Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A covered bridge is a bridge with enclosed sides and a roof, often accommodating only a single lane of traffic. They have typically been wooden, although some newer ones are concrete or metal with glass sides. Mainly associated with the nineteenth century, covered bridges often serve as prominent local landmarks and have long attracted the attention of historic preservationists. Early bridges were often made of wood, especially where it was a plentiful resource. Wooden bridges tended to deteriorate rapidly from exposure to the elements, having a useful lifespan of only nine years. Covering them protected their structural members, thus extending their life to 80 years or more. Most wooden covered bridges employ trusses as their key structural design element. A popular design was the Brown truss, known for its simplicity, but others were also used.