Transport Agency engineers are investigating early-warning measures to reduce chances of overloading Auckland Harbour Bridge with gridlocked trucks.
Although the agency insists the bridge's clip-on girders remain capable of surviving anything less than a one-in-2000-year disaster, a senior official has acknowledged room for improvement in averting such an event.
Consulting
engineers warned the former Transit NZ of a worst-case scenario of a possible "catastrophic failure" if both lanes of a clip-on became jammed end to end with laden trucks, particularly during an early-morning period of heavy northbound container traffic from the port.
That prompted the organisation, which evolved into the Transport Agency last year, to add 760 tonnes of steel to the clip-ons in a $45 million structural strengthening project which is half completed.
Transit CEO Rick van Barneveld told Annette King in 2007 when she was Transport Minister that an end-to-end truck jam would be impossible because of continuous monitoring with CCTV cameras linked to the Northcote traffic management centre.
But Transport Agency northern operations manager Joseph Flanagan told the Auckland Regional Transport Committee on Wednesday that a serious jam could develop over a relatively short period as the bridge was "not as long as it looks".
He said the agency was investigating ways of alerting trucks to prevent them from reaching the bridge in time to avoid such a jam, because "at this stage we don't have a workable solution that can guarantee we will stop the traffic".
Mr Flanagan said the heaviest loads generally occurred between 3am and 5am. Neither of the clip-ons is under any immediate threat of becoming fully clogged by a truck jam, as heavy vehicles have been prohibited from using the outside lanes for the duration of the structural upgrade.
Trucks were banned from using the entire northbound clip-on until this week, but have been allowed back on the inner lane since Monday because of the completion of welding below the road surface.
Mr Flanagan told the transport committee that strengthening work due to start on the southbound clip-on would mean banning trucks from both its lanes in about two weeks. The agency has yet to decide whether to allow trucks back on the outside lanes once the project is completed.
