Well, you can with either painted cardboard, my choice, or with Masonite. Go look up roadways in the Archives.
The width depends on whether you are using 1/53 Trucks and trailers or 1/43rd cars; Typically 2-1/2" to 3" per lane. So a two-lane highway is either 5 to 6" wide. On rural roads or turnpikes leave room for the shoulder. In cities leave room for the curbs and gutters.