[Zak]

Maybe there's better research available, but this DOT report found that changing speed limits has fairly little effect on how fast people drive: http://www.ibiblio.org/rdu/sl-irrel.html

My take on it is that when the speed limit doesn't match the 85th percentile speed of actual drivers, something should be changed. Sometimes, (often, in my view) it's the speed limit. Sometimes, it's the design of the road, especially in cases of neighborhood streets that are shaped like highways. Sometimes the answer may be highly-visible traffic cameras, as the advance warning and certainty of punishment will slow down almost every driver.