Deaths per passenger mile is the stat that air travel happens to achieve spectacular figures for, so of course it's the stat that the airlines have always touted as evidence for the safety of flying.

In some ways, though, it's a silly measurement. In many (most?) cases, flying and driving are not equivalent alternatives. You can't drive to Hawaii and you can't buy a ticket on Northwest to the grocery store. A comparison of the deaths-per-passenger-mile stats for those two trips only sounds meaningful.