Speed limit signage laws

Is there a minimum distance required between speed limit changes? I’m asking because there’s a 30 mph sign approaching the airport nearby, then probably 25 or 30 yards after, a 15 mph sign. There are always cops hanging around there handing out tickets (no, I’ve never gotten on there). Doesn’t seem right to me…

It all depends on juristiction. Where (what state) is this?

Maryland, Anne Arundel County