Core: Closure signs C: who, what and which number is barred

The C category (Book p.239-240) closes a road to a specific kind of traffic, and the C-codes fall into two families. The first family regulates dimensions and mass with an explicit number on the sign: C17 (length), C18 (width), C19 (height) and C21 (total mass or the sum of the axle loads). All of these thresholds count the load — a truck that stays under the threshold empty but goes over it once loaded still falls under the ban. C22 belongs to a second family: no number, but a category. The sign excludes vehicles carrying certain dangerous goods, regardless of how large or heavy the vehicle is — a small van with the wrong load is caught just as much as a large tanker. Within the 'who may not pass' signs there is another precision trap: C6 does not exclude 'all motor vehicles', but specifically motor vehicles with more than two wheels. A motorcycle (two wheels) may therefore simply drive through a C6 sign, while an ordinary car may not. Cyclists, under a closure sign, are by default also caught by the ban (they are, after all, vehicles) — only an under-sign with the word 'uitgezonderd' plus the cycle symbol explicitly exempts them.