The actual U.S. Supreme Court decision (from the 1980s) is that the police can stop you for 3 or more over the limit -- the amount the justices thought was noticeable to the driver. The justices actually went out driving in Washington D.C., to decide the case, after a drug dealer claimed the police had stopped him for a trivial violation. What they effective said was, "When you are 3 mph or more over the speed limit, you know it."
Five over in Wisconsin sounds unusual. I know people say the same about where I work, but no one has ever been pulled over for anything close to that. Usually, people say, "The cop stopped me for 9 over!" They were actually going 22 over, and the officer reduced the speed to 9 over. It's called a "street plea," and is very common. After they get a ticket, the driver's defense mechanisms kick in, and they tell their friends they got popped for "going 9 over," to make it seem like the cop was just out to get anyone -- not that they were driving too fast.
It's easier to understand if you realize police really do think in black and white. You either committed robbery, or you didn't. You either shot somebody, or you didn't. You either went over the speed limit (which was 25), or you didn't. "By how much" really doesn't matter. Have you ever heard anyone in prison for armed robbery say, "But I just robbed her a little bit, I used a little gun, and only beat her up a little bit?"
And if the officer stops you for 22 over and reduces it to 9, then hears that you're taking it to court or complaining to his boss, what he thinks is this: "I cut that guy a major break, saved him $200 so his kid in the back seat could have Christmas, and stopped him from getting a suspended driver's license for reckless driving and losing his job, and now he's trying to take my job."
And yes, I have had someone (more than one person, actually), try to kill me over a speeding ticket.
Speeding is one of the few things everybody does almost all the time, and it's the only violation of the law they rationalize by suggesting it should be tolerated. The great race car driver Mario Andretti wrote a great piece in USA Today a few years ago to talk about why people speed -- all the social and economic reasons, vehicle engineering, etc.
On a lighter note, I cut a big break to a guy once because of his bumper sticker. He was flying -- 25 or 30 over. He was very polite, and his bumper sticker said, "Drive it like you stole it."