The National Transportation Safety Board recently recommended that the threshold for drunk driving be lowered from a BAC of 0.08 to 0.05. In 1980 it was 0.15.
My initial hunch is that we are past the point of picking the low hanging fruit and that further lowering the BAC from 0.08 to 0.05 will have costs that outweigh the benefits.
The article claims that a the lower limit will save 500 - 800 lives per year (a 37.5% decrease in the BAC will save .08% more lives). Assuming 800 lives saved at $6 million/life (value used by the transportation department as of 2001) the saved lives benefit would be $4.8 billion. If we double that to allow for property savings, jail savings, injury savings, etc. we get a total benefit of $9.6 billion. I don't know how accurate that is but it's a number to work with.
So what are the extra costs? Well first I bet it is very hard to notice if someone is at .04 or .05 just based on their driving so I assume the use of checkpoints by police would increase in order to adequately enforce the new standard. I personally think that these checkpoints are already a complete violation of privacy and are often used by police officers to harass citizens, but that aside, they certainly are not free and police officers at DUI checkpoints are police officers that are not doing other things like patrolling neighborhoods. The extra cost of enforcement for already cash strapped precincts and the opportunity cost of having additional and more frequent checkpoints needs to be taken into account.
The article also mentions using passive alcohol sensors "which police can use to "sniff" the air during a traffic stop to determine the presence of alcohol. The sensor is capable of detecting alcohol even in cases where the driver has attempted to disguise his breathe with gum or mints. If the sensor alerts, it is grounds for more thorough testing." I am sure these are not cheap and what are the additional costs of testing, time, etc. if these sensors go off because of passengers who have been drinking rather than drivers? This scenario will happen more often with a threshold of 0.05 rather than 0.08.
I think that the lower standard will lead to more false positives in the short run. Police officers who see people driving between 12 - 2AM will know that if they pull someone over all they need to do is catch them with a 0.05 BAC. That is a pretty low standard and can come from as little as 2 drinks in an hour. If you were a police officer and you knew you could generate $10K (the sum of most current fines) in revenue for catching a drunk driver why not take a chance pulling a driver over? There is a relatively high probability that someone out that late has been drinking and the probability that they are over the limit is also likely higher under the new BAC.
One could also assume that people would drastically change their behavior and would cut back alcohol consumption if forced to drive that night, lowering arrests and false positives in the long run. What affect will this have on the restaurant/bar industry? If people who have to drive quit drinking altogether because they are afraid of being caught and paying a $10K fine, having their license revoked, etc. alcohol sales will fall drastically at many places. This could have a large effect on workers in that industry in the form of lower tip revenue and loss of jobs.
Revenue from drunk driving arrests would also fall in the case of reduced drinking. Governments may claim that this is what they want but many of them rely on revenue from arrests and without it they will suffer. Government revenue is not a true benefit from an economic perspective (it is just a transfer) but it is still something for the governments themselves to think about. The loss of revenue from DUI's may lead to higher fines in other places, such as speeding tickets, running red lights, etc.
With the BAC already so low compared to 1980 it is hard for me to believe that the benefits from lowering it further will outweigh the costs of additional enforcement and individual privacy as well as the adverse effects it will have on the restaurant/bar industry. If I have time in the future I will try to get some real numbers and do a more detailed analysis.