I regularly keep track of a number of statistics about transport and one of those is traffic volumes from the NZTA. Recently I noticed an anomaly with the figures for the Auckland Harbour Bridge. Previously volumes were reported as:
- Centre Span
- Left Clip-On
- Right Clip-On
The monthly data for March and the annual data for 2014 (released in March) was different, instead reporting just Northbound and Southbound traffic volumes – the annual data also included the clip-ons but not the centre span. That in itself isn’t such an issue however the total traffic volumes were quite different, even for previous months/years. An example of the difference is shown in the chart below of annual traffic volumes. You’ll also notice that the volumes are up slightly – although they are still less than they were in 2005 and in percentage terms is low considering Rapid Transit services like the busway are growing by double digit figures. The chart also includes the traffic volume predictions found in the most recent business case for another road harbour crossing.
So seeking an answer for discrepancy I asked the NZTA why the figures were different. The answer is below.
The original site was a National Telemetry Site with loop detectors on the two clip-on sides and an infra-red detector over the four centre lanes. This equipment used on the centre span could not determine directionality and loops could not be used due to the steel deck (the clip-on counters are on the concrete deck north of the main span).
The Auckland Motorway Alliance (AMA) established a count site just north of the bridge some years ago to collect directional data, but it was noticed that the AMA counts and the Telemetry site counts were drifting apart. The problem was with the centre span equipment, which was missing more vehicles as time went by. Therefore, it has been decided that the data from the centre span counter was too unreliable to use.
The Telemetry site was life expired anyway, so the AMA site will become the new Telemetry site. I am told that the clip-on counters are still providing reliable data, so there is no need to decommission them.
That seems a pretty reasonable explanation however as the monthly data released so far only extends back to March 2014 I asked if any further data was available. What I received back surprised me. I did receive some extra monthly data but far more interestingly I also got two years of hourly data by direction – from 1 January 2013 to 31 December 2014. Below are some of the insights we gained from that.
First up results by the day of the week. I was quite surprised to see that traffic builds up over the course of the week with an average difference between a Monday and a Friday of over 15,000 vehicles per day. The busiest single day over that two year period was December 19, 2014 when over 200,000 vehicles crossed.
Breaking that down further by time the table below shows that while Fridays have the highest overall volumes, the strongest peaks occur earlier in the week which I can only guess is due to a lot of people rushing to get home whereas on a Friday the peak is smoothed a little, perhaps from people leaving work earlier or staying at work longer socialising. You can also notice that the late night/early morning volumes over the bridge are much higher than other days of the week from people ou
Showing traffic volumes over the course of the average weekday we get the chart below. I was quite surprised to see that the afternoon peak was stronger than the morning peak.
The data allows us to break that down further including by direction
While volumes peak in the morning and afternoon I was interested to see how things compare on a per lane basis as the moveable barrier on the bridge means that in the peak direction there is an extra lane available. It is often stated that a single motorway lane can move about 2,000 vehicles per hour. As you can see the volumes on the Harbour bridge fall short of that and peak at around 1,700 per lane. It’s also interesting that at times when the bridge is in a 5-3 configuration that lane volumes are similar.
Note: I’ve estimated the times that the barrier is moved as I’m not 100% certain.
I suspect it will be very hard for the bridge to hit any maximum capacity as it is limited by the motorways either side of it. That is also one of the major flaws of any plans to build and additional harbour crossing. You’d have duplicate or at least widen much of SH1 to either cope with the volumes or allow the connections to be used to their potential.
Lastly it’s worth considering the role that buses now play in the Harbour Bridge. Over the two hour morning peak (7-9am) around 200 buses cross the bridge southbound yet they carry around 9,000 passengers which is well more than the bridge carries in an a single morning peak hour. That points to one of the big benefits of PT investment, it’s capacity abilities. By having a strong, congestion free route it allows us to take the edge off volumes and move many more people at a time they want to travel. Imagine the impact there would be if tomorrow all the PT users who currently cross the bridge by bus instead tried to do so by car.
Overall fascinating data so thanks to the NZTA for providing it.