Hey Mr. Green,

What's better for the environment, driving or flying?

—Greg in Dearing, Georgia

A: I wish there was a simple answer, but your question contains a few too many dicey variables. Aircraft efficiency differs, as do the conditions on different routes—a domestic airliner can get anywhere from 45.5 to 77.6 miles per gallon per passenger, with an industry average of about 51 miles per gallon of fuel per passenger.

Motor vehicle variations are much greater, such as SUVs that average 12 mpg to hybrids that get almost 60 mpg. (Jet fuel also emits somewhat more carbon dioxide per gallon, and its global warming impacts are greater than from ground-level emissions, as previously noted here.)

But the biggest wild card of all is the number of passengers in a motor vehicle. Every additional 100 pounds of weight only reduces fuel economy by about 1 percent. So just two people in a fairly efficient (30 mpg) car generates lower emissions per person than the average plane, and even the most gas-guzzling, lumbering, reprehensible monstrosity of an SUV becomes competitive with aircraft if it carries four or five passengers. So, if time is not an objective, the motor vehicle can be the greener choice.

Which leaves me moping over the vanishing breed of hitchhikers. Picking just one of them up could greatly enhance the efficiency of any vehicle “on the road”—as the immortal Jack Kerouac titled his famous novel. Today, I fear, poor Jack might not even get on the road, but languish with his bottle of whiskey in the shade of a freeway sign.