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The UK's rules for what to do when there isn't a sign: https://www.gov.uk/speed-limits

Of course there's the unofficial and regularly observed "I think this is 60, but I can't see (or have no idea what is) a NSL sign so I'm going to do 45-50 to be safe"

It varies state by state and can depend on what kind of road you are on. Most states define a default speed limit for roads and highways.

Most states have rules such as: "35mph unless otherwise posted".