Imagine a world with one million fewer cars on the road. We do. Every day we are working towards a place with less dependence on personally-owned vehicles. Why? Because it matters. Less cars on the road means less congestion, less pollution, less dependence on oil, and cleaner, fresher air to breathe. Some folks don’t believe it’s possible for car sharing to have such a huge impact.

http://www.zipcar.com/business/is-it/greenbenefits