This seems like a lot of hand wringing. Taxi drivers give rides to, and deal with, people in various stages of inebriation every night of the year. In fact, collecting drinkers at night is likely the best money making time of any every day.

The legal blood alcohol limit in basically any developed country is usually so low that people can readily hit it with a drink or two without feeling any sort of intoxication from it, let alone having their abilities grossly impaired.

I don't think this publicity stunt carries much danger for anyone involved beyond the usual dangers associated with operating a taxi at night. Drinkers will still get cabs, free or paid for, but this may cause more people "on the fence" to get a cab rather than driving their car home.

Even getting them into a line for a free cab may spur them to just take a paid one since they've made already made a moral choice to not drive after drinking by lining up for the cab in the first place.

It's a good idea overall, I think, so long as the cab drivers are properly compensated for it. I would also wonder about how it will affect the drivers in terms of lining up to collect these customers versus simply taking whatever is going through the app. There's an opportunity cost involved for them if they're sitting in a queue versus driving around waiting to be hailed with or without the app.