In the US on major roads, "everyone" drives 10-15mph over most of the time. Some roads that are physically identical to interstate highways marked 55-70, are actually marked 35-45, so people are generally speeding by quite a bit.
Actually driving the speed limit would be quite dangerous - the speed differential between lanes, coupled with the impetus for those behind you to do a lot of lane-changing, is not pleasant.
If we genuinely believed the speed limits mattered, we could enforce them uniformly against everyone using ANPR cameras and timestamps. Instead, squad cars pick a few random unlucky souls (out of the hundreds of people going roughly the same speed in their vicinity) to be slapped with a fine.
They are more likely to do this at the end of the month, when ticket quotas are due. In some random jurisdictions you may pass through on a road trip, they are more likely to ticket travelers passing through, who aren't voting on the next sheriff. And yes, in some cases they are more likely to pick you if you're black (particularly if also driving a nice car).
I'm not objecting to ticketing of drivers who are going substantially faster than traffic, weaving through lanes, etc. I am objecting to the fact that the normal daily driving behavior required to not cause an accident (i.e. go with the flow, 10-15mph over the speed limit) is criminal and enforcement is "random" with some opportunity for bias against people the local cops don't like.