As I mentioned yesterday in my look ahead, if it sticks to the timetable laid out in the NZ Upgrade Programme (NZUP), Penlink is due to start construction later this year. But a question that is going to continue to swirl around the project is if it should be two or four lanes wide. When announced as part of the NZUP, and confirmed by Waka Kotahi less than a month ago, Penlink will be a 7km tolled two-lane highway and is expected to cost $411 million. There will also be a walking and cycling path alongside it.
However, locals are keen for a four-lane highway such as in this article running with the classic Harbour Bridge analogy:
The Government and Waka Kotahi NZ Transport Agency are being accused of repeating the blunder of the Auckland Harbour Bridge by fixing one of the city’s worst bottlenecks with a two-lane highway.
It is also suggests that “surely common sense demands a four-lane road” and many on the likes of local facebook pages four lanes is the “smart thing to do“. But as can often be the case, “common sense” often isn’t all that sensible.
To assess if they’re right, we first need to answer two questions
- Does Penlink need to be four lanes, now or in the future
- If the answer to #1 is yes, should we build all four lanes now.
So let’s look at those and to start with, let’s look at how many people might use the road.
The main users will be those living east of where Penlink joins into Whangaparaoa Rd and can be represented by the Statistical Area Units in the map below. Combined, these areas have an population of just over 26,000 people.
The population has grown from about 16,000 people 20 years ago through relatively sustained growth, however, there is good reason to believe that the area won’t see substantially more growth in the coming years. Firstly, much of the peninsula is already developed. There will still be pockets of development but not likely anything on a large scale. Secondly, the Unitary Plan locks in low growth, primarily by only allowing for single houses to be built, thereby preventing the development of townhouses and apartments.
Of note, the main justification given for Penlink is not about growth on the peninsula but that it takes peninsula traffic off the Silverdale Interchange to give space for all the vehicles that will use it from the nearby new greenfield areas.
Penlink will take congestion pressure off the Silverdale interchange, Hibiscus Coast Highway and Whangaparāoa Road. The new transport link will also provide greater capacity, supporting planned growth in Silverdale, Wainui, Dairy Flat and the wider Hibiscus Coast in the next three decades.
This is somewhat highlighted by these images from a presentation a few years ago before the current extent of the greenfield growth areas was made clear. They show the scale of household and employment growth which is fairly small in the area for the area that benefits from Penlink.
Let’s now break down that population a bit further. Of the 26,000 people living in the area, about 6,600 are under the age of 20 and another 4,500 are 65 or older. These two groups are the ones who will not be driving or unlikely to be driving at peak times. That leaves about 15,000 people between the ages of 20 and 65 and notably a third of this group are 50-64 years old and so will fall into the retired category in the not too distant future. But not all of these people will be working/travelling or more importantly, not be travelling to Albany or further south at peak times which is the crucial thing needed to understand the two or four lane question.
Next we can look at Stats NZ’s Commuter Waka which uses the journey to work and education questions from the census to see where and how people travelled. The data isn’t perfect but it is useful to get an indication of travel so we can see for example, how many are travelling to places like Silverdale compared to how many are travelling to Albany or further south.
Looking just at the journey to work data it tells us just over 3,000 people who live in the areas below in green, also worked there, including at home. Meanwhile about 5,400 left to one of the areas in various shades of red. Of those, 1,600 of those were the areas around Silverdale and Orewa, for which Penlink won’t be of any use, leaving just 3,800 heading south and not all of them are driving.
What this suggests is that even if we scale up the numbers a bit, we’re probably looking at 5-7k per day that might use Penlink but
- not all of those will travel at peak
- some will be put off by the toll
- likely an increasing amount will catch a bus or ferry (currently around 9%).
This happens to be not too dissimilar modelling done in the past for the project, which suggests two-way volumes of 12-17k per day in a situation where the road is tolled. The modelling also suggests peak hour flows of about 1,200 per hour, well within the capacity of a two-lane road.
In the table above, scenario A & C relate to whether SH1 is widened or not but it makes little difference. However, it is another important point and still the biggest issue with Penlink in general. The motorway north of Albany is only four-lanes wide, two each way. Yet the motorway south of Albany is frequently congested. Penlink doesn’t do much other than just get a few cars to the congestion point a little bit faster. If anything, building Penlink as a four-lane road it would likely just make this worse.
Perhaps the one thing we may want extra lanes for, and they likely only need to be at the ends, is likely to be for bus priority. Auckland Transport’s Regional Public Transport Plan suggests that once Penlink is complete that they’ll extend some NX2 services to a new Whangaparaoa Station. On its own this would likely help encourage use but could also become part of longer-term strategies to encourage mode shift.
All of this suggests that if tolled, like the government have said it will be, a two-lane road is more than sufficient to handle expected volumes for the foreseeable future. It also means there’s no need to answer the second question I posed. If they still really want a four-lane road then perhaps one thing they should do is start by getting together to call for more housing in the area by upzoning the peninsula.