That line isn't defined by the law--that's my point. If you drive with a blood alcohol level over the legal limit, even if you don't cause an accident and don't do anything erratic that gets a cop's attention, you are still breaking the law, just as if you drive at 56 mph in a 55 mph zone. But everybody knows that the actual law is not enforced as it is written, because it can't be. Instead we rely on vague limits imposed by the resources available to the police and their individual judgment about what counts as behavior that is abnormal enough to get their attention. And we also have to put up with the fact that the cops can use the letter of the law as a revenue source if they want to; they can always post some extra cops with radar guns in places where they know people routinely speed, and harvest a bunch of tickets any time they feel like it.

Personally, I would prefer not having laws that can't be enforced as written in the first place. Everyone knows it's impossible to catch everyone who exceeds the speed limit on a highway--so the speed limit should not be a law in the first place. It should be advisory; the actual law should be something along the lines of "the posted speed limit is the government's best judgment of a maximum safe speed; just exceeding that speed, by itself, does not violate any law, but if you cause an accident and are found to be exceeding the posted speed limit, you are presumed to be at fault and face greater legal liability". A similar provision would apply to drunk driving: just driving with BAL over the limit would not break any law, but if you cause an accident, you are presumed to be at fault and face greater legal liability. That way the actual law could be enforced as written, and the vast majority of people who break the letter of the law as it is written now but never cause any actual harm wouldn't have to worry about playing cat and mouse games with cops.