If you are going to use a through truss to support a highway, you would be better off using an HO scale bridge. Road lanes are at least as wide as normal-clearance rail lines, and highway clearances are intended to be 16 feet or better in any post-Eisenhower construction. The N-scale bridge, if used as a through truss, would be a light-load 12 foot clearance one-laner suitable for a county road in a county too strapped for cash to replace it. Don't forget the yellow "low clearance" and load limit signs on the approaches.
OTOH, if under-bridge clearances allow, that N-scale truss could be inverted, wider cross-beams installed on top of it and a two lane deck could go in on top of that. However, the structural members of the N-scale bridge would be awfully light for that kind of loading.
Chuck (modeling Central Japan in September, 1964)