In plain English
30 is the ceiling, not the target. It's the limit because people live, walk and cross here — at 30 a pedestrian usually survives a collision; at 40 they usually don't.
Where you'll see it
Entering built-up areas. A good clue even without signs: street lights usually mean 30 unless signed otherwise.
The common trap
Treating the red ring as advice. Red ring = legal order. And remember: a limit doesn't make that speed safe outside a school at 3pm.
Memory anchor
Red ring = the law's ring. Whatever sits inside a red ring is what you must not exceed or do.
Quick check