But then you need an up-to-date database of allowed speeds on roads all over the world, and since those already aren't reliable when they're used as indicative data, I don't think that's a viable solution.

It also wouldn't work on road sections with variable speed.

Doesn't need to work everywhere for it to be useful. You can always fail fast, literally. The data is available for wide swathes of Europe, for example. Last time I checked it was very accurate even for temporary construction sites.

Variable speed roads, either make the limit the highest speed that's in the range, or come up with a way to communicate the current limit. Most new cars are always connected, anyways. And again, you can fail fast, at least initially. As with all driving aids, the person in the driver's seat bears the ultimate responsibility.

Yet failing anywhere can be dangerous, even fatal.

An example: A construction site goes away, raising the speed from 35 to 70mph. Someone governed to 35 is suddenly a virtually stationary obstacle. Theoretically (and legally), the onus is on the other drivers to avoid it, but practically speaking it's just begging for an accident.