If you don’t know what the speed limit is then just go at 30. That’s the slowest that any road will ever be unless it’s a suburban housing but then it’s pretty obvious.

The 40 signs are small but they’re also repeated, you get it at the entrance of the 40 and then every couple of metres there’s another 40 time for the entire distance so if you miss one you’ll only be going at 30 for a while and then you’ll see another and it’ll tell you to go at 40.