A new official report on Auckland Harbour Bridge traffic has taken the road-only option off the table and replaced it with rail-only.

It's a big shift in thinking. The report also says we're well behind with the planning.

The Auckland Harbour Bridge will be at full capacity for all transport modes by 2030. But a new Waitematā harbour crossing will take at least 10 years to plan and another five to seven to build.

And as last October's ministerial briefing paper from the New Zealand Transport Agency (NZTA) reveals, that's not the only crisis looming over transport to and from the North Shore.

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The paper has been posted on the NZTA website and it paints a stark picture.

On the one hand, the number of vehicles using the bridge throughout the day, around 170,000, is the same as it was in 2006. And the number of people heading into the central city in cars in the morning peak (7am-9am) has been sitting around 20,000 for 25 years.

But this lack of private vehicle growth overall reflects the existing congestion: there's no room for any more cars. Already, says the paper, "the road connections either side of the Auckland Harbour Bridge cannot carry any more private vehicle trips ... in the peak periods".

NZTA rates some of the roads leading to the bridge as "severely congested", meaning there is a "certainty of flows breaking down in peak periods".

However, while the number of people in cars crossing the bridge is static, the overall number of people who make the crossing has risen sharply.

The growth is in bus patronage. It's up 250 per cent over the last 25 years. There are now more than 1000 bus crossings a day on the bridge and, in the morning peak, 38 per cent of all people using the bridge are bus passengers.

That figure soars to 58 per cent if you count only the people who cross the bridge in the morning peak and finish their journey in the central city.

NZTA expects the Northern Busway to hit capacity by 2030.

The paper suggests that because of this, the city will need a new Waitematā harbour crossing by then. It's just 11 years away.

Traffic congestion on the Auckland Harbour Bridge. Photo / file.
Traffic congestion on the Auckland Harbour Bridge. Photo / file.

But as Transport Minister Phil Twyford told the Weekend Herald, the planning and approvals process for that crossing will take "no less than a decade". And, he said, construction is "likely to take five to seven years". Let's say it will take at least that.

That means there will be a gap of at least five years, perhaps even 10, when on current projections the Auckland Harbour Bridge will simply not be capable of handling demand.

Adding to the problem, the number of trucks is also rising sharply. There are now 11,000 heavy vehicles on the bridge per day, most of them trucks. By 2046, that number is expected to reach 26,000: an increase of 136 per cent.

Next surprise: the NZTA paper reveals there has been a big shift in thinking about what a new harbour crossing might be. The options used to be: road, or road and rail.

Now, the road-only option has been dropped and a new one – rail only – has been added.

"Both those things are new," Twyford told the Weekend Herald.

The modelling that went into the paper's findings was done for light rail, but the planning can be converted to heavy rail, at least in the early stages, if that is preferred. Either way, the planners assume the Northern Busway will be converted to rail use.

"What we want is a better network," Twyford said. "People have to be able to get to where the jobs are. A rapid transit network does that better than a radial system where everything leads to the centre."

Already in Auckland, 53 per cent of people travelling over the bridge in the morning peak go round the city centre, mainly heading for destinations to the south and west.

The paper doesn't make any recommendations about whether the crossing should be a bridge or a tunnel. A bridge would be cheaper and slightly more resilient, but it has other problems. It would disgorge traffic at the water's edge, which means more cars at the choke points. And it would be harder to get consents and other approvals, which means it would take more time to plan and build.

So, road and rail, or rail only? Here's the problem.

It's well known among planners of all kinds that the more you provide of a service or facility, the more it will be used. The more memory your company devotes to email, for example, the more you will store old emails. The more they build roads, the more people will drive on them.

People drive less when it's preferable to use another option. The factors creating that preference might include cost, safety and convenience, health and fitness, a concern about climate change or the quality of life for pedestrians in the city.

But for many people, looming over all those factors is the anguish of traffic congestion. Once you decide there's got to be a better way to live than getting stuck in traffic twice a day, you'll look for other options. If they exist, you'll start to use them.

That's what's happened on the bridge. Despite rapid growth in the number of vehicle owned in Auckland, there has not been anything like the same growth in the number being driven. In 2016 and 2017 an average of almost 172,000 vehicles crossed the bridge each day: only very slightly more than the 169,000 that crossed in 2006.

The number did grow by about 12 per cent over the five years before 2006, but then it stopped. It fell with the global financial crisis in 2008, and then steadily climbed back. Now it is at capacity and seems to be stuck there.

The difficult-to-grasp secret about Auckland traffic is that the congestion levels we experience now are about right for pushing commuters to choose other options.

Another way of looking at that? Congestion may never be fixed.

Add more roads and you'll encourage people back into their cars, so the new roads will quickly become as congested as the existing roads, and we'll all be back where we started.

That's what is likely to happen if we add another road crossing to the Waitematā.

The paper identifies other problems with adding a new road crossing. It will generate more cars in the city centre, and already there is nowhere for them to go. This, at a time when the strategic plan for the city centre is that it loses a great number of the cars it has now.

A new road crossing will also require extensive widening of the existing access routes. The paper identifies many of them, including the motorway from Northcote Rd to Constellation Drive, Esmonde Rd west of Barrys Point Rd, Onewa Rd and the Victoria Park section of the motorway south of the bridge.

All of this suggests the smart option is not just to dispense with the road-only option, as NZTA has already done, but to opt for a rail-only crossing, probably a tunnel, as part of a bigger and stronger rail network.

When asked, Minister Twyford was not prepared to commit to that.

Heavy vehicle use of the Auckland Harbour Bridge has grown by 30 per cent since 2012. Photo / file.
Heavy vehicle use of the Auckland Harbour Bridge has grown by 30 per cent since 2012. Photo / file.

Still unresolved: the question of freight. The paper says it would cost $40 a truck to reroute using the alternative Upper Highway Crossing. That's not going to be popular: it translates to the freight industry having to spend – and pass on – hundreds of millions of dollars more in costs, each year.

Which points to heavy rail. Has the time come to look at re-establishing the rail link to Northland? Should that go around the west of the city, as the old line still does, or should it run alongside State Highway 1, as the busway does, and through a new tunnel under the Waitematā?

Should that tunnel perhaps have the capacity for light and heavy rail? And where should it be located?

Despite the NZTA paper, the whole question of a new harbour crossing languishes at the back end of the 30-year joint strategic agreement between the Government and Auckland Council, called the Auckland Transport Alignment Accord (ATAP). But on this question at least, the paper implies ATAP is already out of date.

There are many decisions to be made. As the paper proposes, we need a business case – which does not yet exist – so we can start making them.