The big idea — the variety is the defence

A sign charge looks simple: a sign existed, and you allegedly disobeyed it. But unlike a stop sign (one thing — a complete stop), a disobey-sign charge can arise from dozens of different sign types, each prohibiting a different act, in a different place, at different times, sometimes only for certain vehicles.

That variety is the source of the defence. The Crown must prove the specific sign, the specific prohibition, and the specific act — and under s. 144(9), the specific intersection geometry. Every one of those is a place the case can break down.