Such as being over the drink limit? How could any camera spot that? How could a human being for that matter?

After all, most people who are over the drink limit don't veer all over the road in an obvious way. It's more a question of having slower reactions, so that in a crisis they take longer to stop or take evasive action, and they crash into something. Similar to what happens when a sober driver is driving too fast - the same result, but achieved by a different mechanism.