I've written about that in the past, such as this entry, "Pedestrian Safety and the proposed DC 15mph speed limit
Often, people take context cues--width of roads, amount of traffic, placement of buildings vis-a-vis the roads, condition of the pavement--and adjust their speed--usually higher--accordingly.
Another problem is that suburban residents, where speeds typically are higher, often aren't familiar with places, like center cities, where posted speed limits are lower across the city. For example in DC, unless otherwise posted, the speed limit is 25mph. In many other cities it's higher, 30mph, even in residential areas.
In Europe there has been a campaign for a couple decades to reduce speed limits in cities to 30kph and 50kph, about 20mph and 30mph respectively. This started in Graz, Austria, and the reduction in speed has been effective in reducing accidents generally and injuries from accidents specifically.
Some boroughs in Montreal are adopting similar policies.
So fine, speed cameras are great.
But they are still very controversial because people resent not being able to break the law without penalty, even though they claim their opposition has to do with concerns about over intrusive government, and how it can be a form of privatization of law enforcement, because typically such cameras are installed by a private sector firm (local governments often lack the money to do it) which reaps a significant portion of the revenue stream. (Actually I agree that this is a problem. There should be an infrastructure bank which makes loans to local governments for projects like these, not unlike the Bank of North Dakota
, which is a state bank.)
I could be seen as hypocritical about stop signs, because as a fervent proponent of the Idaho Stop for bicyclists
, I treat stop signs (and red lights) as yields, provided that there is no oncoming traffic