Overpasses and underpasses allow for the uninterrupted flow of bicycle and pedestrian movement separate from vehicle traffic. However, sometimes it is more appropriate to use traffic-calming measures or install a pedestrian-activated signal that is accessible to all pedestrians because overpasses and underpasses are costly, visually intrusive, and often poorly utilized when a more direct at-grade crossing is possible.
Overpasses and underpasses must accommodate all persons, as required by the ADA. More information on the specifications for accessing overpasses and underpasses can be found in the Proposed Guidelines for Accessible Public Rights of Way. These measures include ramps or elevators. Extensive ramping accommodates wheelchairs and bicyclists, but results in long crossing distances and steep slopes that discourage use.
Studies have shown that many pedestrians will not use an overpass or underpass if they can cross at street level in about the same amount of time. Overpasses work best when the topography allows for a structure without ramps, such as an overpass over a sunken highway. Underpasses work best when designed to feel open and accessible. Underpasses are significantly less expensive when built as part of a construction or reconstruction project and generally offer gentler grade changes than overpasses. Grade separation is most feasible and appropriate in extreme cases where pedestrians and bicyclists must cross roadways such as freeways and high-speed, high-volume arterials.
Entrances and exits to overpasses and underpasses should be clearly visible to encourage use. The AASHTO Guide for the Planning, Design, and Operation of Pedestrian Facilities recommends that pedestrian overpasses be at least eight feet wide. The width should be increased if the sidewalk leading up to the overpass is wider. If the overpass also accommodates bicyclists, the width should be at least 14 feet. Depending on the length of the overpass, it might be necessary to increase its width to counteract any visual perceptions of narrowness. Similar guidelines apply to underpasses. Minimal widths should be between 14 and 16 feet, but underpass width should be increased if the underpass is longer than 60 feet.
Overpasses and underpasses can provide complete separation of pedestrians and/or bicyclists from vehicular traffic. Overpasses and underpasses also provide crossings where no other pedestrian or bicycle facility is available, and connect off-road trails and paths across major barriers, like freeways, railways, or natural barriers.
- Use sparingly and as a measure of last resort. Most appropriate over high-volume, high-speed highways, railroad tracks, or natural barriers.
- People will not use the structure if a more direct route is available.
- Lighting, drainage, graffiti removal, and security are also major concerns with underpasses.
- Must be wheelchair accessible, which generally results in long ramps on either end of the overpass.
- AASHTO recommends a railing height of at least 42 inches.
- When bicyclist space is provided near railings or near motorized traffic, extra horizontal width or a buffer of at least two feet is recommended to protect bicyclists in the event of a crash or wind blast.
Costs will vary greatly based on site conditions, materials, etc. Underpasses (excluding bridges) range from slightly less than $1,609,000 to $10,733,000 in total or around $120 per square foot. Overpasses (excluding bridges) have a range from $150 to $250 per square foot or $1,073,000 to $5,366,000 per complete installation, depending on site conditions. Wooden bridges are approximately $125,000 on average, and pre-fabricated steel bridges approximately $200,000. More detailed cost information is provided here.