Ultimately, this is supposed to be about reducing the number of people hurt or killed in drunk driving accidents, and I think ultimately it's not going to do that.
Less than 6% of alcohol-related fatalities involved a driver with a BAC lower than 0.08, and almost none with BAC of 0.05 or below.
As for other countries, lowering the BAC limit has had mixed (read: statistically insignificant) success. I'll see if I can find some data on that, though.
Bar owners and alcohol manufacturers are going to be against it, so I'll be a little surprised if it actually goes through, but who knows? 0.05 is actually a pretty common number.
I'm personally concerned about my own liability as the owner of a beer tasting room. I know for myself (quick alcohol metabolism, very high body weight), although I would never get behind the wheel if I had been drinking, or ever suggest anybody else doing it, I can barely tell when I'm at 0.05 and, as Yooper mentioned, could probably pass a field sobriety test at 0.08.
Here are the descriptions of impairment levels at 0.05 and below:
BAC .02
Drinkers begin to feel moderate effects.
BAC .04
Most people begin to feel relaxed, mildly euphoric, sociable, and talkative.
BAC .05
Judgment, attention, and control are somewhat impaired. Ability to drive safely begins to be limited. Sensory-motor and finer performance are impaired. People are less able to make rational decisions about their capabilities (for example, about driving.)
If I can't judge 0.05% in myself, where I know my personal variables, how in the world am I supposed to judge 0.05% in the customers I'm serving? There's very little apart from making your customers blow a breathalyzer that can make that determination, and I can't see that being good for business.
That's a big risk for me, because if I "overserve" someone who has 2 glasses of beer in an hour and then runs into a bus, I could be held liable. People at 0.08 are usually visibly impaired, but 0.05 is a much thinner line to walk as a server.