Speed Limits
I think you will find that a national speed limit of 70mph on all unrestricted roads was introduced in 1965.
In 1973/4 this was changed to 70mph on motorways, 60mph on dual carriageway and 50mph on single carriageway roads.
The limits were subsequently raised about 10 years later to 70mph on motorways and dual carriageway roads, and 60mph on single carriageway roads - except for vans, where the 1974 limits still apply.
Speed and safety depend on the conditions - how many times do you see idiots in built up areas driving at 30mph in thick fog on wet roads because the limit is 30mph, so that is the speed they go. A lot more dangerous than 120mph on open road, little traffic and good road and weather conditions.
Colin