Engineers now say the Hairini Bridge might be able to handle three lanes, raising hopes a congestion-busting "tidal flow" lane might be possible after all.

In March, Tauranga City Council ruled out the traffic light-controlled tidal flow system for Turret Rd pitched by Tauranga architect Mark Wassung and councillor Rick Curach.

Wassung said they went with the traffic light option after being told the bridge could not handle the weight of a third lane of traffic.

Council transport operations centre manager James Wickham said civil engineering firm Beca was asked to make a fresh assessment of the bridge.

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He said part one of their study found the deck of the bridge could fit and handle three lanes of traffic if the footpath was moved and clipped on to the outside of the bridge.

Part two of the report - an assessment of the bridge's foundations, remaining lifespan and earthquake resilience - was due in the next few weeks.

Wickham said Beca was also taking another look at the clip-on options to see whether a 3m wide bi-directional shared cycling and walking path was feasible.

Wickham said the bridge would need to have a life expectancy of another 20-plus years to make it worth the money to build the clip-on and install a tidal flow system.

If the bridge had less life left in it, it may be more cost effective to build a new bridge and four-lane Turret Rd sooner than planned, he said.

The tidal flow option - building the clip-on and adding a reversible middle lane - was preferable because it could be rolled out relatively quickly.

"I am really, really hopeful," Wickham said. "This would be so much cheaper than a whole new bridge and a four-lane highway."

He said adding a tidal flow lane to a road on a section of Whangaparaoa Rd, on Auckland's Hibiscus Coast, cost about $8 million.

How Whangaparaoa Rd's dynamic lane control adjusts for peak afternoon traffic. Source/Auckland Transport
How Whangaparaoa Rd's dynamic lane control adjusts for peak afternoon traffic. Source/Auckland Transport

Wickham said he would like to see Hairini Bridge's flexible lane dedicated to buses and vehicles carrying three or more people to encourage more public transport use and ridesharing.

Both Curach and Wassung, as well as Anna Larsen, a member of the Welcome Bay Transport Forum, were glad the council had kept looking into the third-lane idea.

"It would be a quick and easy solution," said Curach, a sentiment echoed by Larsen and Wassung.

Wickham said council staff were also looking at options to reduce the morning impact of the pinch-point created by Hairini St traffic merging into Turret Rd traffic.

They would be looking at how traffic was flowing after the Maungatapu underpass opened next month.

Extending the third lane up Turret Rd towards the city would be investigated only if it was decided the bridge was feasible, he said.


How would a third lane work? The Whangaparaoa example


Auckland Transport's dynamic lane control trial on Whangaparaoa Rd between Hibiscus Coast Highway and Red Beach Rd fully opened in April after two years of planning.

LED lights embedded in the road surface mark traffic lanes instead of painted lines.

Overhead gantries show drivers which lanes are open.

During peak hours there are two lanes for the direction with the most traffic. Off-peak, the middle lane is a median for turning.

How Whangaparaoa Rd's dynamic lane control adjusts for the morning rush hour traffic. Source/Auckland Transport
How Whangaparaoa Rd's dynamic lane control adjusts for the morning rush hour traffic. Source/Auckland Transport