Ok, so let's say you have a nifty N800 and the killer-app Maemo Mapper. You can use it to download routes (aka: driving directions). But, as you're on a bike, you're not so keen to take the A4 or 280 to get where you're going. Fear not, you can download routes while avoiding highways!
Go to the menu in the Mapper. Select route and then select download. A dialog box opens. At the top of the dialog box, there is a textfield which has a URL in it. Most likely, the URL there points to gnuite.com. At the end of the URL is some CGI stuff, like &blah=%s . Scroll to the very end of the url and append the following text to it:
Then, you can download routes to your heart's content, all of them highway free. If you change your mind about highway avoidance, just remove the added text.
The Mapper application saves your settings, so if you want to stay highway free forever, you will not need to modify this again - with all the usual exceptions to such a statement.