I looked into these several years ago...when I became the "Traffic Committee Chairman" of my community association (OUCH!).
Working closely with the county traffic engineers, other committee members, and LOTS of opinions, my views are (in no specific order):
1. The unspoken goal of many traffic calming methods are to reduce the volume of traffic. That is, make it more desireable for travelers to take another route, and much of the speed reduction seen (usually insignificant) can be attributed to a small number of speeders taking different routes.
2. EVERY method of traffic calming will affect the "calm" driver more than the speeder.
3. EVERY method of traffic calming, save one, will also affect (slow down) emergency vehicles. The exception - POLICE.
OK, so what did my communitee do?
1. We "prohibited" any system of islands and diverters that pushed head-to-head traffic closer together.
2. We funded (and continue to fund) off duty police to show precense and use a radar gun to ID speeders and ticket same.
3. When additional measures were desired, we paid to have speed humps installed. The speed humps are stictly specified by the county, and are ~6 inches tall and 12 feet wide. (I don't know about the claim made by prepak above, but I suspect the benefit he alludes to is very dependent on the individual vehicle)