I recently saw this bridge in Lisburn, Northern Ireland - you have to ask: how big does this sign have to be? I can imagine each time it's hit, they make a new sign, each one bigger than the last.
The problem is that all communication requires a contribution by the reader. On your day off, driving your car, this sign does not address you. But if you forget that you're driving a double decker bus, it doesn't matter how big the sign is. It's all about relevance.