You don't seem to have ever driven on a long, empty, well lit 4 lane carriage way at 4am in he morning. If I am going 70MPH (UK Motorway speed limit) or 120MPH in such a situation makes no difference in terms safety.
In the UK we have variable speed limit roads. When they are busy/obstructions the speed limit is lowered. It is put back to 70mph when the traffic is light / no safety issues.
The safe speed on a road is dependant on the road and the conditions. I've been in situations where driving at faster than 10mph would be dangerous and I've been on the same road and doing 40mph was safe.
Except the average person isn't a highways engineer and isn't aware of the road design limits which means 120mph can be unsafe even on a visually empty road.
Even the German autobahns are only unrestricted in specific stretches where someone will have done the legwork to demonstrate safety at those speeds.
> Except the average person isn't a highways engineer and isn't aware of the road design limits which means 120mph can be unsafe even on a visually empty road.
Firstly there is no such thing as the average person.
Secondly, I don't need to be a "highways engineer" to be able to see there is few / no cars in front of me for over several miles on a long straight, multiple lane highway with no junctions for sometimes miles.
Thirdly, the decision for the motorway speed in the UK is a historical artifact.
Generally most cars (even modern ones) it is unwise to sustain speeds over 90mph for a long duration if the engine is small (coolant systems are more likely to fail, it is hard on engines), it is also not fuel efficient to drive much faster than 60 mph in cars that have engines that are lower than 2.0 litres IME (I've done a lot of driving in different vehicles).
I would prefer they have variable speed limits on motorways / or special toll roads where the limit is higher.
In the UK we have variable speed limit roads. When they are busy/obstructions the speed limit is lowered. It is put back to 70mph when the traffic is light / no safety issues.
The safe speed on a road is dependant on the road and the conditions. I've been in situations where driving at faster than 10mph would be dangerous and I've been on the same road and doing 40mph was safe.