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Minimum speed from skid marks in crash report?

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  • Minimum speed from skid marks in crash report?

    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?

  • #2
    Originally posted by TJx2 View Post
    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.

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    • #3
      Not needed. Sometimes I'll do it for my own curiosity if things are slow, but I'm not putting it in the report.

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      • #4
        Originally posted by AI_guy View Post
        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

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        • #5
          Sounds like you're doing it right.

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          • #6
            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.
            Be courteous to all, but intimate with few, and let those few be well tried before you give them your confidence!

            [George Washington (1732 - 1799)]

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            • #7
              Well said! It would be entertaining though to watch an untrained officer testify to he derivation of the slide to stop formula.

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              • #8
                I was thinking of the wrong skid marks.

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                • #9
                  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.
                  No man is above the law and no man is below it: nor do we ask any man's permission when we require him to obey it.
                  -Theodore Roosevelt

                  The views expressed by this screen name do not represent any civilian,municipal, military, or federal law enforcement agency and are strictly the views of the individual writing. Under no circumstances should someone consider the content of these posts to have anything less than a great deal of sarcasm interlaced throughout. Read at your own risk.

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                  • #10
                    Originally posted by Dingo990 View Post
                    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.

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                    • #11
                      Originally posted by SgtCHP View Post
                      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.

                      Comment


                      • #12
                        Originally posted by TJx2 View Post
                        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.
                        Last edited by t150vsuptpr; 02-15-2011, 12:41 PM.
                        "That's right man, we've got mills here that'll blow that heap of your's right off the road."

                        "Beautiful Daughter of the Stars."(it's my home now)

                        >>>>> A Time for Choosing <<<<<

                        Retired @ 31yr 2mo as of 0000 hrs. 01-01-10. Yeah, all in all, it was good.

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                        • #13
                          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.
                          Last edited by JI603; 02-15-2011, 03:11 PM.

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                          • #14
                            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.
                            Sleeping Giant. They're not fat and happy anymore. They are hungry and increasingly angry. That is not a good recipe for a "Puppies and Rainbows America".

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                            • #15
                              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?

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