It's not the one limit, it's the total package, that counts

We’ve pointed this out before, that it’s not the drink drive limit itself which determines how many people drink drive and thus the accident rate. Human incentives don’t work that way. This is thus not surprising:

Cutting the UK drink drive limit would not reduce the number of accidents on our roads, a new study suggests.

Researchers from the University of Glasgow have found that the lowering of the legal blood alcohol limit for drivers in Scotland in 2014 has had no impact on the number of road traffic accidents in the country.

The research, published in The Lancet, evaluated the impact of the change in Scottish law in December 2014, when the blood alcohol concentration limit for drivers was reduced from 80 mg/dL to 50 mg/dL.

Broadly, the weekly rates have stayed relatively stable, with between 5 and 9 road traffic accidents per 1000 traffic count in Scotland between Jan 1st 2013, and January 31st 2016.

In England and Wales, the number of accidents was proportionately similar over the same period, according to the study in which the University of East Anglia, the NHS and the University of Stirling also collaborated.

What matters, as people like Gary Becker have repeatedly pointed out, is the total package here. Yes, the drink drive limit matters. But so does the punishment for breaching it, so also the likelihood of being caught if over that limit. The expected cost of a crime is, obviously enough, the punishment for being caught times the probability of being caught. A 20 years sentence for burglary combined with a 0.1% chance of being convicted might deter less than 7 days in the cells and a 99% chance of capture. No, we’ve not checked the maths there.

The British system did have - England still does - a limit which is high by international standards. We also have a high probability of being caught. And we have, by those international comparisons, very heavy punishment for the crime. Yes, automatic 12 month licence suspension is, while perhaps entirely righteous, high by the standards of other places.

The end result of all of this is that Britain has a low incidence, again international standards, of the crime itself, drink driving. Unsurprisingly, a low accident caused by booze rate as well.

In fact, that former British system of heavy punishment, high likelihood of being caught, produces what we desire, low incidence of the crime and its corollaries, those accidents. Moving the limit doesn’t seem to change that at all. So, perhaps not moving the limit, given that the system already works, is the right course of action?