Container trucks and all others weighing 13 tonnes or more will be banned from the two outer clip-on lanes of the Auckland Harbour Bridge from Thursday.
Transit NZ has softened an earlier plan to declare the lanes - one in each direction - off limits to any vehicle heavier than 4.5 tonnes unless it is a bus.
The original plan would have banned anything looking like a truck, to avoid what Transit feared may be difficulties explaining the cut-off point of a new bylaw to protect the clip-ons, which were added in 1969. But northern operations manager Joseph Flanagan said yesterday that a high level of voluntary compliance with a request for trucks to keep off the outer clip-ons until the bylaw could be introduced had persuaded Transit to be more lenient.
Transit will count on the police to enforce the bylaw, which will carry a fine of $150 for any breaches.
But Mr Flanagan said the bylaw would allow truck drivers enough room to cross to the inner clip-on lane from the northbound Curran St on-ramp, and to leave the motorway southbound up Shelley Beach Rd.
He said the restriction would not lead to a reduction of the bridge's annual maintenance cost of around $5 million, but would minimise the danger of overloading it and damaging any of the clip-on beams.
He said Transit wanted to ensure the clip-ons lasted "indefinitely" and it would consider adding more steel to them in future years to allow heavy trucks back on their outer lanes.
Transit's announcement in May that it would ban trucks from those lanes sparked concern among Auckland leaders about the longevity of the bridge, and a debate about when to plan for another harbour crossing.
The highways agency was close to applying for a designation to protect a route through the Tank Farm for a possible $3 billion tunnel between Northcote and Spaghetti Junction, before deciding to wait for results of a joint investigation with the Auckland Regional Council and other local partners of the best new crossing option.
Regional council chairman Mike Lee favours a tunnel to the east of central Auckland to take traffic away from the central motorway chokepoint and others such as Auckland City transport chairman Richard Simpson want a road-rail bridge between the Tank Farm and Northcote to replace the existing structure and free up waterfront land along St Mary's Bay.