Plus if speed was really THE main factor, highways would be terribly dangerous. Or they're usually the safest when it comes to accident statistics.

So speed is not the main issue but it's a nice tool when you're a politician or you want to communicate.

For the Autobahn, wiki says:

In 2014, autobahns carried 31% of motorized road traffic while accounting for 11% of Germany's traffic deaths. The autobahn fatality rate of 1.6 deaths per billion travel-kilometres compared favorably with the 4.6 rate on urban streets and 6.5 rate on rural roads.

yeah bad phrasing on my end, knowing the max speed you could interact with a common obstacle (say a highway barrier) could lead to targeting harding to prevent fatalities, as opposed to general harding for all speeds.

Cars could still drive into lakes or off cliffs, but knowing the max speed a car could rear end another car may lead to minor enhancements that could save lives