TL;DR version: It depends.
However, last year we also noted that a pair of more recent British studies cast serious doubt on whether such a move will actually save any lives at all by itself. Those studies found that Scotland lowering their BAC limit from 0.08% to 0.05% in 2014 did not appear to show any net reduction in traffic crashes or fatalities relative to England and Wales (who kept their BAC limit at 0.08%), and in fact Scotland showed a small but significant net increase. Whoops! The lack of any detectable lifesaving effect (and perhaps even a perverse effect) in Scotland was of course chalked up to weak enforcement and insufficient alternatives for transportation, and that may very well be true. But hey, at least they didn't show any significant negative economic impacts (e.g. on the hospitality industry) as some had predicted.
Contrast this with Utah's experience, being the only US state so far to lower their per se BAC limit from 0.08% to to 0.05%. When they lowered their limit in 2018, they saw a 19.8% drop in their fatal crash rate and an 18.3% drop in their fatality rate by 2019, which was significantly greater than for neighboring states or the nation as a whole. More people apparently chose to plan ahead when going out drinking. And yet like Scotland, they did not see any negative economic impacts either.
So why such a difference between Utah and Scotland? Well, to ask the question is to answer it: it's pretty clear which of the two has the bigger and more legendary drinking problem overall. And combine that with relatively weak enforcement and poor alternatives for transportation, and it really doesn't take a rocket scientist to see why.
And the difference is notably NOT due to Utahns under 21 either, as drivers under that age were already long covered by their strict zero tolerance law, which has no equivalent in the UK where not only is the drinking age 18, but also their BAC limit applies equally to all drivers regardless of age or license duration. So in Utah, the change effectively only applied to drivers over 21, while in Scotland, it applied to all drivers. Thus, Scotland should have seen a larger effect than Utah, but they did not.
We clearly need to see the forest for the trees. While 0.05-0.08% does indeed significantly increase the risk of crashing for most people, the vast majority of alcohol-impaired driving casualties are nonetheless from drivers with much, much higher BACs.
Recall, as Darren Grant noted in his famous study debunking the effectiveness of zero tolerance BAC laws for people under 21, it should really come as no surprise to an economist, who is used to thinking on the margin. While driving impairment indeed begins long before reaching 0.08% or even 0.05%, it rises exponentially with each drink. Thus, higher BACs are exponentially more dangerous than lower ones. And the decision to have one's next drink based on trying to stay within the legal limit, even if a lower limit had the same deterrent effect at that limit, would have exponentially less effect that the decision not to cross a higher threshold. (If anything, it's even MORE exponential for younger drivers, not less.) And apparently once when the limit goes down to below 0.05%, any additional lifesaving effect over and above lowering the limit to 0.08% or 0.05% simply gets lost in the statistical noise. That is especially true if the penalty is the same for crossing a lower threshold, since there is literally no marginal deterrence effect against each additional drink beyond that threshold.
(Grant also did a sort of sequel to that study in 2016, again drawing similar conclusions.)
We already know that the vast, vast majority of DUI casualties are concentrated among extremely high BAC drivers, usually 0.15%+ (average 0.16%) and often ones who do so repeatedly and frequently. These drunk drivers are sometimes called "hardcore" drunk drivers, and are the most resistant to changing their behavior. And most of those are in fact alcoholics to one degree or another. They would, of course, laugh at the idea of a BAC limit being lowered to 0.05%, or at the very least, wouldn't exactly agonize over it. This is all common knowledge, and not at all controversial, except of course among MADD and similar zealots.
Thus, if we as a society decide to set the BAC limit at 0.05%, or indeed any number below 0.08%, there should be steeply graduated penalties, with 0.05% (or 0.02% for young or novice drivers) being a mere traffic violation with a modest fine and short-term license suspension, and no criminal record, 0.08-0.10% being a misdemeanor with a steeper fine and longer license suspension or revocation, 0.10-0.15% being misdemeanor with an even steeper fine and even longer license revocation and mandatory jail time, and 0.15%+ being a felony with permanent or semi-permanent license revocation, very steep fines, vehicle forfeiture, and a stiff prison sentence. Repeat offenses of 0.08% or higher would carry the same penalties as a 0.15%+ BAC, as would driving above 0.08% with a child under 16 in the vehicle.
Denmark, one of the most liberal drinking cultures in the world, has the BAC limit at 0.05% in general, but zero if not driving safely. Penalties are at least largely graduated as well. And that is for all drivers.
We also know that swiftness and certainty of punishment, or the perception of such, is a far greater deterrent than severity. Thus, automatic administrative penalties, separate from any criminal penalties, are found to be more effective that any criminal laws on the books, most notably in the Canadian provinces of British Columbia and Alberta.
Of course, we would be remiss if we did not discuss the stunning success of special DUI/DWI courts. These are diversion programs that allow DUI offenders to plead guilty and complete coerced treatment and enforced abstinence from alcohol instead of jail or other penalties. And they work quite well, with far lower recidivism rates than for those who do not enter such programs. There are two types of DUI offenders these days: 1) relative normies who simply make dumb decisions and can be "scared straight", and 2) hardcore drunk drivers. And DUI courts actually get to the root of the problem for the latter.
But first, we need to get these ticking time bombs off of the roads before they kill or maim innocent people. Australian-style random breath testing (RBT) is one blunt way to do it, but that would not likely pass constitutional muster in the USA. And even then, the long-run effects of Australia's RBT were not very different than those of American-style sobriety checkpoints when studied in the 1980s. Tougher and sustained enforcement in general really seems to be the key. And that can of course be done more effectively and cost-effectively with roving and saturation patrols, that look for signs of actually impaired drivers on the roads. Especially in the many states that do not allow sobriety checkpoints to take place, they HAVE to rely on DUI patrols instead.
(Twenty-One Debunked recently came up with an ingenious idea to turn roving and saturation police patrols against DUI into a COPS-like reality TV show called "Operation Rovin' Eyes", complete with ride-alongs for community members as well.)
Seasoned drunk drivers know how to avoid the checkpoints with ease. But with roving and saturation patrols, they will soon learn (either the easy way, or the hard way) that you can run, but you can't hide. "Roving eyes...are watching YOU!" would be a good slogan to publicize the program. The "fish in a barrel" method, that is, parking a police car outside bars, clubs, or parties and catching would-be drunk drivers before they get on the road, would also be a great complement to such patrols as well.
And while we're at it, let's get all of the garden-variety reckless, negligent, and distracted drivers off the road as well. The same patrols will of course get them too.
It's time to stop tilting at proverbial windmills, and finish the job for good. So what are we waiting for?
