Say you need to travel from New York to LA and you are an environmentalist with modest means and plenty of time. For whatever reason or reasons your choice is to drive your own Honda Civic (28 mpg-city/36mpg-highway) or take a flight. Which option has the lowest carbon footprint?
Over that distance, a commercial flight would be the answer. The bigger and newer the airplane the better.
Of course, that’s for average use, whereas if you were going on a drive across the nation, you would be driving at a constant speed on the freeway for much of it, so you’d probably come in about even.
It also depends on whether you’re going solo or taking the family with you. The airplane’s big advantage is that it’s filled to capacity, or nearly so, and it will be regardless of how many tickets you personally buy. If you’re driving across country in your Civic, though, you probably won’t be selling your extra seats to other travelers.