You should check with the road authority because they might be fake.
One day I was driving down a road that I’d driven down many times when I spotted a new speed limit. It was posted in a weird location and the speed was odd.
I contacted the local main roads department and learnt that they didn’t know about these signs. The next day they were gone.
Agreed, having a minimum speed sign on a stroad is just begging for pedestrian deaths. It would be such an obvious liability that nobody in charge of transportation would allow it.
Every municipality has a minimum speed limit so you can’t just have someone driving 1km/h down the road creating a rolling blockade. Most are posted on the entrance into the municipality.
There are reasons for both sides to have and not have specific laws.
I’m a traffic engineer and I’ve literally never heard of such a thing (except on freeways).
