Holy [expletive]! We think landmines have a better ratio.
For some reason, I didn’t want to accept Cracked.com as a primary source, so I tracked down the study. A little googling found that the study was authored by one Ronald Bowman and it is cited throughout the internet by speed bump opponents everywhere. The link, unfortunately, has gone bad (it was hosted at a defunct AOL site), but it’s in the Internet Archive here. I don’t know Bowman’s credentials, but the study exists and looks plausible.
Also widely cited on the internet is a master’s thesis from the University of Texas, Austin by Leslie Bunte, Jr., the assistant fire chief of Austin. That link has also gone stale, but it can be found on the Internet Archive here. It is much more detailed that the Bowman study, but arrives at a similar result (page 152). (Note: if you have trouble opening the file within your browser, try saving it to the desktop.)