That's true in a strictly literal sense, but we really want the demand-destroying effects of a per-mile tax on private automobiles so as to incentivize a modal

That's true in a strictly literal sense, but we really want the demand-destroying effects of a per-mile tax on private automobiles so as to incentivize a modal shift to other transit options. From that perspective, there's nothing better than raising taxes proportional to road mile or road hour.