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