Zipcar, the popular car sharing service, and others like it are often lauded for their environmental benefits. After all, if you can borrow a car when you need one, then maybe you won’t buy one. And if fewer people buy cars, that’s good for the environment, right?

It might be, but it depends. It depends on how car sharing affects driving behavior. Some studies have shown that just one quarter of the greenhouse gas emissions associated with passenger vehicles are produced in the manufacturing process. Most of the emissions occur in the process of driving it. Even if Zipcar reduces vehicle ownership a bit, it might actually increase the amount of driving people do. After all, if you can borrow a car whenever you need to, you might drive more. If this is the case, the environmental impact of Zipcar could easily be negative.

I’ve not seen any really…