In which country or state or province or territory?
Here in Australia, the blood alcohol limit is 0.05% for fully licensed drivers in all states and territories, zero for probationary drivers and drivers of commercial vehicles (with some other state-by-state variations). And, according to this Australian website:
At 0.05 BAC drivers are twice as likely to have a crash as before they started drinking.
NHTSA is the general authority for the topic in the US - that is to say, if you go to a DWI court, you're going to hear a lot about NHTSA standards. A quick trawl through their research and history can give you some background. That said, they recently recommended lowering the BAC limit to .05.
For the US, here is a document from the NHTSA explaining why they pushed for this limit (holding highway funds from states until it was set at said level): http://www.nhtsa.gov/people/injury/research/pub/alcohol-laws/08History/1_introduction.htm
Here is the list of reasons compiled from the organizations that desired the .08 limit (copy and pasted from article):
(1) Virtually all drivers are substantially impaired at .08 BAC. At .08 BAC, there are significant decrements in performance of critical driving tasks such as braking, steering, lane changing, judgement and divided attention.5
(2) The risk of being involved in a crash increases substantially by .08 BAC. Compared to drivers with no alcohol in their blood system, the risk of being in a crash gradually increases at each BAC level, but rises very rapidly after a driver reaches or exceeds .08 BAC.6
(3) Lowering the per se limit is a proven effective countermeasure that will reduce alcohol-related traffic fatalities. NHTSA cites a number of studies to substantiate this claim. For example, a study in California found a 12% reduction in alcohol-related fatalities in 1990, the year .08 per se and administrative license revocation laws went into effect.7 A study by Boston University compared five states that lowered their illegal limits from .10 to .08 with five states that did not do so. Researchers found a 16% reduction in the proportion of fatal crashes involving fatally injured drivers whose BACs were .08 or higher in the five .08 states. That same study showed an 18% reduction in the proportion of fatal crashes involving fatally injured drivers at very high BAC levels (.15 or higher) in those .08 states.8 In a 1995 NHTSA study, four states that adopted a .08 BAC limit experienced significant decreases on several measures of alcohol-related fatalities. Decreases in alcohol-related fatalities ranged from 4% to 40% in those states that were studied.9 More recent studies completed in 1999 and 2000 have continued to quantify the effectiveness of .08 per se laws. A study by a team of researchers from Boston University's School of Public Health concluded that states that lowered their illegal levels from .10 to .08 BAC in 1993 and 1994 experienced post-law reductions in alcohol-related fatal crashes.10 A NHTSA-sponsored 1999 study of 50 states and DC concluded that states which enacted .08 BAC laws experienced an 8% reduction in the involvement of drivers with both high and low BAC levels, when compared with the involvement of sober drivers.11 Another report, which studied the effectiveness of the .08 per se law in Illinois, concluded that the number of drivers in fatal crashes with positive BACs in Illinois decreased by 13.7% after implementation of the law.12
(4) A BAC of .08 is a reasonable level at which to set the illegal limit. A .08 BAC is not typically reached with a couple of beers after work, or a glass or two of wine with dinner. The average 170 pound male would have to consume more than four 12 oz. cans of beer within 1 hour on an empty stomach to reach .08 BAC. The average 137 pound female would need at least three cans of beer in one hour on an empty stomach to reach that level.
(5) The public supports levels below .10 BAC. NHTSA surveys show that most people report they would not drive after consuming up to 2 drinks in two hours, and a majority of those who are aware of BAC levels support an illegal limit of .08 or lower for their state.13
(6) Most other industrialized nations have set BAC limits at .08 or lower and have had these laws in place for many years. Austria, Switzerland, Canada and the United Kingdom have set limits at .08. All of the states in Australia have a .05 limit. France and Germany also have a limit of .05, while Sweden's illegal limit is .02 BAC.
EDIT: formatting