Bridge restrictions for tractors and trailers

A bridge has more than one limit

A bridge can be too low, too narrow or too weak for a tractor combination. Passing one check does not pass the others: a trailer that fits beneath an arch may still exceed a structural weight limit, while an empty machine may be within the weight limit but too wide for the parapets.

Treat every sign and temporary restriction as a separate instruction. This guide uses the Great Britain Highway Code and traffic-sign guidance; confirm local and Northern Ireland requirements for the route you are taking. Do not infer that a bridge is suitable because another tractor crossed it or because navigation offers the road.

Height signs and the highest point of the rig

A red-circle height-limit sign prohibits vehicles above the indicated height. A triangular low-bridge sign warns of restricted headroom. Both deserve a check before approaching the structure; a warning sign is not evidence that your particular machine will fit.

Measure to the highest point in road-going configuration: an exhaust, beacon, folded implement, loader or trailer load may be higher than the cab. Record the measurement in metres and feet and inches if useful, but keep the original measured value rather than rounding down. If displayed units appear inconsistent, use the more restrictive indication and seek clarification.

An arch does not have equal clearance across the carriageway. Approach gradients, resurfacing, camber and movement of the load can alter the available margin. Never assume a published central height is available across the whole width of a wide machine. Do not mount a pavement or move into opposing traffic to chase clearance.

Structural weight limits versus goods-vehicle restrictions

Check both the bridge's applicable gross-weight restriction and any axle-weight restriction. Know the actual loaded weight, plated limits and axle loads for the tractor and trailer, not just the payload. A combination can be within its agricultural gross-weight limit yet exceed a bridge restriction or overload an axle.

A goods-vehicle restriction uses a different sign, showing a goods vehicle, and generally concerns maximum gross weight. It can be an environmental restriction rather than a bridge-strength limit. The sign, supplementary plate and underlying restriction determine the treatment of vehicles, combinations and exemptions. Do not assume every 'tonnes' sign means the same thing, or that each unit being below a number makes the whole combination lawful.

An 'except for access' plate must not be treated as a blanket exception to a structural bridge restriction. Nor can an escort, police movement notification or a farm delivery address authorise crossing a weak structure. If unclear, ask the highway or bridge authority before travelling.

Width restrictions and the swept path

The combination must fit the usable carriageway, not just the gap measured at one point. Account for tyres and implement edges, bridge parapets, barriers and a trailer's off-tracking on the approach bend. A long rig can need more space than its straight-line transport width suggests.

Check the entry and exit geometry as well as the bridge itself. A narrow bridge immediately after a sharp corner may be impractical even when its signed width looks adequate. Do not rely on a soft verge as extra carriageway; it may conceal a ditch or unsupported edge.

What to do if clearance or capacity is uncertain

Stop before the last safe turning or waiting place, not under the bridge. Contact the highway authority or bridge owner with the exact location and the rig's dimensions, actual loaded weight and axle information. Ask whether temporary restrictions, works or a closure apply.

Choose an alternative road that has also been checked for the machine and load. A diversion for ordinary cars may include a narrow lane or another restricted bridge. If you cannot establish a safe and lawful route, do not proceed on the assumption that missing data means unrestricted access.

• Keep a measured transport height, width and overall length available in the cab.
• Know actual loaded combination weight and axle loads.
• Check signs on the approach and at the bridge, including supplementary plates.
• Confirm a safe place to stop, turn or wait before reaching the obstruction.
• Recheck official local notices close to departure; restrictions can change.

If a strike occurs

Stop safely and follow emergency procedures. A strike can damage the structure even when the tractor seems unharmed. For a railway bridge, use the contact details displayed at the bridge to report it immediately and contact the emergency services if there is immediate danger. Do not drive away without reporting the incident.

AgriNav warnings depend on available data and the measurements supplied. They are a planning aid, not a clearance certificate or a live inspection. Real signs, road conditions and instructions from the responsible authority take precedence over a map.