it's simple. asking people to do the right thing (i.e slowdown), out of the kindness of their hearts, is laughable.
if you build the street in such a way that driving above it's design limits is impossible, then people wont do it. surprisingly, the threat of their car being damaged or totaled will force compliance with the intended speed limit. if this was done correctly on a large scale, speed limit posts wouldn't even be required - the street layout would naturally dictate the speeds you can drive at.
for UK streets, this can be retrofitted with chicanes, curb extensions, raised pedestrian crossings, etc. increasing the amount that a driver has to think to drive down a street, automatically makes them slower. oh, and none of those stupid painted chicanes and bumps either, you think anyone cares about those? lol. actually build the damned curb extensions