Interesting idea, and one I've seen before, but there's little incentive to contribute to this system. Reminds me of gasbuddy. In order for this to be crowdsourced, there would have to be an incentive to input quality data (see http://www.factual.com/ a local data API that gives you credits for uploading quality data).

A better system might be to scrape foursquare, twitter, facebook checkins, and just measure the ratios. It would be more automatic (no behavior change required), but would also be skewed (say, for example, women tweet more than men do).