Core: Warning signs J: triangle, not a command

The J category (Book p.245-246) warns of danger without ever giving a command — all J-signs share the same shape: a red triangle with a black symbol. Precisely because the shape is always identical, the knowledge that matters is the code-symbol pairing, not shape recognition. The level-crossing family splits in two: J10 (with barriers) and J11 (without), and is separate from J8, which announces an ordinary dangerous crossroads — no track, no barrier. The roadworks family splits into J16 (roadworks, general) and J17 (specifically a road narrowing). The vulnerable-road-user family splits into J21 (children), J22 (pedestrian crossing — the location, not the people themselves), J23 (pedestrians in general) and J24 (cyclists and moped riders) — four different codes for four different situations, even though they feel like one 'vulnerable traffic' category. What all J-signs have in common: they are not an order or prohibition, so they prescribe no exact speed. Instead, the general rule of the law applies: you must drive so that you can stop within the distance you can see. A J17 road narrowing does not, for example, mandate '50 km/h' — that requirement appears nowhere on the sign.