The issue with impairment is not just down to concentrations, it's down to experience, tolerance, saturation levels and frankly whether someone can use common sense. I've known people who had no right being behind the wheel while intoxicated after one hit and others that you couldn't tell had actually consumed anything. It really does come down to the individual and other things like how much experience they have of being intoxicated and their (mental) concentration levels while in that state. I also think age is a big factor. Younger drivers are much more heavily influenced by testosterone and more likely to exhibit risky behaviour, whether intoxicated or not. While many older people with more experience drive in a more relaxed manner.
