same divisor as human comparision.

let me ask you the opposite, what divisor(if any ) would make you question it.

Per hour?

VMT is a common divisor used in human traffic comparisons. Like when you're trying to see if a given area is safer than another area to drive, you'll often compare by VMT to have a more fair comparison.

As for bad divisors, accidents per car? Accidents per paint color? Accidents per average number of donut shops in the area? I dunno, tons of things.