Covered bridges have existed for nearly all of recorded history, with the first such structure credited to the ancient Babylonians over the Euphrates River about 783 B.C., and they remained a common bridging method through Medieval Europe before reaching their greatest use and design variation in 19th-century America. Although Americans did not invent them, American builders advanced covered bridge development through innovations and craftsmanship that combined utility and aesthetics, sometimes adding scrollwork trim, pedestrian sidewalks, exposed trusses, or bright colors, though most rural communities wanted sturdy, long-lasting structures rather than embellishment. Bridges were covered because uncovered wooden bridges trapped dirt and moisture in their many nooks and crannies, making decay likely and limiting their normal life expectancy to around 10 years, while roofs protected the trusses from the elements, helped calm farm animals crossing, provided emergency shelter, and added strength by reducing sag and creaking. In the United States, covered bridge building declined around the turn of the 20th century as the steel and iron industry expanded, engineers relied more on metal structures, traffic increased, and heavier rail and truck shipments made wooden bridges obsolete, with many surviving bridges bypassed by new roads and metal spans. Toll bridges raised money for construction and upkeep, but in the 1800s few laws regulated tolls, so private builders could charge what they wished, though churchgoers on Sunday, people attending town meetings, and doctors on call were exempt in some cases, until government eventually imposed controls and took over all toll bridges. In the 1800s and early 1900s, the shelter of covered bridges also made them favored places for brightly colored posters advertising goods, services, and events.