The UK's Ministry of Transport has been cranking out some fantastic commercials about transportation safety. Below is one about speed limits, which I found on Streetsblog. Warning: the video is graphic and disturbing. Whether or not you watch it, there is more text further down.
I've become increasingly opposed to over-reliance on posted speed limits. Many roads are designed for speeds in excess of the speed limit as a way to account for driver error; but the exact opposite should be done on city streets. Motorists should feel constrained by the road design, and should want to slow down naturally. This can sometimes come down to a matter of feet or even inches: one simple strategy is to narrow travel lanes. An oft-heard concern about this has to do with the widths of trucks. But several existing streets accommodate trucks just fine; Lexington Avenue here in New York is a great example. Near my office the lanes on Lex are 9–9.5 feet wide, compared to the standard 11 ft. that the NYC DOT engineers prefer.
Motorists respond to their environments, and just as they go faster than the posted speed limit when a road is designed inappropriately fast, they also travel under the posted speed limit when it's the posted speed limit that's inappropriately fast. I was just in Washington State's Methow Valley, a beautiful region littered with quaint towns that, even where sidewalks exist, there is a blurring of distinction between car space and people space. I noticed that the posted speed limit in Winthrop, WA, is 25 MPH. But the cars actually go slower – and it's a good thing, since the tipping point between injury and death is about 18 MPH.
City and town streets should be designed for everyone, with preference for people on foot. The term of art in planning is "complete street" (and "naked street" or "shared space" when streets are designed intentionally to lack signage and distinction between cars and pedestrians – of course, this is not happening in the United States). This means that streets' roles as public spaces should be enabled, and instead of pedestrians fearing for their lives, motorists should be much more cautious.
In his wonderful book, Mental Speed Bumps, David Engwicht points out that, in his experience, motorists do not get frustrated when they drive through lively town and city streets (note: this is not about highways or arterials). Instead, they are engaged in their surroundings. Engwicht proposes that, as a result, motorists' sense of time even changes: rather than feeling pressed to get from A to B, they are happy to go slowly if their environment is interesting.