I'm much more concerned about someone going 40mph in a 25 zone than someone going 110 in a 75.
if carOnAutoBahn { setLimiter(155) }
What reason is there ever for a car to go above 40mph? The obvious answer to your question is: quality of life. People like getting places faster. The purpose of governance is to balance quality of life with public safety. No matter how slow the speed limits, some people will die each year, so we're not haggling over the concept itself, but rather were we draw the line.
For context, it's important to remember that the Autobahn is actually safer than U.S. highways despite the lack of speed limit (https://www.ncesc.com/geographic-pedia/is-the-autobahn-safer...). In fact, it's even safer than other German roads (https://www.ncesc.com/geographic-pedia/what-is-the-accident-...). Speed does not appear to be a primary contributing factor in accidents and fatalities insofar as the Autobahn is concerned. Meaning arguing to reduce or restrict speed provides marginal social benefit at comparatively larger cost.
This reads like "I speed all the time and how dare you say it's a bad thing" cope.
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.
Even the German autobahns are only unrestricted in specific stretches where someone will have done the legwork to demonstrate safety at those speeds.
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.
https://readcars.co/2017/06/20/history-speed-limits-uk/
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.