Underpasses

An underpass is a passage that allows pedestrians, vehicles, or trains to pass beneath another road, railway, or other barrier. By separating levels, underpasses reduce at-grade crossings and can improve safety and traffic flow in dense urban areas. They are common at busy junctions, near railway stations, and where space constraints make grade separation advantageous.

There are several main types: road underpasses designed for vehicular traffic; pedestrian underpasses that enable people to cross under a street or rail line; and combined structures that accommodate both. Some underpasses pass beneath elevated expressways or bridges. The term is often distinguished from tunnels used for through traffic, though an underpass may have tunnel-like features.

Design considerations include structural support such as retaining walls or box culverts, drainage and flood protection, waterproofing, lighting, visibility, and ventilation in longer passages. Accessibility for people with disabilities, emergency egress, and security measures are also important. Maintenance typically covers drainage cleaning, waterproofing, lighting replacement, and monitoring for water intrusion and cracking.

Advantages of underpasses include reduced vehicle conflicts, smoother traffic flow, and safer crossings for pedestrians away from busy roads. Drawbacks can include higher construction and maintenance costs, potential for flooding and water ingress, and safety concerns if the passage is poorly lit or policed. Modern practice emphasizes reliable drainage, lighting, accessibility, and regular inspection.