I'm 56, now living in California, but originally from Indiana. I joined a Mothers Against Drunk Drivers (MADD) chapter back there, in the mid-80s. I joined after a close family friend was killed by a drunk driver, and the cop on the scene didn't, for some reason, issue a DUI citation. So I went on an activist tear to collect evidence and then get the prosecutor to go arrest him; and then get said drunk driver what was, at the time, the toughest jail sentence in Indiana history for the then-not-even-thought-much-about problem of drunk drivers. I must interrupt myself to go on a brief tangent: The bitter irony of the story is that the judge who issued that sentence died, a few years later, in a single-car DUI accident in which he was the under-the-influence driver. But now I digress. Sorry.
Of course, MADD and other groups and initiatives have made it very unpopular, indeed, to drive drunk, these days (as it should be, of course); and have toughened the laws on it in every single state. Even since back in the '80s, MADD and others have issued little wallet cards which help one calculate what is their likely BAC... that is, if they, first, bothered to actually look at it; and, second, were honest with themselves.
And, of course, we've long had the whole designated driver, thing. And then there are activists, like me, who will actually take the keys out of a person's hand at a party or in a bar; and I'm a big enough guy that they can't get their keys back; and I always offer to either take 'em home myself, or get 'em a cab. But I'll put my nose right in their business and make 'em want to beat me to a pulp to keep 'em from getting behind the wheel, if I have to. I've been hit, too... but, of course, they got hit back harder, and so that was always the end of that.
So, then, with all that knowledge and experience being an anti-drunk drivng advocate/activist, I'm here to tell you that only the most inherently responsible and self-examining people will even bother to use this or, if they did, would stop themselves from driving if the app showed their BAC was too high. Sadly, that's a tiny portion of the population.
This is an app that is long overdue; one that I suggested someone should write (with an accompanying plug-in hardware device, exactlly like this one) several years ago. Now it's here, and that's great; but while I think it could actually become fairly widely used, I predict its recommendations will also be fairly widely ignored. In the end, the only thing that really works is one of those interlocking devices that the Courts usually order for repeated DUI offenders wherein they must breath into a tube while sitting in the driver's seat, and pass a BAC test, before they may start their car.
Er... well... that and putting 'em in jail for a few years.
All that said, of course I hope this thing will catch on. Even if it doesn't, though, if it only saves one life out there somewhere it'll be worth it, I suppose.