The Traffic Accident Reconstruction Origin -ARnews-

Re: Vericom VC2000

Andrew Rich (74031.1360@compuserve.com)
Wed, 28 Aug 1996 22:20:49 -0400 (EDT)

I must commment on Mr. Serth's statement that with a short expanition (sic) of applied calculus explains why the police drag sled does not work. First I would like to see Mr. Serth's applied calculus. Second does Mr. Serth mean to imply that only the police drag sled does not work? What if the drag sled were built and used by someone who is not a police officer? I would think that he means to indict all drag sleds.

Let us deal with reality. My Accident Investigation Unit under my supervision has performed many tests in order to substantiate the use of our drag sled. We have constantly found that the drag sled provides a lower speed estimate when using the (30df)**0.5 equation. Percentages range from 4 to 11 percent LOWER than the actual vehicle speed.

While attending Texas A M's Pedestrian Bicyclists Accident Reconstruction School I had the opportunity to test our drag sled against their fifth wheel. The wheel provided a drag factor of .81 while the drag sled provided the exact same reading. Coincident to be that exact? In this case maybe. Considering the totality of the circumstances for this drag sled however I feel confident in conducting criminal investigations and testifying in court with this sled.

There is one thing I think I should mention which may explain the higher readings many people seem to get from drag sleds. I have personally witnessed experienced reconstructionist / investigators using the drag sled and found that many of them take readings when the sled first begins to move. The do not wait for the sled to achieve a constant velocity. We all know (or should know) that this practice yields the static friction value and not the dynamic value we are looking for (in the case of conventional brakes). The static friction value will be higher. Remember; no matter how good a tool is it is only as good as the person using it.

Consider the following case I am reconstructing at this time:

A pedestrian is hit by a car in a 50 mph zone. The braking tiremarks are 90 feet in length. All four wheels are braking. The drag sled yields an f of .81. (30df)**0.5 yields 46.7 mph. Let's say my sled was terribly high and the actual f was .71. The speed estimate is 43.7 mph. I don't think 3 mph is whole lot to be bickering about. The driver of the car was still not speeding.

The reality of the situation is that (30df)**0.5 is not
Andrew Rich
74031.1360@compuserve.com

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