Cable corrosion forces M48 Severn Bridge weight restrictions
National Highways has announced it is to implement a 7.5t weight restriction for heavy goods vehicles (HGVs) on the M48 Severn Bridge from 27 May. Routine surveys have revealed corrosion of the main bridge cables is worse than anticipated. By banning heavier vehicles National Highways says it can keep the motorway suspension bridge open for most traffic.
image credit: National Highways
The problem is attributed to exposure to hostile marine and weather conditions all year round. Furthermore the bridge wasn't designed and built for today’s levels or weight of traffic – and since the removal of tolls in 2018 there's been a 34% increase in traffic.
Traffic modelling, says National Highways, suggests most HGVs over 7.5t have a destination within 30 miles, meaning the weight restriction will have a localised impact. Annual average daily traffic across the bridge was 31,936 vehicles in 2024, with HGVs over 7.5t accounting for approximately 10% of the traffic.
The Severn Bridge was opened in 1966 by Queen Elizabeth II. For thirty years, the bridge carried the M4 motorway. Following the completion of the Second Severn Crossing, the section of motorway from Olveston in England to Magor in Wales was designated the M48. The bridge was granted Grade I listed status in 1999.
Weight restrictions are not, in fact, new to the bridge and some have been in place for much of the structure’s life following the discovery of structural weaknesses in the late 1970s. The first intrusive inspection of the bridge’s main cables was undertaken in 2006, following reports in the USA that that some of its older major suspension bridges were showing signs of deterioration, and since then HGVs have been restricted to a single lane in each direction.
National Highways expects the 7.5t weight restriction will remain in place for an estimated 12 to 18 months while it develops a medium-term solution. This is likely to be based upon technology to control access to the bridge and ensure vehicle loads from heavier HGVs remain within safe limits at all times.
In the longer-term National Highways is looking at the feasibility of adding more cables, which it estimates could cost between £300m and £600m.
In the meantime, National Highways has assured stakeholders that the multiple structural health monitoring systems active on the bridge indicate the bridge is currently safe to use. Continued use of the bridge by heavier vehicles could however ultimately see it closed to all traffic.
During the weight restriction period, the bridge will remain open to all buses and coaches, emergency vehicles, gritters and recovery vehicles.
All HGVs over 7.5t will need to use the M4 Prince of Wales Bridge to cross the Severn Estuary, a diversion of 14 miles.
For planned overnight closures of the M4 bridge, National Highways will allow HGVs over 7.5t to use the Severn bridge but will put control measures in place to make sure there aren't too many heavier HGVs on it at any time.
If there is an unplanned closure of the Prince of Wales bridge, HGVs over 7.5t will be routed via the M5 and the M50, a diversion of approximately 100 miles.