The Design Manual for Roads and Bridges TD 27/05 “Cross-Sections and Headrooms” gives guidance on design headroom for new bridges in Section 6. Table 6-1 (reproduced below) indicates overbridge headroom is a minimum of 5.3 m, or 5.7 m for footbridges or structures vulnerable to vehicle impact.
| Type of Structure | New Construction Headroom (m) | Existing Structure Headroom (m) |
|---|---|---|
| Overbridges | \(5.30 + S\) | \(5.03 + S\) |
| Footbridges, sign/signal gantries and other structures vulnerable to vehicle impact | \(5.70 + S\) | \(5.41 + S\) |
| Free standing temporary structures | N/A | \(5.41 + S\) |
| All Permanent Structures over High Load Routes | \(6.45 + S\) | \(6.18 + S\) |
where \(S\) is the maximum deflection plus any construction tolerances.
Note that this headroom is to be maintained above any area a vehicle could feasibly reach, e.g. above footpaths and central reservations.
NR/L3/TRK/2049 gives clearances (in mm) for structures over railway lines