Originally Posted by
Saracen
Well, I'd say that that "definition" given by that police site isn't really a definition but more of a description of common characteristics.
Read, for instance, the Highways Act 1980 and it's definition of trunk roads, special roads, etc, and you'll see why such public info sites don't give the legal definitions, but merely a description that ought to mean everyone understands what they're talking about. If they tried to use legal definitions, the public would have dozed off or gone crazy re-reading the very precise legal terminology trying to work out what they heck they were trying to say.
Really, all a driver needs to know is what he ought to know to get through a driving test (i.e. the relevant bits of the Highway Code) .... that the signage is blue, and the restrictions and stipulations that apply to his/her usage of motorways. Anything else is there if we care to go read it (I strongly suggest not bothering), and would merely confuse most people.