> They instead drive at a natural speed for the road and conditions.
except they don't, they reliably frequently drive above what is save
heck what is safe isn't even always obvious
Especially if you look at urban settings where you have pedestrians, bicycles and motorcycles this is quite common.
Similar people, including some otherwise experienced drivers, tend to fail to internalize that the emergency stopping distances increases exponentially (roughly squared).
A grate example for why you should never base care speeds on percentiles is a street close by to where I live, it looks like it's safe to drive ~50km/h but even when driving just 30km/h you will find it hardly possible to safely keep with other traffic laws, like e.g. precedence on crossroads. It also has funny parts like subtle getting smaller in some areas (including after turns). And playground, primary school and a retirement home are close by. And in difference to some other countries it not uncommon for children here to travel alone (or in groups of children) to primary school and sometimes playground. Potentially as young as 6 years of age.
And that doesn't even include special situations like idk. roads which are especially unsafe during rain due the kind of dust they get exposed too and similar.
> drive according to posted speed limits
sure, but if you have appropriate enforcement most will at most drive slightly above them limits as they don't want to pay high fines or lose their license. If you raise the limits they will again drive slightly above the limits and due to the roughly squared increase in breaking distance this is more then just slightly bad.
> his is why in countries with sane traffic laws, the speed limits are set to the 85th percentile
except it's kinda the opposite
most (not all) countries with very high levels of road safety do not use a percentile system. Most common is a system with fixed speeds for urban, "country side", and "highway" and then assess safety for given speed based on expected reaction time and maximal road safe emergency breaking distance and if it's not safe reduce it (als in general not gradual reduction but based on a preset of "speeds", e.g. in Germany urban walking speed, 30 or 50 for urban streets. The fixed choice of speeds seems a bit strange, but people are animal of habit so it's a better choice, it also makes it easier for everyone (for drivers to not get confused about the speed limit and for state to manage it).
> Get the cars off the road faster and the distance between vehicles will naturally increase.
But if you drive a lot you might have realize that it doesn't increase as much as it must to be as safe. People vastly miss-estimate distance traveled during reaction time and emergency breaking distance all the time. E.g "traffic jam out of nowhere" are not uncommon on German highways and are most times a consequence of people not keeping the required safe distance between cars leading to a chain reaction of people breaking much more then they should need to leading to a traffic jam (as in most commonly a long blob of cars going much slower then they normally would, like just 80km/h instead of 130km/h).
> Germany’s autobahn has no speed limits and while collisions happen, they are relatively rare.
This is not quite right 40% of German highway road has speed limits, because it not being safe to drive faster in that areas.
In addition the recommended maximal safe driving speed is 130km/h, if you drive faster it can lead to reduced insurance coverage and in case of a crash the amount of fault attributed to you might increase, too (depending on you actual speed above 130km/h).
Then there is the thing that the highways are (in the areas without speed limits) designed to allow very fast driving. For one there is the rule that you need to keep on the right most lane as long as you aren't overtaking someone/it's viable. The rule that you aren't allowed to take over on the right side (except in traffic jams). The fact that all highways entrances and exits are strictly only on the right side. And some other smaller stuff which are essential parts of why no speed limit kann be safe. So you really can't easily compare it to other highways.
Lastly in Germany safe driving trump any "rights" speed limits give you and similar. E.g. lets say there is no speed limit but it's relatively crowded and you still insist on driving 200km/h. You might then still get pulled over for non safe driving. Through to be fair ignoring idiots this normally works itself out just fine without such rule on highway. Through the general idea of "looking out for others on the road" being a indoctrinated during driving lessens still does matter.
> maxim is that if you make something idiot proof
Which brings us back why you would want speed limits based on what is actually safe not what feels safe/what people feel like driving. Or why you would only want to have a few categories of speed limits. Or why there are studies which show that building road to wide and safe looking isn't the best idea.