Its a common way of analyzing these kinds of statistics especially on large scales.

What other divisor makes sense to you?

same divisor as human comparision.

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

	▲	brokensegue 10 hours ago \| parent \| next [-]
		Per hour?
	▲	vel0city 10 hours ago \| parent \| prev [-]
		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.