Good point. The really old ones are iron. ETA: Also, the ones that have survived aren't road bridges. They're either railway bridges (and thus were built stron

Good point. The really old ones are iron. ETA: Also, the ones that have survived aren't road bridges. They're either railway bridges (and thus were built strong in the first place) or have been relegated to historic preservation/pedestrian use only. Modern cars and (especially commercial) trucks are far heavier than early vehicles. A road bridge built in the 1850s was only designed to carry foot traffic and horsedrawn vehicles.