The Traffic Accident Reconstruction Origin -Article-

[Home] [ARnews] [Contents] [Classified] [Advertisers] [Approach Angles] [E-mail Directory] [Feedback] [Organizations] [Reference Library]

Error Analysis for Skid Tests


Method 3- Skid Tests

Skid test convention recommends driving the vehicle at a known speed, locking the brakes, measuring the stop distance, then calculating the result. The results should be within 5% of each other. The following data was obtained in a skid test on a paved surface.

Time: Surface: Asphalt Roadway, Time: 4:15 PM, Date 2/28/96, Vehicle 1994 Chevrolet Caprice (ABS defeated), Test Speed 40 M/H, Number of Samples: 2.

Distance Data (Feet)



We will use the same equation to determine uncertainty:

Equation 1


= 73 - The average (mean) of the tests

= 2.82 - The Standard Deviation of the tests

= 2 - The Number of tests in the sample

= 4.30 - The t estimator from the table with 10 samples at 95% probability (note: this value does not appear in the accompanying Student-t Table. Instead, it was taken from a more extensive table published in another text).

= 13.1 Our answer for uncertainty in Distance - The range in which the actual value of distance lies at 95% probability.

Assuming the speedometer is calibrated, the uncertainty in speed is estimated at 1 M/H (certainly true with a digital speedometer)

Next, we must examine how these uncertainties propagate through the equation to drag factor.

The Equation for drag factor from a skid test is


Uncertainty propagates to the drag factor through the partial derivatives of this equation. Those partial derivatives are

Using the average value for distance calculated as 13.1 feet, and the speed of the vehicle as 40 M/H. yields the following sensitivities.

Using the uncertainty value for distance calculated above as 13.1, and an uncertainty of 1 for uncertainty in Speed yields the following for uncertainty propagated to the calculated drag factor.

Uncertainty in Distance propagated to drag factor is

Uncertainty in Speed propagated to drag factor is

And finally, the total uncertainty in drag factor is the sum of these two values or .2+.039=.24.

So, we can express the results that there is a 95% probability that the true of the drag factor is .73 +- .24.

Return to Article

Copyright ©

[Home] [ARnews] [Contents] [Classified] [Advertisers] [Approach Angles] [E-mail Directory] [Feedback] [Organizations] [Reference Library]