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The guideline I was taught was <10% over on surface streets and <14mph over on controlled access (interstates, etc).

FWIW, I've never been pulled when I was following those guidelines (driving in GA + SC).

One of the rules of thumb taught to cops is "over by five, let 'em drive, over by 10, get out your pen"

The variation I always heard (as a driver, not a police officer) was, "Five, you're fine, ten, you're mine!"

Where I grew up (eastern WA): "Eight is great; nine, you're mine."