PDA

View Full Version : Minimum speed from skid marks in crash report?



TJx2
02-14-2011, 06:54 PM
We have some officers who use the skid mark distance with an approximate coef. of friction to determine minimum speed..say in a rear end collision. They will then cite based on that speed and articulate in the crash report the speed.

Do you think this is a good idea since there are so many other variable in play? Not to mention the coef. of friction could be way off leading to an artificially high number...

One of my supervisors has been wanting to start doing this, but it seems to me, that I would be just asking to get called into a civil suit and looking like an idiot by a defense attorney who is more knowledgeable about traffic reconstruction than a simple formula.

What say ye?

AI_guy
02-14-2011, 07:05 PM
We have some officers who use the skid mark distance with an approximate coef. of friction to determine minimum speed..say in a rear end collision. They will then cite based on that speed and articulate in the crash report the speed.

Do you think this is a good idea since there are so many other variable in play? Not to mention the coef. of friction could be way off leading to an artificially high number...

One of my supervisors has been wanting to start doing this, but it seems to me, that I would be just asking to get called into a civil suit and looking like an idiot by a defense attorney who is more knowledgeable about traffic reconstruction than a simple formula.

What say ye?

I don't think its a good idea at all. First off a regular state crash report is just that, its not a reconstruction report. How are they getting the correct coefficient of friction? Are they using published data. Do they account for grade, or superelevation? Do they account for ABS brakes or a four wheel lock-up? There are too many variables that come in to play.... Are the other officers trained in accident reconstruction? If not, I'd bet any attorney worth the polyester used to make his fancy suit, would get that thrown out.

Dingo990
02-14-2011, 07:13 PM
Not needed. Sometimes I'll do it for my own curiosity if things are slow, but I'm not putting it in the report.

TJx2
02-14-2011, 07:13 PM
I don't think its a good idea at all. First off a regular state crash report is just that, its not a reconstruction report. How are they getting the correct coefficient of friction? Are they using published data. Do they account for grade, or superelevation? Do they account for ABS brakes or a four wheel lock-up? There are too many variables that come in to play.... Are the other officers trained in accident reconstruction? If not, I'd bet any attorney worth the polyester used to make his fancy suit, would get that thrown out.

That is why I haven't done this... best I know they get their friction numbers from a published range of numbers and use the one most beneficial to the at fault driver...no account for grade, braking efficiency, etc...

Most I have done is measure the skid marks and put an approximate distance in the narrative and ho many wheels were locks front/back if I can tell

AI_guy
02-14-2011, 08:01 PM
Sounds like you're doing it right.

SgtCHP
02-14-2011, 09:33 PM
Unless an officer is properly trained in the use of "Speed from skid marks" through an accredited class, s/he should refrain from using the data for the purposes of seeking prosecution. It may be used in reflecting an opinion; but not as fact. Too many variables are present for an accurate representation and without a concerted effort to establish or eliminate those variables, data can be misleading and inaccurate. Officers who are untrained in the discipline of reconstruction are presenting themselves for critical review and cross examination in a court - criminal or civil. Their reputations can be severely harmed by testifying to fractional data that is unsupported as to fact. Unless the incident is of a major occurance, that data should be cursory at best.

AI_guy
02-14-2011, 09:44 PM
Well said! It would be entertaining though to watch an untrained officer testify to he derivation of the slide to stop formula.

MDPOLICE
02-14-2011, 10:26 PM
I was thinking of the wrong skid marks.

http://www.headinjurytheater.com/spwn%20clown%20skid%20marks.jpg

Zamkn
02-15-2011, 12:44 AM
I have done this, but I'm in the Accident Investigation unit and I've been to the classes. A PD local to us teaches lots of their patrol officers in Intermediate Crash Investigation which includes this and other equations and I hear they do cite for speed based on skid marks if they've been to that class.

just joe
02-15-2011, 09:32 AM
Not needed. Sometimes I'll do it for my own curiosity if things are slow, but I'm not putting it in the report.

What he said. I wouldn't mess with it unless there were significant injuries, in which case you call out somebody trained in crash reconstruction. You don't need to establish a minimum speed in order to write at fault vehicles. If a guy rear ends another car it doesn't matter if he was going 3 MPH or 100 MPH, he still gets cited for Assured Clear Distance.

just joe
02-15-2011, 09:40 AM
Unless an officer is properly trained in the use of "Speed from skid marks" through an accredited class, s/he should refrain from using the data for the purposes of seeking prosecution. It may be used in reflecting an opinion; but not as fact. Too many variables are present for an accurate representation and without a concerted effort to establish or eliminate those variables, data can be misleading and inaccurate. Officers who are untrained in the discipline of reconstruction are presenting themselves for critical review and cross examination in a court - criminal or civil. Their reputations can be severely harmed by testifying to fractional data that is unsupported as to fact. Unless the incident is of a major occurance, that data should be cursory at best.


+1. My brother did accident reconstruction for a few years and went to all kinds of classes that I couldn't even begin to comprehend. Someone like him (a retired police officer/court certifed expert witness) could make a patrol officer who is "playing" reconstructionist look like a fool. Not only will you lose that part of your case, your reputation will be damaged.

t150vsuptpr
02-15-2011, 01:38 PM
We have some officers who use the skid mark distance with an approximate coef. of friction to determine minimum speed..say in a rear end collision. They will then cite based on that speed and articulate in the crash report the speed.

Do you think this is a good idea since there are so many other variable in play? Not to mention the coef. of friction could be way off leading to an artificially high number...

One of my supervisors has been wanting to start doing this, but it seems to me, that I would be just asking to get called into a civil suit and looking like an idiot by a defense attorney who is more knowledgeable about traffic reconstruction than a simple formula.

What say ye?

I was a trooper for over 31 years and over 20 of those years, on our reconstruction team. I've written parts of courses to teach, I've been to schools, I've put into practice and conmpared methods of speed determination useing radar as a check. I couldn't guess how many reconstructions I've participated in with great accuracy, been a bunch including wrecks, plane crashes, and crime scenes.

Basing a minimum speed estimate on a "estimated" drag coefficient so one can write a site is at best, a guess. At worst, it's just looking for an excuse to write a cite and pad the numbers .... while screwing over the public.

It is not a search for truth and presenting guesses as basis for charges in courts is one sure fire way to eventually see solid efforts barred from that court as well when a good defense atty rips a guess apart in fromn of a judge. If it's a guess, it's a guess ..... and a road officer's estimate of drag factors based on his reading a book or just "how it feels" is nothing but a guess.

No way would I ever suggest that someone attempt it. The supervisor who pushes you to make such guesses is uneducated in accident reconstruction. Your department trained you in early basic what the difference betwen good evidence and bad evidence is. That's what I'ld be guided by as when that day comes that this man is no longer your supervisor, at least you'll still have credability ..... your most valuable asset.

JI603
02-15-2011, 04:02 PM
Dumb, dumb, dumb.

The easiest equation based on the slide-to-stop formula takes into account that the stopping vehicle brakes from a given speed, say on an average roadway at a coefficient of friction (f) value of .7 or so, and then is completely stopped at the end of the skid marks. You can get pretty close with an average F value.

If you are out there measuring skid marks at accidents, without taking into account the residual speed of the skidding vehicle still traveling forward (because it smashed into the rear of the other car obviously) then you are doing it WAAAAY wrong. You can't just use a 30DF formula, because it only gives you the amount of speed scrubbed off in braking TO A STOP. How are you coming up with the residual speed loss during the collision... crush analysis or conservation of momentum? Once you have that value, you can't just straight up add the residual speed to the speed loss during braking.

To figure it out properly you need to move into the conservation of momentum stuff, and be able to know the weights of both cars involved... m1v1+m2v2 = m1v1+m2v2 and all the rest of it. FORGET it if you start talking angles. Most of the cops I know can't make correct change, so forget having them do this, and back up their math on the stand...

This is a dumb idea from a supervisor who has no idea of everything involved. Write the failure to control speed to avoid an accident ticket and forget it.

mavriktu
02-15-2011, 04:47 PM
You rear ended someone? you at fault (99.99% of the time) cite for COMV , have a good day.In the 70s we were trained with a Nomograph thingie ,where you had to have a test skid etc. etc. ,at the time I thought it was smoke and mirrors and no way could it come close to determining speed,still feel that way.

mallninja
02-15-2011, 05:41 PM
While I am on the serious collision team here and certified reconstruction, on a standard accident I would still issue careless. Easy enough to articulate in court and fines and penalties go up if involved in an accident depending on circumstances. Isn't there another ticket to write besides speed?

mikeymedic
02-15-2011, 06:33 PM
While I am on the serious collision team here and certified reconstruction, on a standard accident I would still issue careless. Easy enough to articulate in court and fines and penalties go up if involved in an accident depending on circumstances. Isn't there another ticket to write besides speed?

Bingo, I just cite for careless. If your "estimation" is off then you look like a jack-a55 on the stand in defending the ticket for speed. While you may not intentionally be "falsifying" the report, there is a huge probability using the method prescribed by your brass that you will be unintentionally.

t150vsuptpr
02-15-2011, 11:08 PM
You rear ended someone? you at fault (99.99% of the time) cite for COMV , have a good day.In the 70s we were trained with a Nomograph thingie ,where you had to have a test skid etc. etc. ,at the time I thought it was smoke and mirrors and no way could it come close to determining speed,still feel that way.

Oh, make no mistake about it, if you get a proper drag factor and if you measure the marks right, if you account for grade properly ..... the nomograph is spot on as long as you can see well enough to read it. It's simple math.

:)

Part of our continual training involved exercises where we knew the speed via independent sources and the minimum speed to lay thos marks on that surface was always just under that. And that's what it's billed as, "minimum speed". There's the point where the brakes are applied right after the throttle is released where there are no discernabkle skids yet the vehicle is losing speed.

I'll tell you also that true speed from critical speed scuffs is true, but again it depends on correct info, not guesses.

The problem comes in when people start guessing.

Resq14
02-16-2011, 03:37 AM
We rarely cite for crashes.

rpd1794
02-16-2011, 09:52 AM
I always found it to be more appropriate to photograph the scene and vehicles and measure the marks. When testifying, I would simply tell the court what the posted limit was, the distance of the skid marks, and then introduce photos showing the damage of the vehicles. This was usually sufficient in our courts. The only issue that I ever had with that is that the striking vehicle will generally incur a great amount of damage if the driver attempted a full lock panic stop. The nose of the car would dive sharply just before impact, forcing the bumper under the bumper of the other vehicle and leaving the much weaker grill/hood area to absorb the force. This could result in significant damage even at relatively moderate speeds.

saranac
02-16-2011, 10:11 PM
I would not be so quick to discount the published reports on drag factors and skid distance formulas to obtain speed. I used it on an 18 wheeler fatal, multiple surfaces, using the slide to stop formula taught at recon school by Northwestern. The truck hit the smaller vehicle breaking the frame in half. The smaller vehicle was at a dead stop. There is no crush data to account for this so I simply used the post crash skid marks. Using the published data without an accelerometer, I came up with a range of 68-72 MPH. I put all the information in the report including exactly how I did the formulas and why. The crash was caught on an ATM camera that took them two months to analyze. The speed according to the camera was 72 MPH. It was a 45 MPH zone. The driver was convicted of negligent homicide.

SRT936
02-17-2011, 07:50 AM
I've done it, but only out of curiosity. I would never consider my "guess" good enough to do anything with. Usually, my estimate is pretty close to what the reconstruction guys come up with, but I've also been wildly off.

Blizz
02-18-2011, 12:23 PM
I went to basic, intermediate, advanced TCI and Recon I. I usually don't do formulas for crashes unless the T/C is a fatal, and I'm doing reconstruction. If there's something weird or off about the T/C, I may do a separate report on the tire friction marks and my opinion, but only if that directly contributed to the cause of the collision. I don't get to do T/C investigation or reconstruction that much any more...so every time I had a complaint of pain wreck I was going all-out on investigations...I was told to stop doing that, fatalities only =P

ChopperCopper
02-28-2011, 11:43 PM
I just call FHP.