Where a designer is not able to provide appropriate treatments for the at-grade pedestrian crossing of a roadway, or where the nature of the barrier being crossed does not allow an at-grade crossing (e.g., freeway, water body, depressed rail line, etc.), a grade-separated crossing should be considered. It is not practical to develop warrants governing the construction of pedestrian grade separation facilities. Each situation must be considered on its own merits.

The need for a grade-separated crossing must include an engineering analysis of all alternatives, including safety improvements to nearby at-grade crossings. When the construction of a pedestrian grade separation is considered, an investigation should be made including studies of pedestrian crossing volumes, type of highway to be crossed, proximity of adjacent crossing facilities, the predominant type and age of persons who will use the facility, and the cost of constructing the pedestrian grade separation.

The effectiveness of grade-separated crossings depends on their perceived ease of accessibility by pedestrians. An overpass or underpass will not be used simply because it improves safety. Pedestrians tend to weigh the perceived safety of using the facility against the extra effort and time needed. The degree of use of overpasses and underpasses by pedestrians depends on walking distances and the convenience of the facility. shows an example of a pedestrian underpass.

The primary location for pedestrian overpasses/underpasses is often an urbanized area but may be appropriate in other contexts. Such a pedestrian crossing may be considered when the following conditions exist:

Pedestrian demand to cross a freeway or expressway is moderate to high;
A large number of children need to regularly cross a high-speed, high-volume roadway (particularly near schools);
Conflicts that would be encountered by pedestrians are considered unacceptable (e.g., on wide streets with high pedestrian crossing volumes combined with high-speed traffic);
One or more of the conditions stated above exists in conjunction with a well-defined pedestrian origin and destination (e.g., a residential neighborhood across a busy street from a school; a parking structure affiliated with a university or other campus; a high-volume, multi-use trail; a major entertainment destination; or an apartment complex near a shopping mall); or
Grades on either side of the proposed bridge or underpass are conducive to building the structure.

A pedestrian grade separation should only be constructed when the need for the safe movement of pedestrians cannot be provided in a more efficient manner. Additional guidance concerning pedestrian grade separations can be found in an the

Grade

- Bridges must comply with ADA guidelines: not exceeding a 5 percent running slope or 2 percent cross slope. Where space constraints result in necessary steeper grades, bridges must be designed to a maximum slope of 8.3 percent, and landings are required for every 30-in of ramp rise. Landings must be a minimum of 5-ft in length and can have a maximum 2.0 percent slope measured parallel and perpendicular to the path. Assuming a full 30-in rise at 8.3 percent and a 5-ft landing at 2.0 percent slope results in an “effective grade” of 7.4 percent. It is recommended that this be used for short, constrained segments only if needed rather than a basis of design for long, sustained segments.

Width

- The minimum inside clear width of pedestrian bridges, exclusive of railings and handrails, is 8-ft. If the adjacent sidewalk widths are greater than 8-ft, then the minimum inside clear width must match the adjacent sidewalk widths. See for recommended and minimum widths on shared use paths. Where width of a bridge is sufficient to permit vehicle passage, designs should include some type of deterrent with the preferred choice being a forgiving bollard since more robust bollards may present safety issues for pedestrians and bicyclists. Greater deterrents are recommended where the structure would not support the weight of a vehicle.

Vertical Clearance

- See for vertical clearance requirements for pedestrian crossover structures.

Lighting

- When provided, lighting for pedestrian and bike bridges should allow users to see one another in nighttime conditions to avoid collisions and improve the perception of personal security

Grade - Underpasses must comply with ADA guidelines(not exceeding a 5 percent running slope or 2.0 percent cross slope.) Where space constraints result in necessary steeper grades, underpasses must still comply with ADA requirements for ramps with landings. Underpasses grades and lengths should allow for clear sightlines at the approach and through the underpass.

Width

- Width recommendations are 14- to 16- ft, but underpasses longer in length and expecting high use in urban areas should be wider. Where width of an underpass is sufficient to permit vehicle passage, designs should include some type of deterrent with the preferred choice being a forgiving bollard since more robust bollards may present safety issues for pedestrians and bicyclists.

Vertical Clearance

- A minimum vertical clearance of 10-ft is required. Designers should consider additional height when it would provide clear sight lines through the underpass,

Lighting

- When provided, lighting for pedestrian and bike underpasses should allow users to see one another in nighttime conditions to avoid collisions and improve the perception of personal security.