Hmm. So you're saying that if I'm just "speeding," then it's not a danger to others. But if I'm speeding a lot, then it's a danger to others such that I can be arrested or ticketed? Huh. That seems pretty similar to the concept behind DWI laws.
Sure -- a vehicle that travels at 85 miles per hour is more "dangerous" by any objective measure than one traveling at 65. But a vehicle traveling at 65 is more dangerous than one traveling at 45, 35, etc. For that matter, a vehicle traveling at 5 miles per hour represents an infinitely greater threat to public safety than one that is stationary. So let's just outlaw motor vehicles altogether, eh?
I never said that, but because the cost of determining risk in every single instance is far too high, we line draw. For the most part, our legislatures have chosen to draw the line at 65 or 70. If they choose to draw the line at 55 or 45 or even, as you suggest, 5, that is the decision of the legislature. But, in the same way that we choose to draw lines in terms of speed limits, we also choose to draw lines in terms of operating a vehicle while intoxicated--.07 sends you on your way home. .08 puts you in the pokey.
And by the way, since you want to nit pick on the violations, try this one: policeman sitting in a bar/grill having dinner. Over the course of his meal, which takes an hour, he observes a patron consume several mixed drinks; say, half a dozen. Policeman leaves and sees patron walk to his car, get in, and drive off. Fourth Amendment violation to stop the driver?
Under my standards, YES. Under yours (and the ones in place right now), NO.
Keep in mind that the primary objection to laws against drunk driving is that they are completely arbitrary and subjective -- not in even in the way that a speed limit is arbitrary and subjective, but because the level of blood-alcohol content at which a person becomes impaired will vary widely. One person at 0.04% may be far more impaired than another at 0.12%, and yet the latter person is considered "legally drunk" but the former is not.
This was really borne out to me in my driver's ed class in high school, when they showed a video that illustrated the effect of alcohol on a driver's motor skills, judgment, etc. They had four drivers drive through a course laid out in a big parking area, marked off by traffic cones. Each driver was given a score that was based on how many cones they knocked over on the course. First they drove through it completely sober, then after two beers, after four beers, etc.
The video told a compelling story about the impact of alcohol on a driver's abilities. By the time they drove through the course for the last time, every driver was knocking over cones all over the place. But one of the drivers actually got a better score after two beers than he did when he was completely sober.