Adrian wrote:
Why are speed limits enforced in such a black-and-white fashion?
Because they can be. If the limit is 30 and you are caught doing 47,
then there's nothing to debate.

A big bloke, used to a few beers, would be far more competent whilst
over the limit than somebody smaller and unused to alcohol
This is precisely the "logic" that causes people to die on the roads.
The more familiar one is with alcohol, the less one has the capacity to
judge how competent one is. Those who are most familiar with it of all
have IME only passing contact with reality, and even when they haven't
had a drink for days, they may be very dangerous indeed in a car.

Being a big bloke makes no difference. The limit is measured
independently of body mass.