Quite often i will have a pint at the pub and drive, if i am at a family do for 5/6 hours with a meal i have been known to push it to three.
Personally i think everyone knows their limits (in a serious way, not "i'm gods gift" etc) and mine in most circumstances is no more than 3 units~
You cannot put an arbitrary figure i.e. 5 units as it is down to the individual and over what time period.
If i am honest, i think the attitude towards drunk driving (over the prescibed limit) isn't always representative of the facts but i can't deny it is for the greater good.
Personally i am quite happy they way the law stands, i can have a pint of bitter or several the night before and drive (within reason).
Those who want to drink 3/4/9 pints in a short space of time always will unfortunately.