I think for this map to be salvaged you need to fix the denominator and not use political boundaries for aggregation. Hexagons or raster because the entire point is to make it arbitrary. The formula would need to be something like (Auto Deaths per capita)/(Road Miles Traveled Per capita), or maybe (Auto Deaths per capita)/(Sum of length of roads per cell).

Regardless, its still a population map because the denominator gets really small in rural areas, and the map still doesn't tell us anything about how safe or dangerous roads are.

#EvenThatMapEndsUpBeingAMapOfPopulation,ButMoreOfABoolean,BecausePeopleDon'tDieWhereThereAren'tPeopleAndItTakesTwoToTango