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