It's pretty irrelevant to make the comparison against automobiles because you're assuming the travel will occur regardless of mode. Most air travel is arguably

It's pretty irrelevant to make the comparison against automobiles because you're assuming the travel will occur regardless of mode. Most air travel is arguably frivolous, and wouldn't occur if required to do so by automobile for a variety of reasons (time, inconvenience, et cetera). But if you must make the comparison, automobiles are obviously more efficient than aircraft per passenger for shorter distances. You wouldn't burn anywhere near the amount of fuel a jet uses taxiing to drive a comparable distance, to illustrate the extreme end of the spectrum favoring automobiles. Actually teasing out where exactly the inflexion point lies depends on the specific vehicles being compared, occupancy levels, and distance being traveled. These things have been studied and compared, Google it if you're actually interested in knowing when to drive instead of fly. It's far more prudent to simply cut down or eliminate air travel altogether, IMHO. There are other difficult to measure impacts from flying. For example, most people I encounter in the SF Bay area like to travel frivolously by air and broadcast to the world that they're doing so in their in-person conversations and social media posts. This amounts to advertising and promoting more frivolous air travel throughout their audiences, and wouldn't really be feasible if they hadn't taken the flight in the first place. There's a greater collateral damage with air travel, because it's so accessible and convenient, all it takes is a photo of a wing or some beautiful destination you flew to and anyone who sees the photo in your friend circle can likely afford to repeat the activity - there are effectively no barriers. Social networking has exacerbated this effect _tremendously_, it's somewhat horrifying.