Productivity Shock posted a great abstract on this topic. I would guess that any number of the following might be true and also contribute to the phenomenon in question, but might not be detectable by the existing data.
1. Bumming out cigarettes is more likely when you have to go outside to smoke because smokers are put in a more socialized setting — smoker solidarity. Smokers have to stock up mid-way through the night, thus driving is increased.
2. Most cities don’t let you take your drink outside when you go outside for a smoke. This resulted in a date rape drug problem in New York when women left their drinks unattended. Tobacco companies also pushed smokeless tobacco campaigns because it was so inconvenient to go outside of large night clubs. In addition to the above my guess is that smokers are more likely to order shots and hard spirits rather than beer with smoking bans because they can drink their drink go out and smoke, come back and repeat. My guess is that their on average more drunk than they otherwise would be.
3. The costs to leaving a bar in search of another (better) bar are lower with smoking bans. You’ve stepped outside, you don’t have a drink in your hand. The costs of hopping in your car and checking out another bar to inform your social group about once you’re there are lower than if you’d stayed inside. In other words, it’s not just traveling to one bar across the border that’s more likely with smoking bans but also multiple different locations in a single night – again on net more driving.