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At peak holiday periods at least 120,000 vehicles a day cross Avonmouth bridge. Right from the beginning of the project the prime directive was 'Thou shalt keep six lanes of traffic open at all peak times', a state of affairs only made possible by the provision for widening made by the original designer (NCE 23 November 1995). For the chosen strengthening solution to work, not only had all traffic to be diverted so that it loaded only one box girder at a time, but the very surfacing itself had to be removed from the unloaded box girder to reduce stresses still further.

This put an enormous premium on traffic management on the M5. A battery of CCTV and speed cameras was installed four years ago and so far really serious delays have been rare. 'There are steep uphill gradients both sides of the crossing, which tend to cause bunching and congestion anyway,' explains Hyder traffic management engineer Marian Lewis.

'And while cone 'wipe-outs' are fairly common, there have only been three road traffic accidents on the bridge in more than four years.'

Narrow lanes help keep down traffic speeds through the 50mph speed limit, although the cameras are triggered more than 300 times every day. Rumour has it that the fastest car detected was doing over 150km/hr.

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