New $252 million Petone-Grenada project high priority for Hutt

For all but those people firmly in the "no more spending on roads, put all the money into public transport" camp, the Petone-Grenada link is stacking up as the Hutt Valley's priority transport project.

With public submissions now being invited on the draft Hutt Corridor Plan, NZ Transport Agency representatives Jenny Chetwynd, Selwyn Blackmore and Jo Draper last week gave a briefing to Hutt councillors on the $252 million development dubbed "P2G".

They explained a key factor to win government funding, since projects from around the country were ranked for priority, was the benefit cost ratio (BCR)  essentially the benefits divided by the costs.

The most telling benefits were savings in journey time through a shorter route or less congestion, and to a lesser extent, reductions in crashes.

Any project with a BCR below 1 was considered uneconomic over a 30-year time-frame, "as it's not going to deliver enough benefits to make it worthwhile doing", Ms Draper said.

Petone-Grenada (P2G) has a BCR of 1.8, taking into account the economic benefits of boosting the growth potential of Wellington's Lincolnshire Farm area near Grenada, and Seaview-Gracefield.

For comparison, the BCR of Transmission Gully is about 1.2; for a Melling interchange, 0.5; for the proposed grade separated interchange at State Highway 2 and SH58, 2.

Ms Draper, the project manager for P2G, said the four problems driving the project were congestion on SH1 and SH2; the desire for better access to regional growth hubs; poor East-West connectivity and the snarl-ups caused at the Petone and Ngauranga mergers as traffic from different directions is funnelled into one flow.

On the East-West question, she said SH1 and SH2 were essentially divided by a mountain range.

Since everyone knows when a serious smash or landslip closes the Petone-Ngauranga pinch point, the only alternative routes were the winding and distant SH58, or Akatarawa Rd, which Ms Draper said was so windy that on a recent try of it, "I just gave up".

P2G, she said, had a significant "network resilience" benefit.

In terms of the mergers, other work was being done at Ngauranga.

The problem at the Petone merger was not just the volume of traffic, but the propensity of drivers to jockey for gains by switching lanes, causing braking and a domino hold-up effect.

P2G includes a new, grade-separated (flyover) intersection at Petone, and a previous report has said 12,000 vehicle movements a day could be expected to be removed by P2G from the congested Petone-Ngauranga stretch of SH2.

Other options included upgrading SH58, and improvements to SH1 and SH2 to try to deal with the four problems outlined above.

But Ms Draper told councillors that P2G was the most effective and least expensive solution.

Modelling of anticipated traffic flows between Porirua and Petone by 2026 showed the following journey times for the morning peak and afternoon peak:

Via State Highway 1  32 minutes / 25 min Via SH58/SH2  28 min / 24 min Via a new P2G  25 min / 18 min

Answering questions from Petone Community Board member Tui Lewis and Mayor Ray Wallace, the NZTA representatives confirmed there was no intention to choose a P2G route that went through Korokoro Valley or Belmont Regional Park.

It would be a steep road in places, with a gradient of up to 9.5 (Ngauranga Gorge is 8).

But trucks could manage that gradient and two lanes each way were envisaged to allow passing.

The speed limit may be 70kmh.

As well as savings in congestion/journey times, there was another spin-off environmental benefit from the P2G road.

Mr Blackmore said because there was a "much smaller pot" of money available nationally for cycleways/walkways, investigation/design and construction of the Petone-Ngauranga off-road cycleway may not get funding for years.

It had a good benefit-cost ratio, but other cycleways around New Zealand have a stronger BCR.

However, there were practical and financial benefits from doing P2G, the cycleway and the Beach to Bush (Belmont Regional Park to Esplanade) links at the same time  a contiguous approach would get all three across the line, when otherwise the two cycleways/walkways "might not go ahead for years".

Construction of P2G could begin as early as 2015.

The news was not so good for those who wanted early action on a grade-separated intersection at Melling.

Ms Draper said NZTA was aware of congestion problems at Melling, particularly for vehicles turning off SH2 onto the bridge.

But that turning traffic was a relatively small proportion of traffic that flowed north-south, "so the junction as a whole [in BCR terms] doesn't perform that badly".

Ms Draper said modelling had been done.

"I had that peer-reviewed, and then the peer review reviewed, because I didn't believe it myself."

The three checks all showed that overall, in congestion terms, the problem did not push up Melling's BCR to a point where an interchange might get funding.

There were many accidents at Melling, but the accidents are mainly at low speeds, which do not carry a lot of cost.

Councillor Max Shierlaw suggested the irony was that accidents were mainly low speed because the intersection is so congested, so cars do not move quickly through Melling.

In the shorter term, the right-hand turning lane off SH2 could be extended.

NZTA agreed with Hutt City Council general manager Bruce Sherlock that improvements on the city side of the bridge could improve traffic flow that would relieve the risk/congestion on the highway side of the bridge.

Hutt News