Q: In terms of greenhouse gas emissions, is it better for a family of four to drive a 20-mpg car from San Francisco to Los Angeles, or to fly? At what mpg rating (for the car driven) are the emissions equal?

Asked by Steve Schmidt, ’83, MS ’92, Los Altos, Calif.

The quick answer is pretty quick: Driving your family from San Francisco to Los Angeles emits fewer greenhouse gasses (GHGs) than flying. Acknowledging the lack of good rail service to cover the 400 miles, and despite the strain on your sanity from the bursts of “Are we there yet? Are we there yet?” coming from the back seat, in this case the 5.5 hour drive is probably your best bet. In fact, unless you're looking to venture from Los Altos to LA off-road through state parks the whole way—in a Hummer with Tonka-truck mud tires—driving is generally greener than flying. To tip the balance further, take-off and landing use more fuel than cruising does, so shorter flights like this one end up being even less efficient than a cross-country route.

In your specific case, driving a family of four to and from LA with a 20-mpg car produces about a third of the carbon dioxide that would be your share of the equivalent flight—about 0.4 tons versus 1.2 tons. In order for a car with four people to exceed the CO2 generated by flying to LA, it would need to average about six mpg—Even the largest RVs, in fact, can usually manage about eight mpg.

Not to stir up trouble, but if after a few days of quality time you should tire of the family, the trip back alone in your 20-mpg car would still emit about the same amount of GHGs as flying solo. You could flee by air without additional harm to the planet, though your conscience might start bothering you for other reasons about halfway home. I'm joking of course, but the more passengers per car, the less impact each will have. So pack up the rug rats, and know that the following six hours will have much less impact on the climate than the one-hour flight.

The Bonneville Environmental Foundation provides a carbon calculator, not for specific distances traveled, but rather a yearly summary of your car's CO2 output. And don’t forget efficiency—regular tune-ups, well-inflated tires and thoughtful driving strategies can increase your car's mileage significantly. 

David Carini plans to receive his master's degree in communication in 2010.