I've been collecting SF MUNI's GPS info (including speed, current position) for the past 2 months and comparing it to what should be happening (speed limit, route). This is a heatmap of where MUNI speeds most often.

Full source available at https://github.com/eskibars/Oddball

This is cool--could be interesting to show choke points as well (e.g. high traffic areas, which bus lines to avoid), and maybe some time context so you can see how the buses are on average at, say, 9AM.