Nite Owl:
My sat nav did something similar, tried (and I say tried because one look at the road and there was no [zb] way I’m taking a lorry down there) to take me down inappropriate roads. I changed the route preference from “shortest route” to “fastest route”. It seems to keep me on bigger roads for as long as possible, pretty much what I plan on the atlas really.
■■■■ me, even the ■■■■■■■ TomToms that everyone here loves to dismiss don’t offer the option of “shortest” routes when routing for a lorry!