[GoNative]

Frankly that's just bs nyororin. Sure there may well be algorithms but I don't believe they are used in Japan, well certainly not as they are applied just about anywhere else in the world.
In Australia on country roads as I say the default limit is 100 km/h. If there is a curve in the road then they will post a sign with a lower limit for that particular curve. It will vary with the curve. Obviously using some sort of algorithm like you suggest. Once beyond the curve it defaults back to 100. The posted limits though will give you a good idea of the safe limit for that particular part of road.
Here you can have perfectly straight sections with no curves for as far as the eye can see and the limit will still be 50 km/h on better roads than I've driven over 100 km/h in Australia.
Truly there's no point having a speed limit if you do not enforce it. Create limits that are realistic and police them. Otherwise it creates much more dangerous conditions where you have some few people sitting on the limit and others racing along well over it. Speed limits are not meant to some vague guideline, they should be a true indication of the safe speed to travel. Here they've just defaulted to a ridiculously low limit that doesn't take into account safe limits whatsoever and you have a situation where people just drive at whatever speed they want. That is not safe.