[cameronh90]

In the UK at least, it's relatively simple. If it's a red circle with a number in it, that's the speed limit - anything else is advisory. In my experience, cars with built in sign readers do an exceptional job of working out the current limit (more reliable than me, certainly!).

[dzhiurgis]

It should be trivial to convert every sign to have wireless transponder. Optical guessing sounds awful for wrong angles, wind damage, snow coverage, lighting issue.

Maybe correlate it with central database for sanity. Preferably daily updated git.

[sandworm101]

So no temporary work zones in the uk? What is the rule if the sign isnt there/visible? Do you then get to race through an obvious construction zone?

[Symbiote]

They tie an opaque bag over any incorrect signs, and erect temporary signs (of the same standard, international design) with the new limit. If it's a motorway or similar road, the electronic emergency signs will also show the reduced speed limit [1]. On a motorway, they're often on a gantry, i.e. completely impossible to miss.

I think I read somewhere that it's someone's job to make regular checks that the temporary signs (and covers) are still correct -- they are an important part of the worker safety requirement for the construction crew.

When a speed limit changes, the rule is for the sign to be shown on both sides of the road. There are then repeating signs for the current limit at some regular interval.

The UK is dense enough that having expensive electronic signs on all motorways isn't an unreasonable cost; I understand that's not practical in the USA or Australia.

[1a] https://i2-prod.birminghammail.co.uk/news/midlands-news/arti...

[1b] https://www.ageas.co.uk/globalassets/solved/30072018_road-sa... (possibly this style is no longer used, I drive very rarely in the UK so I'm not sure.)

[InvertedRhodium]

If there's no sign to indicate that the speed limit is changed, how are you supposed to know what the speed limit is?

I've had speeding tickets forgiven in Australia because the sign simply wasn't visible enough due to overgrown trees and a 2 lane off ramp separating the sign from the road it applied to, let alone the sign missing altogether.

[sandworm101]

If you are surrounded by people in orange vests, or traffic cones directing you to a detour, you are in a construction zone and better not be doing highway speeds. The fact that the sign wasnt visible, or even wasnt present, will not help. This comes up in AI cars all the time. The camera is blocked from seeing the sign beside the road, perhaps by a truck in the right lane. Or maybe the temporary folding sign is blown over. You still have to recognize a construction zone. A bad sign might get you out of a basic speeding ticket, maybe, but it wont protect you from a dangerous driving ticket, or going to jail after running over a construction worker. And a great many juridictions mandate vastly reduced speeds when cops/ambulances/firetrucks/towtrucks are beside the road, meaning you have to recognize such situations regardless of posted signs.

[InvertedRhodium]

Well, sure. But in the context of enforcing a limit on a human - by either reporting a breach or actively preventing the car from exceeding the speed limit - and not the car driving itself, I don't see any of that being an issue.