Skip to content

Tunnels and Underpasses

Delaware & Lehigh Trail | Photo by Thom Carroll

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.


Capital Crescent Trail | Photo by Barbara Richey
Capital Crescent Trail | Photo by Barbara Richey

If a corridor includes a tunnel, the first step is to determine its structural soundness. If possible, locate the tunnel’s original engineering drawings. These will describe significant structural features of the tunnel that are not apparent from visual inspection.

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. 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.

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. In the event that you need to seal your tunnel, you can do so with thick wood, steel or concrete. Another option is to install strong, jail-like bars that prohibit entrance yet allow users to look into the tunnel.

Even if your tunnel is found to be structurally sound, make sure to schedule regular inspections to ensure that it remains safe for trail use.

The Cal Park Hill Tunnel in Marin County, Calif., is an excellent example of a converted railroad tunnel. 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. 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.

Occasionally, the need arises for a new tunnel. Most new tunnels are constructed with large, corrugated metal culverts or pre-cast concrete culverts. 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.


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. 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). As with tunnels, striping on the trail surface is important to prevent collisions in the temporary darkness. Additional signs indicating a downward slope, the clearance height and the above cross street may be desirable.

It is possible to construct an underpass with a clearance lower than the recommended guidelines when there is no reasonable alternative. In these cases, consulting with your attorney and liability insurer and requesting a design exemption may be necessary. Again, signage is of utmost importance to prevent head injuries to bicyclists. 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.

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. This was necessitated by the presence of an adjacent streambed. In this instance, the benefits of removing dangerous at-grade crossings of city streets outweighed the potential problems caused by a low ceiling height. Proper signage ensures that trail users are aware of the low clearance.

In some cases, trail users must share a travel lane with motor vehicles in a tunnel or underpass. This is most likely to occur where the construction of a new underpass, tunnel or bridge is prohibitively expensive or limited by topography, and where the existing tunnel or underpass does not have the appropriate width to accommodate both a road and a trail. In these rare instances, AASHTO recommends the installation of both a warning sign and pedestrian-activated beacon at the tunnel entrance to inform motorists that trail users are present. Ideally, shared-lane markings should also be installed.


Water is a potential hazard in tunnels and underpasses. As with any other segment of trail, proper drainage is critical and can be accomplished by digging ditches on the sides of the trail or by adding a layer of well-drained ballast in the center of the tunnel to raise the trail above any standing water. Warning signs indicating that the tunnel or underpass should not be used during high-water events are also recommended, particularly in areas prone to flooding.

Poor lighting is another potential problem in tunnels. Tunnels should have a source of light for safety and security and to show off the interesting elements of the tunnel itself. Install lights in the tunnel, if possible, or post “flashlight-required signs” if permanent lighting is not an option.

Like bridges, tunnels contribute to a memorable trail experience and often act as the signature of their associated trail. Examples of popular tunnels on rail-trails can be found in RTC’s Tunnels on Trails report, along with additional information about 78 tunnels along rail-trails across the country.


‣ Report – Tunnels on Trails: A Study of 78 Tunnels on 36 Trails in the United States

‣ Report – Partnerships in Sustainability: Rail Tunnel Adaptation for Transit, Pedestrians and Bicyclists

TrailNation Collaborative

TrailNation Collaborative is a nationwide peer learning community from Rails-to-Trails Conservancy that brings together advocates, leaders and professionals from across disciplines to establish and accelerate trail networks across America. The collaborative provides proven tools, methods and resources, combined with RTC’s expertise and network of partners across the country, to accelerate the development of connected trail systems. When trails are connected across regions and states, trail networks have a proven transformative impact—they are essential infrastructure that creates thriving, healthier communities.

Topics in this section: