A drunk driver does not pose an immediate threat to you unless they are headed toward you with their car. Stopping them would only be self-defence​ if they were about to hit you. I'm not supporting drunk driving but, if someone makes the decision to drive drunk and makes it home without incident, there was no harm done. The real problem is they may not have the mental ability to make the right decision because of the alcohol and should be stopped because of that, with no penalty. Just send them home safely. Not beca​use they "may" hurt somebody. Fining someone and sending them to prison because they were using the only drug they were allowed to use is just hypocritical. Especially, if that drug takes away your ability to make proper decisions.