Powerful floods, heavy traffic loads, vandalism, and neglect led to the demise of hundreds of historic covered bridges. As vehicles and logging trucks grew larger, covered bridges were built with wider and higher portals. Gradually, the cost of constructing new wooden covered bridges exceeded the cost of concrete and steel spans. Oregon's bridge-building tradition dated back to the 1850s and lasted well into the 20th century, long after these rustic structures were relics elsewhere. By 1925, there were approximately 450 covered bridges along Oregon's highways and country roads, and the state's highway department supplied covered bridge designs to local agencies until the 1950s. Oregon now has more than 50 publicly accessible covered bridges, 45 of them historic truss-type bridges. New vehicular covered bridges are generally built to replace existing older covered bridges, ending a grand era. Lane County was one of Oregon's most prolific bridge-building counties and now has more covered bridges than any other county, with sixteen on the National Register of Historic Places.