Most speed limits, especially the UK's, were set when drum brakes, skinny tyres and 2 ton cars were the norm... it took an age to stop from 70 mph, so the limits made a BIT of sense.
With todays technology they'll stop in a far shorter distance from 120 mph than 70 mph in even the 1970s...
Cars are much safer now than they ever were.. and will probably get safer still.
That might be so but, apart from motorways, the speed limits on most roads in the UK have been drastically reduced over the last few years & this is not likely to get any better. 20 mph has been seriously suggested for built-up areas. Motorways aren't too bad at weekends but at other times there is generally a hold-up of some kind, caused by road works, an accident or simply the sheer volume of traffic so raising the speed limit would be impractical anyway.