On Thu, 22 Mar 2018 18:35:48 -0500, Char Jackson
wrote:

[snip]

Here in the US, especially in smaller towns, as you approach the town
you'll be likely to see two signs. The first might say, "Welcome to
Mayberry, birthplace of Andy Griffith. Population 367." The second sign
might say, "Speed Limit 25 MPH unless otherwise posted."

That provides context for the "unless otherwise posted".

When I see such a sign with no context, what to do?

[snip]

Sincerely,

Gene Wirchenko