Mapping the actual frequency/volume of roadway usage via geotagged Tweets. A cool idea, and a great way to figure out which routes really are popular, and where one might consider expanding mass transit routes. #ddtb
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Infographic Of The Day: Could Twitter Help Us Create Smarter Transit Routes?
Traditional city maps visualize just one aspect of urban design–the city's intended structure, full stop.

So, Tweeting while driving? hmmmm…
Apparently so. I'm sure some are on busses, etc., but I assume most are driving. So maybe a Twitter trail of what roads one doesn't want to be driving on, too.