Once the 0.08 standard took effect nationwide in 2000, 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 sobriety checkpoint aimed at catching motorists violating the new law is a cop not on the highways looking for more seriously impaired motorists. By 2004 alcohol-related fatalities went down again, but only because the decrease [Ed: in the number of fatalities] in states that don’t use roadblocks compensated for a slight but continuing increase in the states that use them.
This is also, by the way, the justification for legalizing drugs. We would still punish the bad behavior that drug-taking can cause, but leave the drug-taking (and, of course, possession), itself, alone.