Despite the extra motorway capacity, the agency is trying to dampen expectations of free-flowing traffic at all times from the Southern Motorway to the harbour bridge from Monday.
Northern highways manager Tommy Parker said yesterday that delays remained possible while drivers "settled down" with the new configuration.
Although tunnel throughput would increase, drivers emerging from its northern portal into St Marys Bay would have to get used to seeing vehicles joining the motorway from both sides of them, from Fanshawe St and Beaumont St.
That was because an extra lane would be opened at peak times from both those streets, to the right of the tunnel, in addition to the all-hours lane already operating from the left of the motorway. Mr Parker said the extra lane would be unique to the motorway network, but would be "gated off" at normal times.
His caution about the immediate impact of the improvements follows traffic queues beyond Otahuhu caused by nervous motorists after the Victoria Park tunnel was opened last year with just two of its three lanes.
Shorter queues of northbound traffic have persisted since then, although the agency says southbound drivers gained immediate time savings of six to eight minutes when all four lanes of the Victoria Park motorway flyover became available for their exclusive use in January.
Mr Parker confirmed a possibility that his organisation may decide to keep an on-ramp to the motorway from Wellington St, just to the south of the tunnel, closed to most traffic.
That would depend on how traffic flows panned out once drivers got used to the extra lanes through the tunnel and St Marys Bay.
He said the agency's priority was to improve traffic movements through Spaghetti Junction, from the port and the Northwestern Motorway as well as State Highway One.
It wanted to guard against further delays from a merger of traffic from Wellington St with vehicles headed for the tunnel.
Mr Parker was unable to indicate the cost of an upgrade to the Wellington St ramp before the cut-and-cover tunnel was dug, but said that was necessary as the ramp was an emergency evacuation route.
A retaining wall above it had to be strengthened for the project and discussions with Auckland Transport may result in its use by buses, as well as emergency and motorway service vehicles.
Although the ramp was used by more than 16,700 vehicles a day in 2006, that was before direct links to SH1 from the port and the Northwestern Motorway were completed, and traffic slumped to less than half that volume before it was closed for the tunnel work.
ON THE WEB
For a video animation of how the new peak traffic lane will work from Fanshawe St, visit www.nzta.govt.nz/projects/victoria-park-tunnel/video-fanshawe.html.
