Dave Winer at Scripting News asks about: Speedbumps and a city's carbon footprint? in particular Berkeley.
As far as I can tell, there are several offsetting factors:
(1) Speedbumps discourage travel by car (less carbon)
(2) Speedbumps encourage remaining travelers to reroute on longer routes (more carbon)
(3) Speedbumps increase fuel use for drivers who stay on the "calmed" route (accel/decel) (more carbon)
(4) Speedbumps lower speed for drivers on calmed route (less carbon)
How this nets out is empirical, depends on the configuration of the network and the extent of traffic calming.
Any studies on this? (Nothing obvious shows up with actual data in a quick scan of Google Scholar)