Can you give up your car for a month? Apparently many people can — though they require an incentive to actually do so. Every year, Zipcar runs a Low-Car Diet challenge, recruiting city dwellers willing to walk, bike, ride transit, and, of course, drive a Zipcar for the year. And more people signed up than Zipcar accepted!

What enticed so many challenge takers to make a month-long eco-living pledge? Perhaps the challenge itself attracted some signers-on, but challenge-takers also received free year-long Zipcar membership worth $50, which likely got more people to sign up.

I suppose $50 is nothing to sneeze at when our economy still isn’t all that healthy, but I was surprised that the free membership promise attracted the challengers that it did. Why? Well, as you know, car ownership in general costs a lot more than $50 a month. And some of the Low-Car Diet challengers were people willing to pay through the nose to own and keep cars in cities where car ownership really, really costs a lot — like New York City! How is it that a mere $50 freebie is so attractive to people who, month after month, shell out hundreds of dollars just to keep a not-very-frequently-driven car on a tiny island?

I guess the bigger question is why some New Yorkers, with their fantastic public transportation system, decide to keep cars around in the first place. Low-Car Dieters were also recruited in other cities with Zipcars. Chicago, Pittsburgh, Washington D.C., and Atlanta were also among the nine cities participating in the Low-Car Diet. The Atlanta Journal-Constitution profiled some of the dieters in its city, pointing out that the real challenge begins after the month-long test “when they’re confronted with the choice to return to their cars or continue a friendlier environmental way of life.”

Curious how the dieters did? Zipcar’s Low-Car Diet website has the car-lite details. And I have a challenge for Zipcar: How about including Los Angeles in the next Low-Car Diet challenge? I’ll be the first to volunteer to sign up!