I'm no fan of drink-driving, but it stikes me that people who drive like tw@ts when they are drunk are quite likely to still drive like tw@ts when they are sober.
People who are normally careful sensible drivers are unlikely to go mad and mow down pedesrtians or whatever just because they had half a shandy with their chicken in a basket.
The big thing to watch out for is driving the day after having had a drink, especially with some modern wines coming out at 14% or even more
A bottle of wine over dinner with a large G&T to start and a decent malt whisky to finish may not seem a huge amount to drink of an evening, but it's 14 units, and if you don't sit down until 8:00PM you'll still be at risk at 10:00 the next day.
