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Buses are at risk of losing their supremacy in Auckland City's most established commuter bus lanes, along 4.5km of Dominion Rd.

The city council's transport committee has overruled staff advice by proposing that cars and other vehicles carrying at least one passenger be allowed into the 11-year-old lanes, as part of a $50 million-plus upgrade of the route.

In a move condemned by minority City Vision and Labour councillors and public transport supporters, the Citizens & Ratepayers-dominated committee has decided to consult business and land owners and "stakeholders" such as the Auckland Regional Transport Authority on whether to convert them into "T2" status. That would mean buses sharing space with any other motor vehicles with two or more occupants, although cyclists stand to gain their own lanes from the four-year upgrade starting in January, 2012.

Committee chairman Ken Baguley said yesterday that although council staff had recommended renewed bus lanes for the project, which will include widening the road by between 1m and 2m, the C&R caucus believed the consultation round should begin with the T2 proposal. But he promised his committee would heed submissions when considering a staff report in August or September.

Authority spokeswoman Sharon Hunter said last night that Dominion Rd was one of the busiest bus corridors outside the central business districts, and allowing cars or vans into the priority lanes would reduce the benefits to bus passengers "to an unacceptable extent or create safety risks that cannot be adequately mitigated".

Mr Baguley said a trial in which a 1.6km bus lane in Tamaki Drive has been opened to vehicles with one or more passengers was proving a successful utilisation of road space, by encourage car-sharing.

An earlier staff proposal for Dominion Rd was for 24-hour bus lanes, which he believed would have been harder for motorists to accept than if they had a chance to reduce journey times by carrying passengers.

Council staff have reported an increase from 24 per cent to 33 per cent in people travelling in vehicles with at least two occupants between 7.30am and 8.30am along a 1.38km surveyed stretch of Tamaki Drive.

A priority lane previously used by 13 buses in that hour now carries 374 vehicles, representing what the staff report calls an increase in corridor efficiency.

Auckland Tramways Union president Gary Froggatt said he was not opposed to the Tamaki Drive trial, but Dominion Rd was a far busier bus route, and opening the bus lanes to other traffic would cause "major safety problems with cars moving in and out all the time".

Bus and Coach Association chief executive Raewyn Bleakley said Dominion Rd was a very successful part of Auckland's public transport network where the lanes should be retained.

* Established: 1999.

* Length in each direction: 4.5km.

* Bus numbers: 53 citybound services between 7am and 9am (one almost every 2 minutes). 53 outbound services 3pm-6pm (one almost every 3 minutes).