How to ensure / encourage responsible social drinking at work? It's a dodgy old topic this one as it straddles a couple of pieces of legislation, one of which is prescriptive (Road Traffic Act) and the other not so much (Health and Safety at Work Act).
The "problem" is that the HSWA is there to prevent the risk of harm from work activities, whereas the question being asked is how you prevent the risk of breaking other legislation by staying under the limit (I am sure the moral side is that no harm occurs to workers or other road users too).
While I wouldn't want to be a fun filter, but due to you having the guilty knowledge of problems occurring in the past, I can't see a way of solving this without putting some clear rules or limits in place for social gatherings, and meaningful deterrents as a consequence of not following these expectations.
If your risk tolerance does cover the possibility of having intoxicated persons on the premises (but not while working), then I would definitely be encouraging lift shares and the use of public transport on drinks day; it could even be a useful PR stunt due to the environmental benefits :wink: