Pedestrian Safety Guide and Countermeasure Selection System
Pedestrian overpasses and underpasses allow for the uninterrupted flow of pedestrian movement separate from vehicle traffic.
Source: pedbikeimages.org - Julia Diana (2009)
Pedestrian overpasses and underpasses allow for the uninterrupted flow of pedestrian movement separate from vehicle traffic. However, they should be a measure of last resort, and it is usually 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 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.5 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.17 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 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 pedestrian use. The AASHTO Guide for the Planning, Design, and Operation of Pedestrian Facilities recommends that pedestrian overpasses be at least 8 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 ft. Depending on the length of the overpass, it might be necessary to increase its width in order to counteract any visual perceptions of narrowness.2 Similar guidelines apply to underpasses. Minimal widths should be between 14 and 16 ft, but underpass width should be increased if the underpass is longer than 60 ft.
The cost for specific types of bridges can vary substantially, based on the specific situation, materials, and other factors and, as demonstrated in the table above for wooden and pre-fab steel bridges. Underpasses (not included in the table above) range from slightly less than $1,609,000 to $10,733,000 in total or around $120 per square foot. Overpasses (also not included below) 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.
Authors and Acknowledgements