Well actually I proposed a scenario completely different than the guy who hijacked my thread. He didn't even bother to read what had already been said so if he feels stupid/ignorant maybe the 'shoe fits'.
Adding a sail is an interesting idea but there are already teams of researchers competing for land speed records with wind-powered vehicles. Of course it's possible but like you said, really impractical for city driving. I can imagine a light turning green and waiting for a gust of wind to start a slow crawl across the intersection.
I've also seen a car in Australia, I believe, that had a retractable kite to help harness wind power - also not practical for mass transit. There is also another neat concept using kites where a very large kite is allowed to un-spool a large amount of wire, spinning a flywheel to generate power. I believe the kite is then retracted when the wind is not as intense.