Drag factor testing

[CuriousCitizen]

When a reconstructionist uses an instrument like the Vericom to measure drag to obtain that factor for the skid speed formula, can the reconstructionist run the tests on a different road, that is, not the accident road? Months and miles away from the original accident? Does Vericom or other such other training instructions direct investigators to test on the accident road? The "owners manual" seems to.

[rickryerse]

The best practice is to do the tests in the same spot/area in which the collision took place. If the tests are done anywhere else a reconstructionist should not hold the findings of the tests, done in a totally different area out, to be representative of the drag factor for the area in which the collision took place.
There are too many variables to take into account.

[CuriousCitizen]

Thank you for the reply! So a drag factor obtained on a road 40 miles away six months later should not be used as evidence in a trial? That makes sense, but is there an authoritative source, such as an SAE or other scholarly paper or article or textbook that could be cited for it?