One also has to look at the context of the design. If you design a residential street to look like a two-lane highway with pavement only wide enough for the two lanes, ditches, no room to park cars, the painted lines, etc, people will drive faster on it than if it was designed to look like a city street (a little bit wider but with curbs, storm sewers, sidewalks, terrace trees and a few parked cars mixed in).
In the former case, my mind subconciously says 'I'm out in the country, I can drive faster' whereas in the latter case, it subconciously says 'I'm in the city and must drive slower'.
One has to be carefull with 'traffic calming' things, too. They are usefull in purely residential side-street areas, but could cause grief, encourage 'road rage' and increase pollution on major streets.
Also, according to those whom I talk with who know about these things, simply posting slower speed limits has little effect on driving speed, only causing drivers to question the wisdom of other speed limits that were set for sound engineering reasons and breeding a more dangerous disrespect for other more 'wholesome' laws in general. Police enforcement will slow things down a bit, but within a short while of the cop leaving, things will be right back to what they were before he/she got there.
It is 100% context of design.
Mike