Myths & Reality #2

Today we have Part II of the "Myths @ Reality" series

You can locate Part I here.

For today we are going to look at what is quite possibly the most popular anti-road argument being.

"Building Roads Doesn't Reduce Congestion"

This one comes in multiple forms but the general idea is that when you build a new road or widen an existing road the reduction in congestion is only temporary and you quickly end up back where you were before. This implies that the work done to improve the road was a waste of time as congestion has not reduced and therefore there has been no benefit.

Background:

Like all myths and legends there is actual an element of truth to this claim; It is true that when you add more capacity to something that has unreleased demand that usage will go up, but the conclusions that there has been no benefit is completely misplaced.

In a previous post I elaborated on what congestion is and based on what was discussed there we can expect that when we upgrade a busy road to have more capacity more people will come start using that road.

What about Public Transport:

Although I have started off mentioning roads this same induced demand effect also effects Public Transport (PT), in fact you could argue that one of the main driving factors of PT usage is the inadequacy of capacity in other modes. For example, since the extensive and disruptive roadworks have been occurring on the Northwest Motorway there has been a notable uptake in rail usage on the western line.

However if you look at PT in isolation; If you had a given bus route that's operating one bus every 15mins, and each of these buses tends to be approaching its limits in terms of capacity you will find some potential users are being turned away. To increase the capacity you can start running buses every 10mins which will make life more comfortable for the existing users until the customers who were previously being turned away come back making the buses just as busy as before.

PT supporters will claim that it is cheaper to increase the capacity on PT routes as you have already done the hard part of building the road, however the issue here is that the costs of running a bus, rail or ferry service is directly related to the capacity you are operating.

The Myth and the Reality:

To test this claim, that when you build a new road you simply attract more users and leave things just as congested as before, we are going to look at the SH18 Upper Harbour Motorway here in Auckland.

This section of motorway was built during the mid to late 2000's and bypasses Hobsonville Road and Upper Harbour Drive. The way this road bypasses the old State Highway is actually very similar to when the Southern Motorway was built between 1950 and 1970 and how it bypassed Great South Road.

So this section of motorway was built in two sections:

1) Hobsonville Section - Bypassing Hobsonville Road 

2) Greenhithe Section - Bypassing Upper harbour Drive

Hobsonville Section:

Hobsonville Road 2010 = 37,000 vpd

Hobsonville Road 2014 = 6,700 vpd

SH18 Motorway 2014 = 35,000 vpd

Greenhithe Section:

Upper Harbvour Drive 2006 = 31,000 vpd

Upper Harbvour Drive 2012 = 4,500 vpd

SH18 Motorway 2014 = 43,700 vpd

So as you can see from the numbers above there has been a significant reduction, >80%, in traffic on the existing route. For those of you that are not familiar with this section of road, the existing road alignment was a two-lane two-way road which that was at its absolute limits of capacity presenting a hazardous environment for cyclists and locals and a very low Level of Service (LoS) for all users.

Biking along this road now is both quiet and pleasant with there being very few cars, nice views and an open environment.

Upper Harbour Drive - Google Maps

Here Comes the Induced Demand:

So the above Upper Harbour Drive section has had 8 years for the induced demand to come and fill up the released capacity, however it seems here we are with 85% traffic less traffic than before. The motorway meanwhile as been growing as can be expected due to the demand that was being held back by the existing route that was at capacity, this will also be driven by cross harbour traffic which is unable to cross the main harbour bridge which is also at capacity.

2010 = 31,100

2011 = 33,700 +8.3%

2012 = 38,900 +15.4%

2013 = 40,900 +5.1%

2014 = 43.700 +6.8%

So yes we can see that motorway is attracting more traffic, but what about the existing roads that have had their traffic flows reduced by over 80%. Well if Auckland stopped changing, the city stopped growing and new business stopped establishing quite possibly these traffic volumes would stay down, however this is not the case.

One of the reasons for building the new motorway through this section was that the existing roads were at capacity and therefore development was on hold. Now that all this new capacity has been opened up the area can now handle more people and business.

The planned growth is shown in the following image of the Northern Strategic Growth Area (NorSGA).

NorSGA - AT

Busted:

In terms of the short term, ie 5 years we can safely say this myth is busted, however one issue we have is that the people who make this claim never actually quantify any of their claims. If their talking about 20-50 year time frames I guess they are correct in that any arterial roads we built in a growing city have a good chance of getting congested, but it makes you wonder what they are arguing against.

Are they arguing against roads or are they arguing against growth?

In most cases the people who make this claim live in large metropolitans and their greatest joys come as by-products of large number of people and the concentrations of wealth found in a growing city yet it seems they don't appreciate that this growth comes with increased travel and therefore congestion for all modes of transport.

Myths & Reality #1

I'm going to start a little series on "Myths & Reality"

It's quite common to see these from a pro-PT anti-road point of view but I'm going to try and make this series cover both sides.

So to start things off here is Number 1.

"Roads are Designed to Eliminate Peak Hour Congestion and are Empty for the Rest of the Day"

This is a common one from the anti-road brigade and is generally used as an argument against road improvements and motorways as it is claimed they are significantly under utilised for most of the time.

The reality is we can't afford to design roads to eliminate the peak hour traffic congestion, what road designers do is design roads to "manage" peak hour congestion. This may very well mean that the average speed is reduced from 100km/h down to 70km/h but the aim is to keep traffic flowing smoothly and safely.

It is the inter-peak hours that we try and keep traffic flowing at optimal conditions.

The following photo shows the Northern Motorway on a Saturday afternoon. You can see here that there are quite a few cars and if it were a video you would note the traffic was flowing smoothly. It is however in a bit of a fragile state in that one crash could cause some real issues, and when you have 160,000 vehicles a day driving along a given section of road the chance of there being is crash is quite high. Given there are wide shoulders here the traffic impacts of a minor crash would be much less than what we have been seeing on the North-Western Motorway (SH1) where a single crash would bring this level of traffic to a stand still.

Northern Motorway (SH1) Weekend Traffic

The next photo shows the South-Eastern Motorway (SH20) on a Sunday afternoon. You can see here that the road is rather quiet and flowing completely smoothly. A minor crash here would be of little consequence in terms of the of traffic flows. This section of road gets about 70,000 vehicles a day and potentially you could question as to why it was recently upgraded from 2-lanes each way to 3-lanes each way. The simple answer to that is forward planning as in 2017 the Waterview Tunnel is expected to open which will result in another 60,000 vehicles driving this route every day.

South-Western Motorway (SH20) Weekend Traffic

So what does any of this have to do with designing roads to handle peak hour congestion?

Well basically these photos are showing the motorway operating in near optimal conditions. The Northern Motorway is a little too close to capacity and the South-Western is a quite a bit under.

When it comes to peak hour when there is two or three times more traffic however, the northern motorway will be in stop start condition and the south-western will be close to flow breakdown. In both cases one minor crash would result in significant delays.

Fixing Auckland's Transport - The "Essential" Transport Network

As many of you will be aware, Auckland is the the middle of sorting out it's 10 year budget and as part of this they are trying to encourage the people of Auckland to take a bold step and pay a little better so that we can start to improve transport in Auckland.

Being the councils transport body, Auckland Transport (AT) has a fair idea of what Auckland transport issues are and they also have a good idea as to how the city is going to grow in the future and hence what new transport issues arise.

This is why we are presented with two separate transport plans as follows.

• Basic Transport Network @ $6.9 billion
• Advanced Transport Programme @ $10.3 billion

Although AT and the Auckland Council have been working away on these plans for some time, the anti-road brigade over at GenerationZero have noted that some improvements to the road network have been proposed and so the next campaign has started to stop improving Auckland's road transport in the name of the "Essential Transport Budget" (ETB).

As an overview, the ETB is the Basic Transport Network with a few key road upgrades being removed, they then and in a bunch of new bus/rail interchanges which are all part of the new bus network. It also includes about $100 million for improved walking and cycling.

Background:

Based on the ETB Report, the Basic Transport Network will result in "serious delays to major transport projects", but other than that they don't identify any issues and are not at all concerned with increasing congestion or allowing Auckland to grow.

The ETB is said to "priorities the desired step change in transport choice in Auckland" which I assume means making the roads so congested people have little choice but the take a train.

The rational is that apparently for the past 50 years Auckland has invested in nothing but motorways. However if you read through my Auckland's Transport History series you will note that Auckland actually has a long history of investing in nothing. It was back in the 50 & 60's that the New Zealand government built some rural expressways outside of Auckland, the only real urban motorways that were built in Auckland were again built by the government with that being the CMJ which was built very slowly over a 40 year period. Most of the urban motorways you know of today are due to Auckland growing and engulfing the rural expressways.

In terms of what Auckland has done for itself, it has only built residential streets and a few 4 lane arterial roads. In reality, Auckland is where it is today due to the council doing nothing but the bare minimum for the past 70 years, waiting for the government to step in and do something. It has only been in the last decade that Auckland Council has started to do something with their first steps being turning the rail network into an effect Rapid Transit Network (RTN).

The repeated trend of Auckland in the past of doing as little as possible is what has resulted in a congested and unpleasant road network. Roads that were meant to be arterial were built as residential streets meaning they had very little capacity and no space to be upgraded. This in turn made them progressively less desirable to walk or cycle along as additional space for traffic was squeezed in.

One of the repeated chants from the anti-road brigade is that traffic volumes aren't growing and more people are using PT, which although true it is due to a simple and obvious reason. If the roads are congested people are going to look for other ways to perform their trip. In addition to this, rail usage has shot up due to the bus network being revised so that existing bus users are now required to transfer to a train.

To show the level of trust you can put in their document, they make the claim that passenger volumes on the Auckland rail network have been increasing by 20% per annum. However if you view the following image you can see that this is not the case; although its very clear that rail patronage has been increasing, it actually declined sharply only 2 years ago and has generally been growing at just over 10% per annum.

Transportblog - 2015

On a similar tune, GenerationZero have made the claim that we reached peak traffic in mid 2000's and will never reach such levels again, this is despite the fact we have reached those levels again, passed them, and have been continuing to grow at around 2.5% per annum when you look places like the southern, northern, northwestern and upper harbour motorways.

Pro PT

In terms of the projects that are in the ETB, I don't actually have an issue with any of them however I don't see them as going far enough.

In true to anti-road campaigner fashion, GenerationZero has removed a number of road projects where the primary benefactors are bus users. This is similar case to a previous post I did where the blind hatred of roads can lead folks to campaigning against projects not knowing that the main benefactors are the PT users.

An example of this is the Lincoln Road Upgrade, currently this road has two lanes each way with no cycling and poor walking provisions. The planned upgrade leaves the same number of general traffic lanes but adds a buslane each way along with new cycle lanes, and footpaths with improved safety along the route.

Similar targets are:

• Te Atatu Road upgrade, which is being done to improve buses, walking and cycling along Te Atatu Road.
• Local road Upgrades for the East West Connections Project, which is essentially bus improvements.
• Long Bay Southern Corridor, which provides benefits for all users.
• Dominion Road Upgrade

Growing Auckland:

The other obvious assumption in the ETB is that Auckland is only going to grow upwards and not outwards with their scrapping of the Mill Road upgrade.

Although this area is expected to have an additional:

• 22,000 homes
• 6,000 jobs
• 80,000 more people

The ETB assumes these people will be more than happy to take the bus along a narrow and congested two-lane road to the nearest train station regardless of where they want to go.

Effects of the Essential Transport Budget:

The aim of the ETB is clear in that they want to improve public transport along with walking and cycling. Unfortunately they have fallen for the all to common mistake of assuming anything with the word "road" in the title is bad and therefore must be stopped.

To this extent the ETB will roll out a suite of new train station and bus interchanges however, these buses will be stuck driving along congested local roads as they have removed all of the projects that were going to upgrade these roads to provide the fast and frequent bus services. The end result of the ETB is that we get to spend more money but have a slower and more congested transport network for everyone.

Funding:

I always enjoy reading the funding cases from anyone who is anti-road as they are always campaigning to stop road projects yet they insist that the people who are on the road should pay for their pet project.

We pretty much have the same situation here where they are pretty content with fuel tax but in a rare case of generosity they have actually supported increasing rates. Normally rates are completely off the cards as this means they will need to pay some money but they seem happy in this case as they have reduced the amount they would need to be to $15.73 per annum.

Fuel Levy:

We are told the fuel levy "affects everyone relatively fairly", well I guess this would be the case if we all drove relatively similar vehicles, relatively the same distance at relatively the same number of times a year. However in reality the person it effects the most is the person who needs to travel for work who doesn't have a company car. With a 7c/l tax that somewhat average person pays about $140 a year on tax towards the new transport network, however another equally average person who takes the train (that the guy driving paid for) to work pays nothing what so ever. And so this levy is in no way shape or form fair unless the person paying the $140 in tax is getting some sort of a benefit out of this additional money they are paying, which in the case of the ETB they get less than what they would get in the Basic Transport Network package.

A case of pay more and get less for road users.

Motorway Toll:

Strangely the motorway toll doesn't get the biggest round of applause because there is some concern that some parts of the city have poor PT provision. We are told places like Te Atatu and Messey, the ones who are getting brand new bus lanes built as we speak, have no choice but to take the motorway despite their express bus services. It is only once these places have greatly improved PT that we can toll them.

Without having to read too hard between the lines it's rather clear that a few of the contributors to the ETB live in these places and are probably not to impressed with their current PT service which is required to mix with motorway traffic due to the upgrades that are being built for them. It will be for this simple reason that they currently drive along here that they don't want a motorway toll, as it does add up to a bit at the end the year.

Similar to the fuel levy, if you make 2 trips a day you are looking at spending about $450 a year on motorway tolls, which once the motorway upgrade is finished in 2017 will reduce to nothing as you can start taking the bus again.

The interesting part for these folk is that the ETB does nothing for them. Once the motorway upgrade is complete buses will be able to drive along the motorway should free of congestion, however the ETB has cancelled the local road upgrades and so once they get off the motorway they will be stuck in congestion rather then using the nice bus lanes they were going to get in the Basic Transport Network.

Government Cash:

By far the most preferred source of funding for any project is from the central government. The general call is that PT projects should be funded in the same way as motorways are, from a big pot of money the government has.

The issue here is that the big pot of money gets its money from fuel tax and various other charges paid my motorists. So if you say you want PT to be funded in the same way motorways are then you are really saying you want to remove all subsidies from PT and have it entirely run as a users pays system.

Of course this isn't what they want, but rather they want road users to pay for PT. Currently about 57c in the litre gets paid towards the land transport fund and so for the average driver that's about $800 a year. The preference from the anti-road campaigner is that this $800 should be directed towards the train user and the road user should be rewarded with more congestion for their efforts.

Conclusion:

In the end of the day, the Essential Transport Budget is trying to push the Congestion Free Network into the 10 year plan. Although it's all well and good to campaign for better public transport, in this case it seems they have picked a battle that isn't really needed.

In it's goal of being cheaper the ETB has shot itself in the foot and removed many of the essential roading projects that are needed to support the public transport network along with many of the walking and cycling upgrades. Roading projects have never been just about cars, and even motorway projects these days are doing more for walking and cycling than many most other projects.

The way I see it, if you want one or more of the following:

• Better Public Transport
• Better Walking Provisions
• Better Cycling Provisions
• Greater Choice
• Great Flexibility
• Reduced Congestion

Then the only option you need to chose is the Advanced Transport Programme.

Auckland's Transport History - Part 1

A few years back I came across a paper called
"The American Heresy: Half a century of transport planning in Auckland"
The paper is a rather interesting read however it came across as rather bias with the conclusions being what the author wanted them to be rather than using the evidence presented which generally implied the opposite.

For this post I'm going to generally use the same information which was used to write this paper and present it as I see it which should make for an interesting comparison for those who read both.

The Early Years

The following chart is quite good at showing how our transport mix has changed over the years with the fluctuating levels of PT usage.

TransportBlog - 2015

As you can see in the chart, back in 1920 there was hardly a car in the country and the total number of PT trips were similar to that of today even though the population was not even 200,000. Back in this time personal transport was extremely limited and most people relied on the tram network to move around and therefore Auckland was a very small and dense city.

Starting in the 1920's car manufacturing started moving into assembly lines and assembly factories started to be built in New Zealand, this resulted in cars becoming not only available for purchase but actually affordable for the average family. You can see this reflected in the chart where the number of vehicles in NZ starts to increase and the number of PT trips being made starts to reduce. This trend continues through to the 1930's where The Great Depression strikes and car sales flatten out and PT trips also take a dive. It's during this time when a few of New Zealands large public works programs went into action such as hydro dams on the Waikato River and the railway from Auckland to Tauranga (which was never completed), for this reason it is understandable that a large number of the men of Auckland would have left to work on these projects resulting in the big decline in trips.

After The Great Depression PT trips jumped back up to 70 million and car sales started to boom. You can see that by 1939 PT trips were flattening out and car volumes were increasing at a great rate, however in September 1939 Briton and therefore New Zealand declared war on Germany, it was at this time New Zealand became devoted to feeding the war machine and PT trips doubled in the space of 5 years to 120 million trips annually.

After the war PT trips went back to declining and the growing demand for automobiles that had started in the 1920's continued. This decline in PT trips and growth of automobile demand had started in the 1920's however it is most prominent from the years 1945 through to 1970 where the annual number of PT trips declined from 120 million to 40 million.

In the following image I have added two black lines for the way I think things may have gone if it were not for The Great Depression and WWII.

• The solid black line shows the change in mode share while Auckland transitioned from a small dense city that only existed around our current CBD into a sprawling city merging with the surrounding town.
• The dashed line is simple a smooth out vehicle purchase profile.

TransportBlog - 2015

There is another spike in PT usage through the years 1970 through to 1990, similar to the increase in PT usage from 1990 on wards, I will talk about both of these later.

So now that we have looked at what the numbers show from 1920 to 1970 in my next post I will look at the polices implemented through this time and see what impact have had. The paper I referred to at the start blames the decline in PT usage squarely on the 1955 Auckland Plan  and similar plans thereafter however looking at the chart we really see the decline in PT usage starting in the 1920's the exact time that automobiles started to make their way onto the market in mass. This pre-dates the Auckland Plan by 35 years which questions the impact this plan really had on the way people changed this choose of transport.

Traffic Volumes - Part 2

To carry on from my previous post, I'm going to look at how the currently proposed crossing could operate in terms of capacity and diversity. For this I will again be using the values from the 2010 study located in the Transport and Traffic Model Report.

For a quick summary, the predicted demand across the harbour are as follows:

2008 = 168,150 vpd

2026 = 197,830 vpd

2041 = 205,200 vpd

Traffic Modelling

For a quick note on how these volumes are arrived at:

These volumes come from a regional traffic model that takes into account a large number of assumptions such as land use changes and public transport. The model is not all that detailed on specific elements, such as does this left turn have a pedestrian crossing, but takes a look at the big picture and the general capacity and demand on a system and so on a macro scale is very detailed. When new capacity is added to the model, such as a new harbour crossing, not only will the traffic distribution change but so does the demand due to the potential for trips being increased.

So using the 2010 Study we can see how the travel over the harbour changes with the increase in capacity. Note that these values are the expected AADT and not the demand, even with the new crossing there are still plenty of constraints on the system in other locations and so we will still have demand in excess of capacity.

2008 = Existing Bridge = 168,510

2026 = Existing Bridge = 95,590

2026 = New Crossing = 134,100

2026 = Total = 229,690 (16% more demand)

2026 = Existing Bridge = 110,070

2026 = New Crossing = 144,240

2026 = Total = 254,310 (23% more demand)

Induced Demand

As you can see, the new crossing has enabled a large number of increased trips over the harbour, 23% or 49,110 more each and every day. Comparing demand and actual volumes we can see that the repressed demand, or demand that can't be provided for, is reduced from 18% to 13% which is a sizeable change.

So what you have seen here is induced demand; in the existing situation the shortage in capacity results in trips not occurring, when the capacity is increased the congestion is reduced which in turn enables the repressed trips from before to come out and join the system which in turn increases capacity. 

In a way you can think about it like the apple display at the fruit store. When the display is full of nice juicy apples everyone who thinks about getting an apple will likely grab one. Once the display starts to get low and only the less desirable apples are left people will think twice and maybe decide they don't want an apple. Eventually you will run out of apples and no matter how desperate someone is to get one they simply can't.

In a large city the transport system is very similar to this fruit stand as it's simply not practicable or financially feasible to provide a system that can provide ideal conditions for everything and everyone all hours of the day. So when you put a few more apples in the bowl or upgrade a road the large number of people who have been held back will quickly take up the new supply.

This brings us into a bit of a debate about how much road capacity should be provided. Although road travel by private car can be very convenient and pleasant, when it comes to large volumes of commuters significant dis-benefits arise making it less ideal. In my view, this is where we need to think about what we do with the existing bridge if a new crossing is built.

New Crossing

With the new crossing we get the following peak hour traffic volumes:

2026 = 5,300 (actual), 6,260 (demand)

2041 = 5,300 (actual), 6,440 (demand)

Capacity = 3 x 1800 = 5,400

Existing Bridge With New Crossing

Next we will look at how much traffic will be using the existing bridge after a new crossing has been built.

2026 = 5,430 (actual), 6,390 (demand)
2041 = 5,800 (actual), 6,320 (demand)

From Appendix G of the 2010 Study the existing bridge has been modelled with 5 general traffic lanes, 3 in the peak direction and 2 in the counter peak direction. 1 of the existing lanes is handed over for active modes and another 2 given to buses.

What isn't clear here is how these lanes are arranged, and so it's hard to know what the capacity of each route actually is therefore I wont comment further on the arrangement in the report.

Future Layout

In terms of the bridge layout I'm going to refer to what could happen which continues on from my Victoria Park post. For this, the idea is to reduce the footprint of the existing motorway through St Marys Bay so that more of it can become open public space as seen below.

Turenscape.com

In this layout I propose to make the existing bridge 2 lanes each way for general traffic with this traffic using two existing clip-ons. The central span would then be converted to provide a single lane in each direction for buses and freight with something like the Skypath being added for active modes.

This would end up being very similar to what is currently proposed for the Pakuranga to Botany Busway and would extend along Fanshawe St into the CBD. 

Auckland Transport - 2014

So how does this work traffic wise?

Well when using the clip-ons the lanes are nice and wide so we can expect full utilisation.

2 x 1,800 = 3,600 vehicles per hour (vph)

Compare this to the current 3 median lanes during peak hour where the shy-line effect reduces the capacity of the lanes.

3 x 1,440 = 4,320 vph

Those 3 lanes are including freight however, if we assume a low number of 5% given most freight will be taking the new crossing we have 4,104 vph.

What you will notice here is that in my proposal we have 12% less capacity for general traffic over the harbour. The question to be asked here is; is this what we want?

If we refer back to the network layout we see that the existing bridge provides for people travelling to or from the north into the CBD and Ponsoby. From my perspective we don't really want large volumes of people driving to these destinations, neither of them have much scope to handle any additional vehicle demand but would greatly benefit from improved PT and less private vehicle traffic. To this extent I'm not really too worried about there being less vehicle capacity over the existing bridge and through St Marys Bay when you look at the benefits that come through doing so.

NZTA - 2010

Summary:

So when we look at what a new crossing could mean for traffic volumes and people moving over the harbour we get given a few questions and options. Certainly the new tunnel provides for those long distance trips that are not wanting to go into the CBD itself but what do we do with the existing bridge. We have the option here to downsize the private vehicle demand and turn some of the road space over to open public space, or we can leave the roadway as is and let people chose how they travel to the CBD.

If we refer back to the traffic modelling, the tested scheme results in 23% more demand on people wanting to travel over the harbour using both the tunnel and the existing bridge due to the improved access. Of that 23% however, how many more do we want driving to the CBD?

AWHC - Traffic Volumes

To continue my series on the Additional Waitemata Harbour Crossing (AWHC) I'm going to have a look at the traffic volumes and potential lane configurations such a project could bring.

Project Layout

Based on the 2010 study I am going to assume that the the project will consist of two 3-lane tunnels which will take over the function of SH1. These tunnel will run directly from the Central Motorway Junction (CMJ) through to Northcote Point. For additional details you can view the 2010 study drawings here.

Auckland Council - 2014

One of the interesting aspects of the current designs for the AWHC is that the cross harbour traffic flows are split to provide for two separate functions. The new tunnel provides for north/south movement through the city and bypasses the CBD with users wanting to go to the CBD and Ponsonby taking the existing harbour bridge. The big benefit of this is that traffic travelling long distances on the motorway network are not forced to mix with commuters making short trips over the harbour to the CBD.

The following image comes from the 2010 study showing the routes each crossing provides for.

NZTA - 2010

Anyone familiar with Auckland will know that the existing bridge is rather large with 8 lanes in total with a movable median barrier enabling the bridge to operate in a 3/5 configuration to provide for the tidal peak flow. 

Stuff - 2011

With the new crossing in place we no longer need this level of capacity provided over the existing bridge and so we are presented with an opportunity to reassign the space on the bridge to serve different priorities.

Traffic Volumes

In terms of traffic volumes I am going to use the 2010 studies volumes.

The important thing to note when looking at the AWHC is that we currently have two crossing, the existing Auckland Harbour Bridge (SH1) and the Upper Harbour Bridge (SH18)

As a base line we get the following volumes:
Annual Average Daily Traffic

2008 (AADT)
SH1 = 168,150
SH18 = 27,160
Total = 195,000

2026 (AADT)
SH1 = 197,830
SH18 = 78,260
Total = 276,000

2041 (AADT)
SH1 = 205,200
SH18 = 80,500
Total = 286,000

What's not shown above but is in the report is that a large part of this growth is expected to come from increased inter-peak volumes. Currently inter-peak traffic is about 60 to 70% of the peak hour traffic flow, this is expected to increase to 80% in 2026 and 90% in 2041 meaning you can expect peak hour conditions for pretty much most of the day.

Something else that isn't shown in the above is the demand; if you're a commuter you will likely have experienced driving on a congested road and there have likely been occasions where you have taken a different route or mode in order to avoid the congestion.

Taking the existing harbour bridge as an example, the morning peak in 2008 experienced 7,640 vehicles per hour (vph) in the southbound direction, however the demand for this link is 18% more than this at 8,980 vph. So from that we have 1,340 vph that are either taking a different route, not making the trip or have changed to another mode.

Capacity

If you're someone that lives on the Northshore and commutes over the bridge in the morning you will likely know that the bridge itself seems to flow somewhat smoothly. The following numbers list the capacity of the roads that feed onto the bridge and then the bridges capacity, as you will note the connections to the bridge act as constraints leaving some residual capacity on the bridge. 

Northern Motorway 3 x 1800 = 5,400
Esmonde Road 6s Signal Time = 1,200
Onewa Road Signal Intersection = 1,000

Total Inflow = 7,600

Bridge Capacity = 5 x 1800 = 9,000

One thing I haven't taken into account above is the affect of thee narrow lanes on the bridge. Currently the clip-on lanes are 3.5m in width which is fine for 1,800 vph however the lanes on the central span are just under 3.0m in width and this reduces their capacity by 20% to 1,440 vph. This reduction in lane capacity comes from something called the "shy-line" effect. Most people who drive over the bridge will know that tight feeling of travelling in the central lanes and the "shy-line" effect is the technical term used; this has the impact of people driving slower, increasing vehicle spacing and shying away from objects such as barriers which then impacts the next lane over.

For some technical reading on "Shy-Lines" Section 6.2.1 of the SHGDM gives a few details.

If we reduce the capacity of the central 3 lanes to 1,440 vph we get a total capacity over the bridge of 7,920 vph, this lines up quite well with the 2008 volume of 7,640 vph crossing the bridge which suggests that only a small increase in traffic volumes would result in flow breakdown and congestion on the bridge.

Conclusion

So in conclusion we have:

• Current traffic volumes put the peak hour near capacity
• The existing connections hold back traffic to prevent the bridge from getting over capacity
• The existing crossing is unable to provide for the current demand with 18% of trips being lost or diverted.

Given this post is getting somewhat long I'll leave talking about the new crossing and how the volumes play over in the next post.

Cheers