I'll chime in.
I did DUI enforcement as a primary responsibility for almost 9 years. I am (was) a drug recognition expert as well as an instructor. I went all over the country teaching DUI/drug impairment to other LEOs. I investigated more than a hundred fatal accidents; many hundred of more serious accidents. I am also a certified reconstructionist for both commercial and passenger vehicles.
When I first started we had one level, .10 BAC. Later they lowered it to .08 and added extreme DUI for those who were over .16% (I think). I have arrested people for DUI for being as low as .06% and as high as .37%. I have seen some .400+ but they weren't driving. I met a nurse at Phoenix Indian hospital who had seen several over .70%. That is mind boggling.
I also put on many "alcohol workshops" I would enlist volunteers to come drink and I supplied the booze. I fed them and their alcohol intake was carefully monitored by scientists. They were tested frequently to see what their BAC was. We would then let the cops in training run them through a battery of field sobriety tests. They had to determine whether they person was above or below a .100 BAC.
I got to see what people behaved like as they progressed.
The first things that happen are inhibitions are removed and judgement is altered. You can't test this with a straight line on the sidewalk. It is impossible to replicate the complicated task of driving a motor vehicle- especially in the mix with other traffic.
I investigated a LOT of accidents where the at-fault driver was a .06% or .07%. Was he legally drunk? No but most state laws describe "impaired to the slightest degree".
A guy can smoke a big fatty joint and nail the walk and turn test yet ask him to estimate 30 seconds and he will likely go beyond 2 minutes or maybe only 5 seconds. That's what weed does to you. This affects your judgement in perceiving threats and situations. You don't look impaired until you are asked to do a task that requires judgement.
Another thing that most people are getting wrong here is that 2 beers at dinner will not put anyone except maybe the most petite person to a .05%.
As some has stated, alcohol tolerance has some affect on you "feel". Your judgement is likely still impacted regardless your drinking history. There are many things that can impair your ability to drive, alcohol is just one of them and it impairs everyone who drinks it to excess. Having sex while driving will also impair your ability to drive. We don't have anything on the books about it though. Texting is gaining some traction and I see a few signs around the country where it is illegal.
I am not in favor of the .05 for the most part in that it is extremely difficult to detect that level of intoxication in the field with rudimentary testing. I am not against charging someone for DUI should they have an incident that is their fault and a chemical test shows a .05 BAC.
Pilots and truck drivers are already at a .04% BAC threshold and have been for a long time. We don't bitch about that!
