An essential requirement for a reliable quantitative reconstruction of many accidents is the determination of a reasonable estimate of the tire/pavement coefficient of friction. The preferred method is, of course, a braking test at the accident site using a similar accelerometer-equipped vehicle with similar tires traveling at approximately the same speed as the accident vehicle.
Using a simple drag sled is a surprisingly popular alternative method for determining a value of the coefficient of friction for use in the reconstruction analysis. This consists of a lightly loaded piece of tire that may or may not be similar to the accident vehicle tires, dragged slowly across the pavement. The light loading produces a very stiff tire segment that basically eliminates the hysteresis effect – loss of kinetic energy to heat as it repeatedly bends
and recovers while sliding across the surface irregularities (macrotexture), thus yielding a friction factor that is much too low. The light loading on the tire diminishes the mechanical interaction between the rubber and the sharp aggregate projections (microtexture), therefore indicating a friction value that is too small. Dragging the sled at walking speed or less generates negligible heat loss compared to a tire traveling at the accident vehicle’s speed. This would tend to yield a friction value that is much too high.
Since the conflicting deficiencies in this method do not remotely replicate the braking mechanism of a pneumatic tire traveling at roadway speed, the measured values of coefficient of friction will be totally unreliable. The results of this procedure would never be used for legitimate scientific or engineering purposes and should never be allowed as evidence in a civil or criminal trial. Traffic accident research has shown that a trained experienced investigator can, by a careful visual examination, make a significantly better estimate of the pavement coefficient of friction than a drag sled.
A further discussion of coefficient of friction is provided in the Evidence chapter in Highway Accidents: Investigation, Reconstruction and Causation available at: Amazon.com. Information about the book and author is available at: www.bmorrow.com