In my jurisdiction, in my day, we permitted ten over except in school zones and residential areas. Of course your speedometer should be calibrated weekly. Kidding! But, factory speedos were notorious for being incorrect, often the higher the speed and more off the needle. That could be compounded when tire sizes were changed from the originals. (Kids in their altered vehicles rarely took that into consideration.) My advice was to stay with the traffic flow.
I can't speak to today's electronics in vehicles nor, do I have the need to. Laser speed measuring units weren't possible, we used radar, calibrated weekly at the shop and two or three times during a shift with a tuning fork. Enforcement tools have moved way past my day and age. I only got to use a radar gun in my last couple of years as a street cop. The old "Decatur" with one direction, hang out the window, dpme was my trusted sidekick unless I was "pacing" another vehicle. I allowed a lot more leeway when pacing. I allowed ten and fifteen over when pacing and wrote the cit for five less than I had paced.
In a nearby town, there is a main street that serves as an artery between a state highway and several other communities. There are four stoplights in the town. One stoplight is at an intersection that, because of a diagonal crossing street, has a rather long span from crosswalk to crosswalk. The light at this intersection was set so that the yellow light was abnormally short. I forget the exact figures, but I measured, timed, and calculated that there was a zone of about fifty feet where it was possible neither to stop safely before entering the intersection, nor to cross the intersection at the speed limit before the light turned red. If the light turned yellow and you were in this zone and the constable was in his stance, you were screwed.
Yes, I got a ticket, driving with zero intent to break the law, which is why I went out and measured and timed, and found that the yellow light was far shorter than it should be according to various law enforcement guidelines. The judge seemed completely uninterested in my research, but at least I got the points thrown out, just as I would have simply by showing up. Clearly it was a revenue enhancement system, and I was merely a resource.
Bad laws (or bad enforcement) makes bad citizens. Good laws (and good enforcement) put people of good will on the same side as the law. Arbitrary and punitive laws put them in conflict. Bans on tobacco importation, for instance. Yesterday it was legal, today it is illegal. With the stroke of a pen we have created criminals where none existed before. And achieved what?
