A friend of mine mentioned that he'd heard that in Austria, people who had some number of drunk driving convictions were allowed to drive only a particular kind of very small car. That struck me as an intriguing idea.
Naturally, to implement this, the legal system would have to decide what the threshold number should be, and also how long they should lose the license for altogether. At one point on the continuum, one might not want to constrain the driver at all. At the other extreme, one might insist on barring the person from driving altogether. Still, I take it that at some intermediate point one might conclude that
(1) someone is enough of a menace that their liberty to drive should be constrained at least for some time even once they're let out of jail, but
(2) it would be too much of an economic burden on them and their families to strip them of a driver's license altogether.
If that's so, why not limit the person to the kind of vehicle that would cause the least possible damage to others, if it is driven drunk? It's true that all motorized vehicles can kill people other than the driver, but some are more likely to do so than others. If the person is to be let out on the road, might as well minimize the risk to bystanders.
One possible mandatory vehicle is a motorcycle, though perhaps there the risk of injury to the driver in case of an accident (which can of course be a non-drunk-driving accident, or even one in which the driver is largely innocent). Another might just be a very small car. True, there's a tradeoff between risk to people in other cars and risk to the driver's passengers. But I take it that most adult passengers are more likely to have in some measure knowingly assumed the risk than are outsiders, since they know they're getting into a small and thus somewhat more dangerous car. I would have some concern for their welfare, but not as much as for the welfare of the drivers and passengers of other cars.
Finally, while this is obviously a restraint on liberty, it strikes me as a pretty justifiable one (assuming it works). Drunk driving is rightly made a crime; the legal system may well constrain a person's liberty for committing this crime, especially since he has shown himself to be something of a menace to others' lives. This solution may preserve more of the drunk driver's liberty than the alternative, since the alternative may well, at least in some situations, be a total ban on driving. One can also try this sometimes-useful (though of course hardly perfect) libertarian rule of thumb for deciding the proper rules for government-owned shared resources: If the resources were privately owned, would reasonable owners who had full information about their customers impose such a constraint on their customers in order to better protect other customers? The answer, I think, would be yes (again, assuming the proposal works), which would cut in favor of this proposal.
But that's just my quick and tentative reaction; I'd love to hear what others think. I'd also like to know something more than just a vague, unsourced rumor about whether any jurisdictions have indeed experimented with this.