I think, especially in Scotland and most of mainland Europe were the limits are lower than in England & Wales they want to do away the idea that you can drink “a little bit” and drive – the limit is more a buffer for things like food with trace amount of alcohol in it, if someone’s just used mouthwash or had a drink the day before or something.
There is no safe way to determine how much you can drink before driving, it’s theoretically possible to be over the limit on a small or relatively large amount, I wouldn’t even trust one of the home testers you can buy, if you get caught over you’ll be banned, cause an accident, even if alcohol wasn’t a factor (in theory) and you might face prison. It’s just not worth it anymore.
Whether it’s fair or not is open for debate, when I was young most people considered 2 pints of weaker lager the limit for driving, when my parents were young it was “Five and Drive – don’t have that 6th pint if you’re driving”, the roads were quieter back then, but lots of people died every year from drink driving.