How wind tunnelling is having a real impact on road safety

A NEW central reservation barrier to increase safety and reduce maintenance on the Second Severn Crossing is being put to the test in model form.

National Highways have created a 1:70 model of the 5,128m (5,608 yard) Prince of Wales Bridge which carries the M4 over the River Severn.

Replacing a steel barrier with a concrete central reservation is normally straightforward but the environment of the bridge, which opened in 1996, provides a unique challenge.

National Highways needed to understand the impacts of installing a concrete barrier, particularly on the 948-metre cable-stay section of the bridge, that is subject to strong winds.

This section of the bridge can be particularly sensitive to wind-induced oscillation – shaking – if changes are made to the road.

To ensure a concrete barrier is right for the structure and users of the road, National Highways’ Specialist Bridges Group drove an innovative feasibility study, aimed at modelling and assessing the effect of installing a new concrete barrier on the aerodynamic stability of the bridge.

The tests were carried out in a wind tunnel in Canada with experts from National Highways using an internet link observe the effects of wind on the bridge model in a controlled environment.

This assessment, as part of the project’s development stage, was crucial because not correctly identifying the impact of a concrete barrier could lead to how the bridge acts during windy conditions.

Matthew Jones, Asset Needs and Programme Development Manager for the Specialist Bridges Group, said: “We are constantly exploring new innovative ways to design, build and maintain our roads and are committed to making them safer. 

“We believe that embracing innovation and working closely with engineering specialists is the path to more efficient and safer projects.

“By undertaking modelling, trials and tests like these, we can ensure we’re making the right changes to see significant long-term benefits and improve the experience of our roads for motorists.

“Upgrading these barriers will improve journeys and significantly reduce the risk of vehicles crossing over from one carriageway to another, improving safety and reducing the duration of incident-related congestion.”

National Highways worked with specialist consultant RWDI, which also has experience with other well-known bridges such as the Golden Gate Bridge in San Francisco.

Studies allowed the proposed design to be tested under different wind conditions and scenarios using the specific dimensions and cross-section of the Prince of Wales Bridge.

Tests showed ‘the new solid median will behave virtually identically to the bridge in its current state’ and means construction is likely to start next year.

The proposed concrete barrier is a significant improvement compared to the existing metal one and will reduce the risk of vehicles crossing over from one carriageway to another, improving safety and reducing the likelihood of incident-related congestion.

The new barrier will last twice as long as the original metal barrier, with far less need for closures for routine repairs.

Patrick Madden, designer for civil engineering firm Amey, added: “Upgrading the existing barrier to bring it in line with current standards is vital, and it has a significant impact on maintenance requirements of the barrier.

“The current barrier requires partial closures of the bridge for ongoing maintenance, and these interventions can impact the bridge structure itself as the current barrier is bolted to the deck.

“The proposed barrier will require significantly less maintenance, as it is less prone to damage that requires interventions, and it does not require mechanical fixing to the deck, reducing the impact on the bridge structure when maintenance is required.”

To find out more about the Severn Bridges, visit