Radley Balko:

Consider the 2000 federal law that pressured states to lower their BAC standards to 0.08 from 0.10. At the time, the average BAC in alcohol-related fatal accidents was 0.17, and two-thirds of such accidents involved drivers with BACs of 0.14 or higher. In fact, drivers with BACs between 0.01 and 0.03 were involved in more fatal accidents than drivers with BACs between 0.08 and 0.10. In 1995 the National Highway Traffic Safety Administration studied traffic data in 30 safety categories from the first five states to adopt the new DWI standard. In 21 of the 30 categories, those states were either no different from or less safe than the rest of the country.

Once the 0.08 standard took effect nationwide in 2000, a curious thing happened: Alcohol-related traffic fatalities increased, following a 20-year decline. Critics of the 0.08 standard predicted this would happen. The problem is that most people with a BAC between 0.08 and 0.10 don't drive erratically enough to be noticed by police officers in patrol cars. So police began setting up roadblocks to catch them. But every cop manning a roadblock aimed at catching motorists violating the new law is a cop not on the highways looking for more seriously impaired motorists.