Hacker News Reader

Sure, if you count 5 minute drives to get cigarettes

Exactly, the per-hour rate is obviously the most meaningful, and then the per-km rate which in some sense includes benefit/risk.

Per-trip is nonsense, since the the different modes entail completely different trip lengths and distances.

If you're trying to figure out whether it's safer do drive somewhere than to fly, the only metric that is relevant is deaths per unit of distance.