The general expectation on American freeways is that speeding 10-15 mph above the posted speed limit will not lead to a ticket. Not only that, speeding 10-15mph above the speed limit is often the social norm, and other drivers will get mad at you if, for example, you are going at the speed limit on a 1-lane road or in the left-most lane of a multi-lane road.
For a driver new to the country or the state, this is quite confusing –– I got a ticket for going 10 over the speed limit in Idaho. I was following the Washington state norms, where I lived back then, but the norms are different in Idaho where the posted speed limits are higher. Selective enforcement of the law is also a problem — you’re breaking the law as written when you go 61mph on a road with a 60mph posted limit, and a policeman with a vendetta could ticket you for it. Even for a relatively seasoned driver like me now in California, there’s always a nagging doubt at the back of my head, “Is today the day I get ticketed for going 72 mph in a 65 zone?”
The worst thing about the norms around speeding laws, though, is that it makes everyone who drives into a law-breaker, and creates the expectation that it is okay to break some laws. This is the moral corruption of man, and of society. A society that tolerates, even encourages, one law to be broken invites the same for other laws. After all, what is the moral difference between going 5mph above the speed limit and jumping a BART fare gate? Or shoplifting from Walgreens?
If the safe and expected speed for a certain road is 10mph above the posted speed limit, just change the posted speed limit to be 10mph higher. Is there a good reason not to?