Pretty sure I've seen exposure-adjusted incidence rates used in clinical trials.

Miles is simply a proxy for exposure.

Given risk here does vary by exposure time and trip length varies so much, it seems reasonable to use - at least in combination with crude rates.

Fair point - a combo might be the best approach.. I understand the idea of accidents correlating w/ miles driven, but it seems to be optimizing for driving safety rather than human life? Does that make sense?