Roads and Bridges are the most basic terrain feature in the game. They are always the quickest route of travel for land units, always costing only 1 Movement Point to traverse per tile, even if it is Raining or Snowing. The downside of Roads and Bridges is that they provide no defensive bonuses to units stationed on them. While efficient for quick travel, roads should be avoided for when holding defensive positions, as any unit on Roads or Bridges will take full damage from any given attack.
It should be noted that while Koal gains increased offense when his units are on Roads, he does not gain any bonus when his units are fighting on Bridges.