The Waterview Tunnel is still making car journeys quicker than before the $1.4 billion project opened a year ago on Monday, but travel times have increased on a couple of key routes since October, figures show.

The much-anticipated twin tunnels — at the heart of a Weekend Herald feature today on the people working in the tunnel, and those making a life above it — were a key part of the Western Ring Route, aimed at providing an alternative to Auckland's under pressure State Highway 1 and better linking the city's sprawling isthmus.

Motorists rejoiced in the below-ground transport link last year, with travel times on key motorways and arterial roads plummeting, according to New Zealand Transport Agency data.

In October last year, four months after the tunnels opened, it was taking an average of 49 minutes to travel from Papakura to Auckland's CBD on State Highway 1 during the morning peak, compared to 46 to 70 minutes before July 2.

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However, average travel times on the same route increased to 56 minutes last month.

Morning commuters travelling from Westgate to the CBD via SH16 also face longer journey times since October, when it took an average of 20 minutes.

Last month the same route took an average of 28 minutes. Before the tunnel motorists could expect a 21 to 32 minute journey.

It was also taking longer since October to travel from Auckland Airport to the CBD via the tunnel in the afternoon peak, 33 minutes last month compard to 25 minutes.

One route had seen a significant drop in travelling time — morning travel from the CDB to the airport via Manukau Rd and Gillies Ave now took an average of 22 minutes, compared to 27 minutes in October. The average before the tunnel opened was 24 to 32 minutes.

Travel times were steady on other key routes — Silverdale to the CBD via SH1 (morning) and the airpt to CBD via Manukau Rd and Gillies Ave (afternoon) were within a minute of October's average journey times of 34 and 33 minutes respectively.

Driving from the CBD to the airport through the tunnel in the morning now takes an average of 26 minutes, compared to 24 minutes in October.

NZTA director of regional relationships Steve Mutton said road works and speed restrictions north and south of the CBD had affected some of the benefits of the tunnel.

Nearly 22 million vehicles have driven through the tunnel since it opened and the tunnel's greatest benefit was the reliability it gave.

"Auckland drivers have spent the last year working out how the Waterview Tunnel works best for them. The greatest benefit is that it's made most journeys more reliable, even for drivers not using the tunnels ... the journey reliability is despite Auckland's growing population and vehicle numbers."

Auckland's population grew by 42,700 in the last year and registered cars increased by about 25,000 a year, Mutton said.

Every day an average of just over 60,000 vehicles used the tunnel, with about 2000 more vehicles travelling north than south. On weekdays, daily totals can exceed 80,000, he said.

For some of those users, however, there was a sting in the tail following their journey through one of the country's biggest roading projects.

In the nine months since the tunnel opened 34,149 speed camera tickets were issued, with 2729 in the first month and peaking at 7245 in December, according to the police.

The nominal value of those tickets nudged just past $3m — a total that if continuing at the same rate would average $4.5m a year.

If the tunnel's $1.4b cost were to be paid off by speed camera tickets it would take 311 years.

There's relief in sight for motorists, with the speed limit on motorways around the tunnel to increase to 100km/h next month, following public consultation.