I don't think it's such a black and white issue. There needs to be specific algorithms for determining speed limits. In America, speed limits vary WILDLY from county to county for equivalent roads. I'll give an anecdotal story...
Where I live, there was a road that had a speed limit of 35mph. All the state guidelines said the speed limit should be faster. It was 35mph because the mayor lived on that road and he blocked it every time anyone tried to raise it. When he left office, all of a sudden it was 40mph. So all the people that got tickets for going 40mph on that road were in the wrong 6 months ago, but are not wrong now? Nothing has changed except a sign. The road is no more or less safe.
In the Gainesville, FL area the surrounding towns know there are a lot of people passing through that don't know the local speed limits (that's where the University of Florida is). That area of Florida is very poor and for some small towns, speeding tickets are a significant source of revenue. They will do things like post a 35mph limit, then soon after a jump to 50mph within a few hundred feet. When people see the 50mph sign they begin to speed up even though 50mph doesn't technically begin until the point of the sign. They hide behind billboards trying to see how fast they can clock you before you've technically reached the new speed limit.
While in most places it's illegal to have an official ticket quota, I know for a fact all tactics just shy of a quota are used. I've heard of whiteboards with all officers tickets for the month on display for all to see. Enforcement programs get very aggressive during times of budget shortfall. There needs to be a disconnect between funds raised by the legal system and where they go. If the money stopped going to police departments and say directly to the federal government, the police departments would lose motivation to try and cheat people out of money.