**SKID MARK ANALYSIS***Accident Investigation for Private Investigators One From The "X" FILES by Chris Voeglie of Forensic Associates, Inc. www.cybercomm.net/forensic*- A recent conversation with a fellow licensed investigator, raised my curiosity regarding those who have ventured into the field of Accident Investigation. I was introduced to this gentleman through a mutual friend. I will refer to him as Investigator X. Investigator X has been a licensed investigator for nearly 10 years and retired from some 20 years of law enforcement experience. Through our conversation, Investigator X was clearly a knowledgeable and experienced investigator, who has probably forgotten more about the private investigation industry than I have yet to learn. That was until he mentioned this great opportunity available to investigators, Accident Investigation. Well, that's where my expertise came into play and my concern mounted. Investigator X began explaining, that although once intimidated by the technical jargon and reputation of the math involved in accident reconstruction, he found it to be a rather simple. So far I was still listening, however this is also where my concern turned to fear. Investigator X at this point has no idea that I am a Traffic Accident Reconstructionist and that my practice specializes in Accident Investigation & Reconstruction.
- He advised me that after reading various publications, which appeared to have been written for insurance investigators or adjusters, he was able to easily determine a vehicle's speed from skid marks left at the scene. Although our discussion became rather entangled and involved numerous topics I wish to address this particular subject matter. The science and procedures of accident reconstruction are based on scientifically approved methods that are derived from the physics developed by Sir Isaac Newton. In recent years many individuals have surfaced, proclaiming to be Accident Investigators and or Reconstructionists. Some of these individuals are indeed well trained and knowledgeable in the field and as this example will demonstrate some not so knowledgeable. This unfortunate case is a perfect example of the old saying "a little knowledge is worst than no knowledge at all" and will prove deadly in a litigation matter.
- When I was working in the traffic division many times I was asked "how can you prove that," when I had determined a vehicle's minimum speed. I will not, and can not encompass the entire subject matter of speed from skid marks, but I will attempt to provide a basic understanding of how it can be calculated and the details needed to make your estimates more accurate and less susceptible to legal attack.
- A vehicle's speed from skidding can be calculated with a multitude of different formulas. The simplest formula to discuss and utilize is that of : The square root of 30df. This was also the method utilized by Investigator X. What this equation states is that the square root of the sum of: 30 x d x f will equal the vehicle's speed calculated from skid marks. What this means is that the formula calculates a vehicle's speed if it had skidded to a stop without striking anything. This formula, when used properly, will determine a vehicle's minimum speed in order to skid the given distance without taking into account the speed of the vehicle at impact.
- This calculation, when used alone will provide a lower than actual speed, thereby providing the driver with the benefit. In most cases, except for impacts which result in little or no damage, it will provide speeds substantially lower than that of the actual initial speed. Elements of the equation are this:

= coefficient of friction or drag factor

**EXAMPLE**- Let's use Investigator X's case as an example to help understand the use of this equation: Vehicle #1 was traveling westbound in a posted 25 mph school zone and skidded 35' prior to striking the rear of Vehicle #2 that was stopped at the crosswalk. After impact both vehicles did not move significantly although there was minor damage noted to both vehicles.
**APPLYING THE FORMULA**- In order to apply the formula we must first determine the values of our variables. We have determined the skid distance from the given example but not the drag factor of the roadway. Let's assume that the roadway characteristics are that of normal bituminous concrete (asphalt) construction, level and dry. Various tests can by performed by the investigator in order to determine the drag factor of the roadway. There are basically 3 ways to determine a roadway's coefficient of friction or drag factor.
**1) Is to perform actual skid testing at the accident site, preferably with the same or similar type vehicle configuration, at the same location and direction of travel.****2) Utilize an investigative tool called a drag sled.****3) Utilize a range of values that have been approved and accepted through previous testing.**- Investigator X simply used the given distance stated in the police report and a value he ascertained from an article he read regarding this topic. After ascertaining the drag factor he proceeded with the equation by substituting the variables with the determined values. 30 X 35' X .80 = 840 The square root of 840 is 28.98 Thus he determined that the vehicle was exceeding the posted 25 mph speed limit. It all seems simple enough right! Well, I could not help asking how he determined the roadway characteristics needed to apply published data to without visiting the site and what his experience was in determining a roadway surface characteristic?. Did you apply a range of values to determine the sensitivity of the results regarding a drag factor? Was there a grade in the roadway that needs to be considered in determining a resultant drag factor? Did you determine the location of the vehicle's center of mass including its occupants and load, and did you apply it in determining weight shift during braking? How did you determine the percentage of braking, were all wheels locked and sliding? If not, which wheels left the skid marks? How much braking can be contributed to the skidding wheels? If the other wheels were not skidding, does it mean they were not contributing to the slowing of the vehicle? What percentage of braking can be contributed to the non skidding wheels? How much additional kinetic energy was possessed in order to facilitate the damage done to the vehicles at impact. How would you convert kinetic energy to velocity or speed? How did you determine the speed at impact? What method was used to combine these findings? Did you simply add them together? The list goes on and on, and I'm sure you get the gist of it. These types of questions mount up quickly depending on the complexity of the accident.
- This example is one of the simplest accidents to determine speed from. Let's throw in maybe a tractor trailer combination with some braking problems, a child chasing after his ball and a speeding motorcyclist who was thrown from his cycle 150 ft with an unknown takeoff angle, and slid an additional 80ft once he hit the ground. Once Investigator X realized he was placed in such an uncomfortable position by a young man nearly half his age and over all experience, he realized he was lucky to have not been put on the stand concerning this "new opportunity." Opportunity yes, but definitely not for everyone. I now provide his office with these services. This is why we spend years in training, attending and participating in independent studies including actual crash testing, not to mention reading every article and book about the subject we can get our hands on. Some of us eat, sleep and drink facts, figures, procedures, formulas and theories of physics. For those of you who are interested in, or have ventured into this field use caution in providing your findings and conclusions. There is a fine line that once crossed will transform you from Investigator to Reconstructionist. This instant change in title will expose you to a world of experts most of which posses extensive formal and specialized educations, and who are knowledgeable in forensic engineering, ready and willing to pounce on easy prey.
- If you wish to conduct accident investigations by all means do so, but be conscious of the fact that after spending years building your reputation and clientele you may be the next "Investigator X "on the stand. There is a lot more involved than what first meets the eye. I strongly suggest employing the services of a Reconstructionist when presenting any evidence whether physical or assumed which will be considered above the evidence gathering and reporting level. For some I may have discouraged you, and for others, peeked your curiosity. My intentions are neither.
- For those interested in pursuing this investigative opportunity I have a few friendly words of advice. One must craw before he can walk, and walk before he can run. A great deal of credible reference material is available to you without breaking the bank. Most reference books and courses are indeed geared toward the police officer who has the luxury of investigating the accident when it happens. We, as Private Investigators on the other hand, may not get to see the case for years. For this reason I mention fellow NAIS member Jack Murray. Jack Murray has written several books, two of which, and he tells me a third is in the works, encompass accident investigation in the private sector written exclusively for private investigators. Jack was very helpful to me, in my transition from law enforcement to the private sector. Although I was extensively trained in accident investigation and reconstruction and could be admitted as an expert in court, I did not know how my potential clients needed their information presented. Jack was more than willing to assist me and for this "Jack I thank you very much!" Both of his books "Accident Investigation in the Private Sector Vol. I," which was also named "Best Investigative book of 1994"; and "Accident Investigation in the Private Sector Vol. II," which was just released, to which I posses the first issue sold along with a friendly hello from the author, are well written and are easily understood. Jack has a way of writing factual and informative material in such a way as to allow you to read on! Both of these books are available through Thomas Investigative Publications, Inc. PO Box 33244, Austin Texas 78764, www..pimall.com/nais/home.html. I have read both his writings and strongly suggest to those new to this field to purchase them. An additional initial necessity will be the "Traffic Accident Investigation Manual Vol. I," by Jay Stannard Baker & Lynn B. Fricke, Northwestern University Traffic Institute, 405 Church St. PO Box 1409, Evanston, IL 60204, www.nwu.edu/traffic/. The combination of these three books will undoubtedly provide you with a wealth of information and best of all, will not cost you a whole lot of money. All three publications will probably run around $120 and an excellent addition to any investigator's library.

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