i think that i'll steer clear of the larger constitutionality debate, and just weigh in on what constitutes 'impaired.'
as you all may know, i live in Vegas, and work in the Service Industry. it is required that all persons in Las Vegas who serve alcohol, be it in a gas station, liquor store, bar, restaurant, or casino, periodically take alcohol classes...
the thing that you learn in these classes is that there are a LOT more factors which determine a person's impairment than BAC. take your garden variety functioning alcoholic: every day he drinks a bottle of Jack. say there are 25 ounces in the bottle. NOW, it generally takes ~1 hour per ounce of 80 proof liquor to metabolize the alcohol. therefore, the man wakes up the next morning over the limit. then, he goes about his day, and hits the bottle again after work. due to the fact that it will take him 25 hours to metabolize the 25 ounces of Jack, by the time he hits the bottle the next day, he is STILL over the limit. the next day, he's even further over the limit. and so on. yet, he is functioning without any noticeable impairment, due to the fact that he is ALWAYS over the limit. this person is rarely the one who gets the DUI. the DUI's generally go to people who 1) don't generally drink often or in large quantities, and 2) don't have any idea what their limit is. i can state honestly that i know for a fact that i've driven when i'm legally over the limit- for the simple fact that i just drank three 20 OZ draft Fat Tires. but i'm not impaired- yet. partly because i drink a few beers after work pretty often, but more so due to the fact that my body hasn't had the chance to metabolize the alcohol yet. maybe i will be impaired in an hour or so, but not right now. now, when i sit at the bar for five hours, and have a few shots of Patron along with my Fat Tires, then i know that i need to either call a cab, or start walking. i have no problem leaving my car at the bar and picking it up in the morning...
but the amount and the regularity that one drinks are not the only factors. others include sex (women are more greatly affected by alcohol), body type, metabolic rate, and the percentage of body fat that one has. as well as food. protein stays in the stomach longer than carbs, thus, any alcohol that goes into the stomach with the protein will be absorbed at a much slower rate than if the person ate a bunch of bread. this is a common misunderstanding: Bread makes you get drunk FASTER. carbs don't stay in the stomach long. they just get dumped straight into the intestines, and the alcohol gets dumped in with the carbs... the intestine is where the largest portion of alcohol is metabolized.
but it's not just the food that determines the metabolic rate of alcohol absorbtion. mostly it has to do with the individual's matabolism: i know that i can sit with my brother, and we can each drink 6 Budweisers, yet he will ALWAYS be more banged up than me, because his metabolism is a lot faster than mine. so, i may end up drinking a couple more beers than him, but i'm the one that has to drive.
it's just the way that it is. impairment should be determined on a per person basis, and not by some number that the Gov't decides. shoot, from a strictly technical standpoint, i'm legally over after ONE Fat Tire, but i sure as shit ain't drunk... now after four or five, i'm definitely impaired... yet i know that i've passed the Field Sobriety Check when i was 'legally impaired.' because for me, over the limit is not the same as the nest guy...
the point is that it's all subjective...
and now for some interesting trivia: it was recently determined that at any given time of day, 26% of all drivers in Las Vegas are over the limit... on big Party Weekends such as New Years, or Labor Day, or St Patrick's Day, that percentage spikes to over 50%... just some food for thought...
and here's a fun little story: i was driving back from The Orleans last night around 4 AM, and a car drove past me on I-215 THE WRONG WAY. this guy was clocking 65 MPH, travelling westbound in the EASTBOUND fast lane. on a 6 lane, divided Interstate Highway... i got the fuck off the highway with a quickness and called 911... as soon as i said, "i'm eastbound on the 215," she said, "the guy going the wrong way, i know." so i hung up... i don't know what happened after that, but one thing is certain: THAT guy was impaired...