You will have to mark up the byways yourself.
I use an HP IPAQ with GPS jacket. This is loaded with Memory map. I then use memory map on my desktop to mark the byways and lanes and routes and push them to the maps on the IPAQ.
road angel adventurerer 8000 will run memory map (digital os mapping prog). As will lots of PDA's. You can either get one with a built in GPS receiver, or one which you can use a bluetooth GPS dongle