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
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.
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.
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.
Summary:
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 |
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?
No comments:
Post a Comment