Imagine a world where the blood alcohol limit for driving is 0.05, instead of the national standard of 0.08. What would that be like? Who would be the most likely to suffer from this rule? Most likely family-oriented people; decent, hard working people who just want to enjoy a drink to calm down after a long day, and then be able to drive home with little (if any) impairment.
One of the major reasons the blood alcohol limit is set at 0.08, in the words of one official for Mothers Against Drunk Driving, is that a driver's impairment is "definitive" at that level. Under 0.08, and the impairment is no longer definitive -- and thus, the matter is subjective.
The reason this 0.05 BAC limit is an issue is due to an announcement made back in May by the National Transportation Safety Board, in which they suggested that a reduced BAC limit could save hundreds of lives every year. The suggestion is not a formal proposal, and it is unlike to become one in the near future. In fact, even MADD is refusing to support the idea.
Making the BAC limit 0.05 would only increase arrests and hit good, honest people with a criminal record. It probably would not do much to reduce the number of drunk driving car accidents, instead amplifying their visibility. It is a controversial idea, to say the least. There are already plenty of questionable drunk driving arrests, where people are pulled over without probable cause or the police violate their rights.
Source: ABA Journal, "Should drunken-driving limit be lowered to .05? Researcher uses simulator to gather data," Debra Cassens Weiss, Sept. 4, 2013