Aha. That explains something that I have noticed (and mentioned on here quite some time ago), where a generated route takes you off the highway, then makes you enter the same highway immediately, because distancewise (and timewise) it calculates as a little bit shorter (even though I know for sure from experience that staying on the highway is much, much faster). But the fact that link roads are always mapped to highway, explains that particular issue. It also explains why taking an exit off a highway does not show up particularly well in the textual directions (Also see this post I made earlier and this one ).
It would be nice if this could be improved. As it is I would not dare to give these kinds of instructions to some people I know (and might have difficulties with them myself), if all they had were those textual instructions.