Department of Unintended Consequences, a continuing series

A rigorous statistical examination has found that smoking bans increase
drunken-driving fatalities. One might expect that a ban on smoking in
bars would deter some people from showing up, thereby reducing the
number of people driving home drunk. But jurisdictions with smoking
bans often border jurisdictions without bans, and some bars may skirt
the ban, so that smokers can bypass the ban with extra driving. There
is also a large overlap between the smoker and alcoholic populations,
which would exacerbate the danger from extra driving. The authors
estimate that smoking bans increase fatal drunken-driving accidents by
about 13 percent, or about 2.5 such accidents per year for a typical
county.

That’s coming out in the Journal of Public Economics, so it might even be true.  Here is the short source article, which surveys other interesting results as well; worth a read.