As with bridges and overpasses, safe accommodation should be made for bicyclists to use roadway tunnels and underpasses to prevent impediment to free movement across freeways, railways, and other barriers. Access from adjoining streets should be as direct as possible to reduce out-of-the-way detours for bicyclists, and designs should endeavor to minimize conflict points at entrances and exits. Space should be continued through the facility, with extra consideration for issues such as lighting and personal security. Separate tunnels may also be provided, particularly to connect multi-use or bike paths (also see "Path Intersection Treatments").

Most existing roadway tunnels have, however, been built to accommodate motor vehicle traffic, and retrofit measures may be limited if extra space is unavailable to accommodate bicyclists. Planned improvement or tunnel reconstruction projects are an ideal opportunity to improve conditions for bicyclists. In the absence of major reconstruction, some retrofit measures that may improve bicyclist safety or comfort include providing warnings to motorists that bicyclists are present in the tunnel and providing extra lighting, call boxes, and other measures to improve visibility, safety, and personal security. To activate a "bicyclist present in tunnel" flashing warning light, a bicyclist pull-off area and push button are typically provided before the tunnel entrances (see case study #3). If there are no suitable alternate routes, and safe access cannot be provided through a tunnel facility, creative measures may be called for, such as providing transit or shuttle service through the tunnel on a scheduled basis or at certain high-use periods, or other solutions.

New roadway tunnels and underpasses should incorporate planning to accommodate bicyclists. There are at present no specific design standards relating to bicycle accommodation in roadway tunnels. General design standards for bicycle facilities would likely apply, but consideration should be given to providing significant extra width for shy distance from walls or other barriers. Bear in mind that bicyclist speeds will be affected by grade, and extra width may also be needed on steep grades. As previously mentioned, lighting and personal security are issues in tunnels, and designs should maintain good visibility without "hidden" recesses or unlit areas that invite security concerns. Other issues, such as air quality, may be particular to tunnels, but should be addressed from the bicyclist’s perspective.

If separated bike and pedestrian tunnels are provided, vertical clearance of 3 m (10 ft) is recommended for bicyclist comfort.³ Following general AASHTO structure guidelines for shared-use paths, the Iowa Department of Transportation recommends a width of at least the trail width plus clear zones, or a minimum of 3 m (10 ft) if emergency vehicle access must be provided, but the wider the better for lighting and comfort.⁴ Security issues must also be addressed in separated facilities. Generally, bicyclists are more comfortable if they can see "the light at the end of the tunnel" when they enter, but appropriate lighting should be provided to ensure good visibility both for security and to view the bicycling surface. Diversion of water away from the tunnel and good drainage and non-slippery surfaces in underpasses are also important design considerations to prevent water from becoming a hazard for bicyclists. The City of Davis bicycle plan also provides some guidance for shared-path underpasses.⁵