so on.

[fekason]

I have some experience in the aviation operations field.

It is probably true that in general speed can approximately be calculated by skid mark distance.

However, there is a phenomonen called "rubber reversion", which is a potential hazard in a severe skid. In this circumstance, the heat generated in the locked tyres caused the rubber to effectively melt. The end result is akin to aquaplaning. The skid will continue with effectively no retardation.

I would assume that investigating officers would check the tyres to see if rubber reversion had occurred as part of their investigation.