![]() In some cases, trail users must share a travel lane with motor vehicles in a tunnel or underpass. Proper signage ensures that trail users are aware of the low clearance. In this instance, the benefits of removing dangerous at-grade crossings of city streets outweighed the potential problems caused by a low ceiling height. ![]() This was necessitated by the presence of an adjacent streambed. In 2007, the existing MKT Trail in Columbia, Mo., was rerouted under three city streets, with the new underpass clearance in a couple of places below the recommended height. Signs instructing bicyclists to dismount, stop or duck, painted warnings on the trail surface, paint on the low clearance, directional signs to alternate routes and a swinging bar before the underpass to indicate the low height are all good solutions. Again, signage is of utmost importance to prevent head injuries to bicyclists. In these cases, consulting with your attorney and liability insurer and requesting a design exemption may be necessary. It is possible to construct an underpass with a clearance lower than the recommended guidelines when there is no reasonable alternative. Additional signs indicating a downward slope, the clearance height and the above cross street may be desirable. As with tunnels, striping on the trail surface is important to prevent collisions in the temporary darkness. ![]() When topography or other factors necessitate the installation of an underpass, an 8-foot vertical clearance is generally recommended (longer tunnels should have a minimum of 10 feet). Underpasses are a common solution to avoiding at-grade crossings of roads and active railroad tracks, although the American Association of State Highway and Transportation Officials (AASHTO) recommends bridges, where feasible, due to their security and water drainage advantages. The tunnel’s vertical clearance should be at least 10 feet, and the width of the tunnel must be at least as wide as your trail, plus a 2-foot-wide shoulder on each side. Most new tunnels are constructed with large, corrugated metal culverts or pre-cast concrete culverts. Occasionally, the need arises for a new tunnel. Half of the tunnel was also improved to serve a future train line, making the Cal Park Hill Tunnel and the trail that it serves one of an increasing number of rail-with-trail projects around the country. Originally built in 1884 to transport trains hauling lumber and freight, the tunnel reopened in 2010 to carry bicyclists and pedestrians between San Rafael and Larkspur. The Cal Park Hill Tunnel in Marin County, Calif., is an excellent example of a converted railroad tunnel. Another option is to install strong, jail-like bars that prohibit entrance yet allow users to look into the tunnel.Įven if your tunnel is found to be structurally sound, make sure to schedule regular inspections to ensure that it remains safe for trail use. In the event that you need to seal your tunnel, you can do so with thick wood, steel or concrete. ![]() If the tunnel is found to be structurally deficient, and you are not immediately able to make repairs, it is best to close the tunnel temporarily and reroute the trail until repairs can be made. The engineer should inspect the structure and provide you with a written report of the tunnel’s condition, a list of what needs to be done to make it safe for trail use and a cost estimate for any repairs. To make sure your tunnel can be safely and successfully adapted for trail use, obtain a certified report from a structural engineer describing the current condition of the tunnel. These will describe significant structural features of the tunnel that are not apparent from visual inspection. If possible, locate the tunnel’s original engineering drawings. If a corridor includes a tunnel, the first step is to determine its structural soundness. Tunnels are among the most striking physical features of a trail and are often the most memorable aspects of a trail experience, while underpasses offer safe crossings of roads or other barriers where an at-grade crossing would be dangerous. However, a tunnel or underpass on a trail can also present challenging structural, design and management issues. Howard Tunnel on the York County Heritage Rail Trail - Photo CC Rob Carlson via Flickr
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