Underpass is a grade-separated crossing that carries pedestrians, cyclists, vehicles, livestock or wildlife beneath a road, railway or other linear infrastructure. Underpasses remove conflict between traffic streams without the ramps and exposed structure of an overbridge, and are commonly constructed as buried concrete boxes or corrugated steel arch and box structures.
Types of underpass
- Pedestrian and cycle underpasses (subways) — providing safe crossings beneath busy roads and railways.
- Vehicular underpasses — carrying minor roads, accesses or agricultural traffic beneath major routes.
- Accommodation and livestock underpasses — connecting severed farmland.
- Wildlife underpasses — designed to DMRB LA 109 and ecology guidance to maintain habitat connectivity for mammals and amphibians.
Buried corrugated steel underpasses
Corrugated steel arch and box profiles — such as ViaCon’s ViaPlate® and SuperCor® systems — are the standard low-disruption method for creating underpasses beneath live infrastructure. Plates are bolted together beside or beneath the route, the structure is backfilled with compacted engineered material, and traffic loading is carried by soil-steel composite action. Spans up to around 25 m accommodate everything from a small mammal tunnel to a full vehicular underpass, with installation typically completed in days and no bearings, joints or deck to maintain.
Design considerations
Key design drivers are clearance and headroom (2.4 m minimum for pedestrian subways, more for shared and equestrian routes), natural light and sightlines for personal security, gradient and drainage — underpasses are low points, so pumped or gravity drainage and flood exceedance routing must be resolved — plus durability and waterproofing of the buried structure. For wildlife underpasses, substrate, light and noise conditions determine which species will use the crossing.
Related ViaCon solutions
See ViaCon Buried Bridges and Flexible Steel Culvert Solutions for underpass structures, or explore the full Bridges & Culverts range. Related glossary entries: wildlife crossing, box culvert and footbridge.
Frequently asked questions about underpass
What is an underpass?
An underpass is a grade-separated crossing that carries pedestrians, cyclists, vehicles, livestock or wildlife beneath a road, railway or other infrastructure. It is typically built as a buried concrete box or a corrugated steel arch or box structure.
What is the minimum headroom for a pedestrian underpass?
UK guidance requires at least 2.4 m of headroom for pedestrian subways, with 2.7 m or more preferred for shared pedestrian and cycle routes and greater clearances for equestrian use. Generous headroom also improves natural light and personal security.
How is an underpass built under a live railway or road?
Buried corrugated steel structures are bolted together from prefabricated plates adjacent to the route, then installed and backfilled during short possessions — or jacked/slid into place — so traffic disruption is limited to days rather than the months required for cast in-situ concrete.
What is the difference between an underpass and a subway?
In UK usage a subway usually means a pedestrian-only underpass beneath a road. Underpass is the broader term covering pedestrian, vehicular, agricultural and wildlife crossings beneath any linear infrastructure.