Speed Limit in UK

OH dear, the government is going to impose 20mph on all A-roads!!! That would mean a drop of 10mph as compared to the usual 30mph speed limit currently. I don't really understand whether it is viable or sustainable for people to drive at 20mph for most of the time? Oh, I Don't know. I don't think it is wise though because it is just too slow!!! I don't mind driving at 30mph but 20mph? I always get cars coming close to me even when I am driving at 30mph along A roads. That is bad enough.