In many cases those experts are quite dumb and speed limits are arbitrary. You can see it most clearly in Europe where different countries have set different speed limits for similar road conditions. For example in one country the speed limit in highway tunnels is 80 km/h, but in others 90 km/h while the width of the road is similar, or even mostly wider where the limit is lower. The same kind of highway can have 30 km/h limit difference over countries. Some countries recognize higher variance of speeds in the cities than 30 or 50 km/h others don't.
In France, some *départements* adopted the 80km/h limitation while some other kept the 90km/h one, this has to deal with a compromise between safety, co2 emissions and current road behavior and population requirements.
The decision to transition from 90 to 80 has been backed by some test which tended to show a reduction of mortality by 12%.
Initially, most of the places where 90km/h is kept are huge countryside regions in which the traffic is low and the population has to drive actually quite some time in order to reach any city.
The difference may seem insignificant in terms of travel time, but it felt like it was a huge topic back in 2018-2019 where the new limitations appeared.
Now, more and more départements are asking for derogations to fall back to the 90km/h limitation, the main reason for this being 58% of the population in france doesn't follow the 80km/h limitation and doesn't consider 90 more dangerous than 80 (no test implied this time, 58% of france population is "optimistic").
So maybe for each country, before experts gave roads their current speed limitations, the way the population drove was taken into account by those experts or was taken into account *in place* of experts analysis, in order for the limitations to be a negotiation/compromise between current habits and experts analysis, in the best cases, and only an affirmation of the population *sentiment* in the worst cases.
Some départements opted for another solution by putting some 70km/h limitation on dangerous sections of a road mainly limited to 90. The problem is that often, when I drive at 70km/h in these sections, some deadly stupid person is stuck full speed right behind me, nearly kissing the back of my car, flashing me with a stroboscopic lights-on lights-off motion because they do probably think that 90 is not less safe than 70 (ability to judge if a situation is safe : eternally damaged).
The problem is not the road experts analysis in EU, but the political decision to apply or not the analysis, and the population refus to follow the limitations.
We have entire highway segments that have variable speed limits that change with traffic density and weather conditions. The speed limit signs are digital and update whenever a change is necessary.
