I think they make things more dangerous, and put lives in danger. More than once I've been following a car, at a distance, doing less than 30 km/hr, less than the posted speed, and come close to rear-ending them because they think that they have to cross a speed bump at 5 km/hr. I don't know the bump is there, so it's only that I've left a proper distance, and driving safely, that has stopped their being an accident. There are no speed bumps on my street, but are on many of the streets nearby. As a result, there is some traffic avoiding other streets to use mine; but I've not noticed any speed differences. I have noticed erratic driving on roads with speed bumps, as the cars drift to the edge of the road to avoid the highest part of the bump - surely that is dangerous to children on the sidewalk, that might step out suddenly onto the edge.
I have no idea how one would think that speed bumps make thinks safer.
Well, that's the joy of living in a nanny state.
Denmark and Holland have toyed with removal of signs and road markings from roads and discovered that accidents and fatalities actually declined. Imagine that? Let driver's judge for themselves and they will actually become MORE cautious!
I doubt 4-way stops and speed bumps 'calm' anyone. Frankly, I drive faster on speed bumps because they piss me off. And it has been documented that driver's will 'rub the curb' to avoid the worst of the speed bump. It works quite well, actually.
But all of this is just one more symptom of the slow, unerring degradation of the infrastructure in this city.
Recipe for disaster:
1) Build a city on a one mile grid.
2) Make those roads 4 lanes only.
3) Add parked cars to choke traffic to one lane in each direction.
4) Act surprised when motorists take side streets.
5) Choke said side streets with speed bumps, unnecessary 4-way stops.
6) Put useless, arbitrary cross-walks every 50 meters just to really mix it up.
7) Put streetcar ROWs on 4 lane roads to make everything slow more.
8) Tear down or strangle existing freeways.
Presto! Instant ghost town.