Long-suffering Welcome Bay commuters are tipped to become the big losers in a trial to re-route peak morning traffic heading into the city from Tauranga's coastal suburbs.

''Are we going to go back to where we were before the underpass opened,'' Welcome Bay resident and city councillor Bill Grainger said.

He was responding to the NZ Transport Agency's announcement of a trial to stop west-bound traffic from using Maungatapu roundabout's off-ramp down to Turret Rd.

Thousands of motorists, including from Maungatapu, will be forced to detour around to the Welcome Bay underpass in a trial introduced to improve traffic flows on State Highway 29.

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The 10-day trial hopes to ease the queues of city-bound traffic using the Maungatapu and Welcome Bay roundabouts.

Grainger said the trial would double the number of traffic lights that motorists coming from Maungatapu and Mount Maunganui had to go through to reach Turret Rd - from two sets of lights to four.

But his biggest worry was the impact the extra traffic would have on the intersection at the Welcome Bay side of the underpass - only three weeks after Welcome Bay commuters finally got relief by the opening of the $45 million underpass.

Grainger said it meant traffic coming from Welcome Bay would be held behind red lights for much longer to accommodate the extra traffic coming from Maungatapu and the direction of Mount Maunganui.

''How will throwing the extra traffic around that loop affect our community at Welcome Bay,'' he asked.

''Are we going to go back to where we were before the underpass opened. And what will it do for the people from Mount Maunganui and Maungatapu if it becomes permanent.''

Grainger said the underpass had eased congestion for Welcome Bay commuters. His experience was that morning rush hour traffic now backed up to the underpass from Turret Rd.

He said the agency should have anticipated what would happen at the roundabouts during planning for the underpass.

''The NZTA is starting to see that there was not as much relief on the highway as it thought. The underpass was built to relieve traffic on the state highway.''

He said highway traffic had begun banking up along SH29 towards Pyes Pa since the new roading arrangements came into force and the former Hairini St link to Turret Rd had closed to all vehicles except buses.

Although roading authorities said the closure was for safety reasons, Grainger believed that traffic could merge safely and he wanted the Hairini St link reopened.

''Let's see what it would do for the flow of traffic on the highway.''

Grainger said, in his view, more should have been done before the trial to encourage traffic exiting Ohauiti Rd to use the underpass, rather than heading into town via SH29 and the Maungatapu roundabout off-ramp.

''It would have put a bit more pressure on Welcome Bay traffic but not as much as this will do.''

Transport Agency project manager John McCarthy said that since the underpass opened, it was highway traffic heading to the city at the Maungatapu roundabout that had slowed efficiency.

''We need to trial ways that may help us find a way to ease traffic flow.''

He likened the trial changes to the path motorists took when they exited a motorway - turning left and going back under the path to join a local road.

''We are trying to quicken the trip for people on the state highway so they are not impacted by the local road traffic.''

McCarthy said that while re-directing the traffic may be a little longer in distance, it should result in a smoother flow along SH29A in the morning peak, and a quicker time to get into the city.

Variable messaging signs would be used to help people navigate their way through the changes, with the phasing of traffic lights changed to support the new movement of traffic.

''It will take a couple of days for the changes to bed in but we need the results to see what further enhancements need to be made before making any permanent decisions,'' he said.

Changing the route in the morning peak for city-bound traffic coming from Te Maunga should result in ''a more efficient journey for all'', Graham said.

The changes will not stop eastbound traffic on SH29 accessing the Maungatapu roundabout off-ramp. However, traffic coming from Te Maunga wanting to access Maungatapu Rd will need to use the Taipari St on-ramp.

The 10-day trial to re-route traffic
- Starts Thursday, July 19.
- Monday to Friday, 6am to 10am.
- Routes revert to normal at all other times.

The trial to re-route Tauranga's coastal commuters through the Welcome Bay underpass has been branded ''a nonsense'' by Maungatapu resident Gary Graham

Graham believed the NZ Transport Agency and city council had not given the new system a fair shot.

The underpass opened on June 22, with a new dedicated city-bound bus lane on Hairini St becoming operational last week.

The retired real estate agent said there was going to be significantly more traffic coming down the underpass than ever before.

Graham said the changes would be inconvenient for Maungatapu residents who, even at 8am, did not find it too bad getting out of the suburb.

''I don't think it will be any quicker.''

There would be more traffic lights and big queues waiting for the lights to turn green by the underpass, he said.

''How will that impact on Welcome Bay?''

Graham said the bottom line was that huge amounts of traffic would still be coming up the highway from Mount Maunganui and Papamoa in the morning peak.

That was why some of them shot up through Maungatapu to miss traffic banked up down the highway, he said.

''Volume is volume. Re-routing traffic is not going to change the volumes.''

- By John Cousins