Totally depends on the street. All drivers drive at the speed they feel safe at for the given road conditions, right up to the point where concern about a ticket supersedes. Cops who are just being cops get that. They know that everybody drives 75 on this section of highway, so they're only busting the people at 80. Cops who are told to go out and write tickets today also get that, so they find chunks of roads where the speed limit is lower than the average accepted speed. That's always been my problem with speed enforcement. When it's just intended to be a cash grab, it's never done in an area that would actually benefit from it. They're set up in places where people are naturally inclined to exceed the speed limit, which is almost always in a place that's quite safe to do so.
In fact, of all of the speeding tickets I've gotten (which is a startlingly high number), I can only think of one that was a genuine public safety issue. The only ticket I've ever plead guilty to.
BTW, I got a ticket for 40 in a 30 a few years back. Hardly a residential thing. Three lanes each way, super-wide median, no intersections for nearly a 1 mile stretch. The natural inclination for any driver is going to be about 45 or so. Excellent place to write 30 or so tickets over breakfast.