CROSS Safety Report
Mobile message board in strong wind
This report is over 2 years old
A reporter raises concerns over mobile message boards adjacent to motorways after one moved into the motorway lane in windy conditions.
Key Learning Outcomes
For asset owners and service providers:
Supplier’s instructions regarding safe deployment should always be followed
Consider implementing a wind management plan which can assist in the safe operation of mobile message boards
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A mobile variable message sign was set up on a hard standing away from the hard shoulder on a motorway. When the wind caught the sign it acted as a sail and moved onto the region of the hard shoulder and lane one.
The manufacturer’s instructions advised that the sign should not be erected when the wind is over 25mph (11 m/s). The reporter suggests that this is not a safe approach. It mirrors an approach taken in the entertainment industry with stage safety. (See comments below on this aspect).
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This emphasises the need, and obligation, for a risk assessed approach. A moving sign on a motorway could have had catastrophic consequences and is in the category of a ‘high impact, low probability’ event. The 2011 Health and Safety Executive publication Preventing catastrophic events in construction contains the following warning:
If the wind design speed is 11m/sec and it happens to gust to 13 (who can tell accurately at such low speed?) the force jumps to (13/11)2 = 1.4 which might well eliminate any stability safety margin. The standard safety lesson is enshrined in the word ‘sensitivity’. How sensitive is safety to the basic design assumptions?
The Highways Agency made the following statement:
In the entertainment industry outdoor temporary stages are frequently designed in accordance with the Institution of Structural Engineers publication: Temporary demountable structures. Guidance is given on the active management of such structures in the event of high winds. This, however, would not be practical for unattended items such as motorway signs.