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Thanks, TWR, but that makes no sense at all to me, I'm afraid. From what you say, you would happily break the law, because the worst thing that could happen is the roof flying off your trailer!!
Scenario....... lorry crosses bridge which has been closed to high-siders due to high, dangerous winds, driver swerves and takes out car (or cars) killing a number of people. What then? Does he get away with it, because he thought the weight of the vehicle would hold it down?? I don't think so.
If a sign says "low bridge ahead" or "width restriction 6ft 6ins" the driver would find alternative route. To me, "road closed to high-sided vehicles" should mean (to the driver) ROAD CLOSED.