Cars may be squeezed out of Auckland's main street as the city's transport authority looks to modern trams to move growing throngs of commuters.
Auckland Transport chief engineer Steve Hawkins warns there will be insufficient space in much of Queen St for general traffic to co-exist with trams running in each direction every few minutes between the waterfront and Dominion Road.
"For the section between Wellesley St and Customs St, we would essentially have just light-rail vehicles and pedestrians," he told Auckland Council's infrastructure committee.
But he said allowing cars to keep using Queen St south of Wellesley St "would be possible" and there would be enough room for a traffic lane each side of tram tracks along routes such as Dominion Rd and Fanshawe St.
Mr Hawkins was updating the committee on an investigation, due for consideration by Auckland Transport's board at the end of this month, into using trams for the first time since 1956 as a mass transit alternative to buses threatening to clog constrained downtown routes.
Heart of the City business association central manager Tania Loveridge said the organisation was "compelled by the idea of a transport network that significantly reduces the environmental impact on the city".
A study last year found high pollution readings - notably from diesel bus emissions - which indicated air in Customs St at the bottom of the Queen St valley risked failing World Health Organisation standards.
Auckland Transport chair Lester Levy said in disclosing the light rail investigation in January that he wanted to attract private investment to kick off a staged development, starting with a link between Wynyard Quarter and Mt Roskill via Queen St and Dominion Rd which he hoped could be built for less than $1 billion.
Transport planners envisage four isthmus tram routes, three converging on Queen St and the other from Onehunga feeding into Symonds St.
A Y-shaped route from Wynyard Quarter to Queen St via Fanshawe St would be a prerequisite to trams running in 2019 up Dominion Rd and then west along Stoddard Rd to a depot near Richardson Rd in Owairaka.
That would be followed by trams along Sandringham Rd in 2021, then from Onehunga via Manukau Rd in 2030 and on Mt Eden Rd in 2035.
Mr Hawkins said that although Symonds St was envisaged as a second CBD route, it was already carrying about 150 buses an hour at peak times - almost twice the optimal number for efficient operations - and the number needed to be eased before digging it up to lay tram tracks along a 7.5m-wide concrete slab across two lanes.
That meant funnelling up to 18,000 tram passengers an hour into Queen St from Dominion Rd and Sandringham Rd to reduce the number of buses using Symonds St.
Without introducing trams to complement the proposed $2.5 billion city rail link, buses arriving in downtown Auckland were forecast to reach their capacity of 420 an hour by 2023.
His team was looking at light rail systems not requiring overhead lines, along which trams could be re-charged from surface induction plates or by super-capacitors providing 20-second bursts at each station.
Auckland Transport strategy and planning chief Pete Clark said the council body had also started re-looking at the capacity of the Northern Busway to assess whether and when light or heavy rail should be added to another Waitemata Harbour road crossing.