The Traffic Accident Reconstruction Origin -ARnews-

Accuracy of rounding caution

Jim Willey (AxdntRecon@AOL.Com)
Sun, 7 Feb 1999 21:16:52 -0500 (EST)

I just completed a reconstruction and testifying in Federal Court on a case involving a slow moving piece of railroad equipment. During the investigation I found that some experts have a habit of rounding all their numbers up versus rounding down or as in my case, truncating. In the average reconstruction this would not pose much or a variance. An example is the conversion from MPH to FPS. I use 1.466 while others use 1.47. The difference is almost negligable. However, in this particular case, we were dealing with a very slow moving piece of equipment crossing the highway. Tests show the vehicle took 11 seconds to accelerate from a stopped position to a distance of 53.6 feet. This produced an acceleration rate of .88595. I used the acceleration rate of .88 versus .89. With the aceleration rate the difference was again negligable in the final result. However, when I determined the acceleration factor f = a/g the rounding of the answer would make differences of 10% to 16% in
the time it takes the vehicle to travel distances between 0 and 53.6 ft. Examples: The time required for the vehicle to accelerate for 44 feet with an acceleration factor of .02 is 11.6 seconds. With an acceleration factor of .03 the time is 9.5 seconds. With an acceleration factor of .0273, the time is 10 seconds. My point is if you are ever using acceleration factors with very slow moving vehicles, it just might be worth your time to explore the differences in the times versus your imput data. In this particular case it was very critical as to the amount of time it took the vehicle to reach the edge of the roadway and a difference of 1 second would be decisive in positioning the approaching truck in relation to its ability to stop before reaching the point of impact.
Jim Willey
AxdntRecon@AOL.Com

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