PaperTerrain have released a cardstock Modular Bridge Set
From their announcement:
Rivers and canyons come in all sizes and shapes–now your bridges can be built to match them! Our modular bridge set allows you to build bridges of variable length and height. Each kit has enough parts to build either one long bridge or two shorter bridges. The bridge can either be the classic “up and over” wargame bridge where it starts level with the table and then arches up and over and back down to table level. Or, if you have a topographic table with your rivers realistically below the table level the kit has end pieces to match this as well.
The 10-15mm set can be up to 40″ long with a roadway up to 3″ wide. The 20-28mm set can be up to 30″ long with a roadway up to 4 1/2″ wide. (6mm is coming soon!) It is even possible to combine multiple sets to make a very long bridge or viaduct. The kits come with options like side buttresses and foundations. The kits are available in brown stone, gray stone, or concrete finishes. Constructing the kit requires foam board or balsa wood as a reinforcement and this is not included in the kit.
This kit comes with complete step-by-step illustrated instructions, but it should be noted that this model is a bit more complicated than most of our PaperTerrain products.