Pedestrian overpasses and underpasses allow for the uninterrupted flow of pedestrian movement separate from the vehicle traffic. However, they should be a measure of last resort, and it is usually more appropriate to install safe crossings that are accessible to all pedestrians. Grade separated facilities are extremely high-cost, and overpasses in particular are a visually intrusive measure.
Such a facility must accommodate all persons, as required by the ADA. More information on the specifications for accessing overpasses and underpasses can be found in the Revised Draft Guidelines for Accessible Public Rights of Way. These measures include ramps or elevators. Extensive ramping will accommodate wheelchairs and bicyclists, but results in long crossing distances and additional time 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 or less. Overpasses work best when the topography allows for a structure without ramps (e.g., overpass over a sunken freeway). Underpasses work best when designed to feel open and accessible. Grade separation is most feasible and appropriate in extreme cases where pedestrians must cross highways or barriers such as train tracks