Here is a fun one from my travels last year.
The Republic of Ireland uses the metric system. Speed limit signs are posted as a white circle with a red outer border and black numbers in the center. 50KPH is a fairly normal speed.
Northern Ireland uses British units. Speed limit signs are posted as a white circle with a red outer border and black numbers in the center. 30MPH is a fairly normal speed.
The border between the Republic of Ireland and Northern Ireland has no demarcation. The speed limit signs look pretty much the same - just the numbers change. There are no units presented on the signs. Driving North at 50KPH I encountered signs saying '30' and slowed down to a comparative crawl of 30KPH - wondering for a bit why everyone was passing me. It took about 30 seconds for me to figure out we had, 'crossed over' - about the same time the GPS figured it out too and flipped units.
I knew that there would not be a formal border checkpoint, but I had expected there to at least be a sign saying, "Welcome to ****!" But - there was nothing visual to indicate the border going in either direction. The part that the border 'is no big deal' was great. But - if you're going to switch units on a road on an island, would it hurt to put some units on the first couple of signs in each direction?
30 is not a typical speed used with Kmh... 40,50 are now used to slow you down near schools 60,80 and 100 are the most common open road speeds.... at least here in Australia. some highways go up to 110 on longer stretches ...