03 November 2005

Transmission Gully - The Real Story - Part 4

In Part 3 I summarised the funding that Transmission Gully cost far more than previously estimated and how Transit and the Wellington Regional Council responded to that – by calling for a more comprehensive study into transport along the corridor. I also summarised the government’s additional funding for Wellington transport, without the Western Corridor. In this part I go through the key study findings and what it really means. As submissions on the draft Western Corridor Plan are due in by Friday 4 November, I thought I better accelerate these posts in case anyone wants to make a submission. Details on the study are available at the Greater Wellington Regional Council website for the project

The Western Corridor Transportation Study

The Western Corridor Transportation Study goes through the basic stages of public policy analysis:

- Problem definition;
- Option identification;
- Option analysis;
- Recommended packages.

The project has three phases:

- Initial consultation (identifying issues, priorities and options that the public wanted considering);
This stage screened out options that were not worthy of further consideration (including electrification of the rail line to Otaki and converting Akatarawa Road into a major highway) and provided the raw data for putting together packages of projects that were complementary.

- Scenario testing (development of different packages of projects and consultation on those).

- Development of draft corridor plan (a preferred package for consultation).

This package has been consulted on by Transit and the regional council, with submissions closing on Friday.

The purpose of the study is to reach a Western Corridor Plan to be adopted by Transit and the Greater Wellington Regional Council. Another part of this is that the findings about the best transport options would inform advice to Ministers about what level of additional taxpayer funding should be provided for the Western Corridor.

Key findings

In terms of problems along the corridor, the key findings are:

1. Ngauranga Gorge has the worst congestion by far.
2. Traffic growth is greatest through Paraparaumu/Waikanae.
3. Trip reliability is the biggest issue of public concern– not safety or regular consistent congestion.
4. Paremata/Mana will be eased by the current improvements, but congestion will be back to current levels within ten years.

Congestion is low between Plimmerton and Mackays Crossing. Yes I know sometimes you get delayed for a short period on some evenings there, but on average, the traffic flows relatively well. Congestion at Pukerua Bay is starting to emerge at peak times, but congestion between Pukerua Bay and Mackays Crossing wont be serious for a bout 10-15 years. The biggest issue for the public is the route remaining open. Congestion is far more serious south of Tawa and particularly south of Johnsonville.

In addition, the main safety concerns largely relate to intersections on the highway (particularly between Raumati and Peka Peka) and the coastal stretch of highway.


The key findings in relation to options are as follows:

1. The main weaknesses in the rail link are the single track sections between Pukerua Bay and Raumati. Passenger rail service frequency can be increased to 15 minute intervals at peak times to Paraparaumu only if some double tracking is carried out from Mackays Crossing to Raumati ($40 million). Further double tracking from Pukerua Bay to Paekakariki would improve reliability (as the line would be at capacity) but would cost another $280 million more – this is hardly worth it.

2. Rail alone wont fix congestion, as rail improvements will shift 500 people a day from car to rail, but only remove 100 vehicles from Ngauranga Gorge at peak times. At best some low cost enhancements (bigger park and ride) and improvements that would happen anyway (new trains and more frequent services) may delay growth in congestion, but beyond that rail improvements are very expensive and will deliver little.

3. Completely free flowing road capacity between Kapiti and Wellington at peak times will reduce rail patronage by 10% and will probably increase congestion south of Ngauranga.

4. North of Mackays Crossing, the Kapiti Western Link Road would reduce congestion in Paraparaumu and Waikanae, and improve reliability by providing a second crossing of the Waikanae River. In the longer term, an expressway following much of the current highway would reduce crashes and further relieve congestion, including providing options to bypass central Paraparaumu and Waikanae townships. There is little debate about this and Transit has the Kapiti Western Link Road – Stage 1 – in its 10 year State Highway Plan.

5. South of Linden, the Petone-Grenada link road would improve linkages for freight between the Hutt and the north, and the Hutt and Porirua, as well as providing congestion relief for the most congested parts of the Western and Hutt Corridors (Ngauranga Gorge and Ngauranga-Petone respectively). Petone-Grenada was the MOST ECONOMICALLY EFFICIENT PROJECT evaluated in the study – it would generate the same level of benefits as Transmission Gully, but for only $180 million. Dr. Cullen and Peter Dunne might want to debate this, but it does show what value there is in politicians keeping out of the way – it is clear that the Petone-Grenada road is well worth building.

Now the real debate is between Linden and Mackays Crossing.

The BIG issue Gully vs Coast- same benefits, different cost

Some of the biggest myths need reiterating here. The best projects on the Western Corridor are NOT on this section. There are far greater benefits in building Petone-Grenada and a Kapiti expressway than either Transmission Gully or 4-laning the coast road. Simple as that. Why? Because the congestion at Ngauranga Gorge and between Hutt and Ngauranga is worse, the congestion in Kapiti is growing faster, and the crash rate in Kapiti is higher than along the coast road (plus the coastal crashes can largely be prevented by a median barrier).

Most roading projects in New Zealand proceed not because of politicians (hilarious seeing local MPs claiming credit for new roads – Winnie Laban and Darren Hughes had absolutely no influence at all on when the Mackays Crossing Overbridge proceeded), but proceed because they are fixing a problem that is worth fixing.

The much maligned Lindale underpass, unfortunately underscoped by Transit and above budget, was built because Lindale was the site of several fatal accidents, and with growth in that area would be the site of more – but Peter Dunne wouldn’t know that. One of the most treacherous sections of highway in Wellington used to be State Highway 2 outside Cornish St in Petone, until Transit resurfaced it with non-slip surfacing, which eliminated a blackspot – but Peter Dunne wouldn’t campaign for such a non-sexy project which actually saves lives for only a few hundred thousand dollars. Petone-Grenada would provide an alternative to the Hutt motorway for people going to the Hutt when that route is congested due to an accident – which happens several times a year – something I would have thought Hutt MPs would care about.

However, back to the central section of the Corridor. The issues are:

Safety: The coastal section head on collisions will be prevented by a median barrier (with some modest widening) which will cost around $16 million. Transit has design funding for this work, which will need resource consents as some of the existing walls and rock structures beside the highway need to be demolished – let's see if the Plimmerton and Paremata Residents’ Associations oppose this one. The other safety issues are intersections at Paekakariki and Airlie Road (Whenua Tapu cemetery), and the exposure to risk in Pukerua Bay and Mana. Paekakariki and Airlie Road are fixed with an overbridge ($25 million) and underpass (maybe $5 million) respectively – Transit is consulting separately on Paekakariki because it does not believe that it can continue with the current dangerous intersection regardless of whether or not Transmission Gully is built, and it is right. Transit also believes that a 2-lane bypass of Pukerua Bay ($50 million) is necessary, regardless of whether Transmission Gully is built – primarily because if it IS built (assuming untolled) 40% of traffic will remain on the current highway. So the main safety issues can be fixed for around $96 million – leaving Mana/Plimmerton still exposed (although safer as the traffic lights provide far safer traffic and pedestrian access for local residents). Transmission Gully would avoid the need for any further work at Mana, and possibly a Pukerua Bay Bypass.

Congestion: Congestion on this section will exist primarily at Mana/Plimmerton, within ten years of the latest improvements being completed, and the merge beyond the yet to be completed Mackays Crossing overbridge. Some modest congestion at Pukerua Bay is also likely where the 4-laning ends. The only way this can be resolved is by either 4-laning the entire route and a bypass at Mana, or Transmission Gully. However, lets get some perspective here. That congestion will be very much peak focused for relatively short periods, it wont be urgent at Mana/Plimmerton for some time, and a 2-lane Pukerua Bay Bypass will ease delays at that point. Congestion relief should not be a priority in the short term, but planning will need to be made to build extra capacity to relieve congestion in the medium to longer term.

Reliability: This is the issue that Transmission Gully proponents talk the most about – they want a third highway standard road into Wellington (there are already two – SH1 and SH2, and arguably Paekakariki Hill Road and Akatarawa Road both provide light traffic alternatives during closure). Another road would certainly improve reliability – but building in such a high level of redundancy for closures that happen only a few times a year, at such a high cost is something worth debating. The reliability problems of the current route are substantially reduced if a median barrier is installed, because it will significantly reduce the rate of crashes – which are the main reason the road gets closed. The crashes will also be located on one side of the road, meaning that traffic should still flow in the other direction.

Slips? These are rare, and the one event in the last year that closed the highway for a considerable period of time was unexpected, but is unlikely to happen again. This is when the option of 4-laning could make a significant difference. 4-laning the highway provides far more road space to undertake work to clear crashes and slips. It is relatively easy to clear a 4-lane highway of any incidents, so much so that the proposed “Reliability” package for the Western Corridor did NOT include Transmission Gully. In short, the reliability benefits of Transmission Gully (remembering it would be steep, with a viaduct along a fault line) are not worth it. Reliability is significantly improved by placing a median barrier along the coastal route and a flyover at Paekakariki, and would be adequate with 4-laning of the coastal route.


Now this is the critical one. According to the study, taking into account inflation and factoring in project risk, the comparison is as follows:

Transmission Gully $1.09 billion
Coastal upgrade $735 million (Comprised of Mana Bypass $220 million, Pukerua Bay Bypass $70 million, Coastal 4-laning $365 million, Paekakariki interchange 4-laning $20 million and Grays Road upgrade $60 million)

So there you have it. Since the study finding, a specific report was commissioned to confirm these costs and essentially, nothing bridges the $250-300 million gap in cost between these options. Both routes are about the same length, so no gain there. Both routes would enable 100 km/h travel between Mackays Crossing and Linden. The key difference is that Transmission Gully is a brand new 27km road built along a faultline in rural farmland vs selective upgrades to an existing road, with a couple of new segments along an established corridor and coastline, with some houses that will need to be moved or demolished.

Now note I have assumed a number of givens – that the median barrier along the coastal highway proceeds anyway, and a flyover (2-lane) at Paekakariki also proceeds anyway, but that the Pukerua Bay Bypass does not proceed regardless.

Transmission Gully’s benefit/cost ratio is now less than 0.5:1, it is a worst dog than ever before.

However, the coastal highway isn’t entirely the best project either. Parts of it are. A Mana Bypass is expected to be worthwhile within ten years, as traffic growth erodes the extra capacity benefits from the just completed upgrade. A Pukerua Bay Bypass is also expected to be worthwhile in saving travel time and significantly improving access around that community. However the coastal segment doesn’t generate many benefits and would struggle in itself to get a positive benefit/cost ratio (though is nowhere near as bad as Transmission Gully). The reason being that the only benefit in 4-laning the coastal section is really network reliability – safety benefits are minor and the congestion relief benefits are low as well, at least at current traffic volumes. The coastal section should be done last.

So what now?

Assuming silly arguments such as not doing Petone-Grenada are dismissed, along with refusing to do anything along the current route for safety reasons, there are three options:

1. Develop and build Transmission Gully. This would mean no pursuit of any coastal improvements after the median barrier is built and the Paekakariki flyover. Transmission Gully could probably be completed within 10-12 years.

2. Develop and build the coastal expressway upgrades progressively. The sequencing isn’t important, but would see the expressway completed within 10-15 years, although segments would be open in advance of that.

3. Build neither within the next ten years, but improve the existing highway to be a safe reliable 2-lane route until the economics of either of the above options improve.

Next: What my preferred option is and what funding the Labour government gave for the Western Corridor?


llew said...

Rather excellent!

One thing I wonder (because I've never seen it it can't exist right?) is the bottleneck really Ngauranga Gorge now?

Since the new bridge at Mana, I've only encountered (not counting retards who cross the centre line & spoil it all for themselves & the rest of us) bottlenecks at Pukerua bay (driving north - peak time, holiday traffic).

So I really do wonder about the Petone/Grenada road.

Tom said...


Thanks for the in-depth analysis. One thing I've wondered about is the oft-quoted claim that "rail improvements will shift 500 people a day from car to rail, but only remove 100 vehicles from Ngauranga Gorge at peak times." Following your link to the GWRC site, I've been able to wade through the reports and analyses to work out what that stat is based on.

It turns out that they're not talking about capacity: in fact it would only take minor infrastructural upgrades to increase capacity by 3300 passengers during morning peak (page 5 of the rail report). They're basing the 500 people a day on their Transport Model (p62) that includes population and demographic forecasts, and some assumptions about improved demand due to slightly faster times, better rolling stock, Waikanae electrification and other relatively minor improvements.

What the model doesn't seem to take into account is any new disincentives to road travel as an option, whether deliberate (tolls, congestion charging, HOV lanes) or caused by external forces (rising petrol prices). It also assumes no changes to land use (such as encouraging Transit Oriented Development and discouraging greenfield sprawl), no other major incentives for rail (extension beyond Wellington Railway Station, aggressive price cuts) and no change in lifestyle and attitudes from our current car dependency.

It seems to me that if we're to find a long-term solution, we're going to have to take a more integrated and open-minded approach that simply relying on engineering solutions and simple-minded traffic modelling.

Libertyscott said...

Llew Ngauranga Gorge faces the worst congestion in the morning peak, largely because the heavy flow from Tawa south is added to by the heavy flow from Johnsonville and Newlands, which is greater than the extra lane added to the road. Then the weaving manouevres in the Gorge to get into the correct lane slow it down too. However, just as bad is Ngauranga to Petone - the merge from Petone in the morning and the merge north of Ngauranga in the evening are both bottlenecks - with no scope for widening to fix easily.

It will be interesting to see whether the Mana upgrade which is now open makes a big difference.

Tom, you make some valuable points. Road pricing in the longer term will change everything, modes, distances and land use. I am very skeptical about Transit Oriented development, as it has almost always been an abject failure in new world cities, such as in NZ and the US - and results in no mode shift whatsoever. Most people don't want to live in apartments near railway stations. Modelling of extending rail beyond Wellington rail station (which means light rail, as underground makes Transmission Gully look good) shows virtually no difference, and light rail and heavy freight rail do NOT mix for various reasons, including safety. More bus lanes and bus priority through the city DOES help - but there is not much scope for more of that anyway. Free public transport doesn't work either, unless you want to half the mode share of walking - the free bus around central Christchurch has, on average, only 1 person at any time on it who otherwise would have used a car - all others were pedestrians or would not have taken the trip - hardly environmentally friendly