The false assumption

Even though the cost-benefit ratio of both bridge (0.6) and tunnel (0.4) options for providing an additional crossing of the Waitemata are pathetically low, I actually think they’re an over-estimate. This is due to the same false assumption that afflicts the analysis of all future roading projects:

That traffic numbers will keep rising

If you have a look through the business case for the harbour crossings, the main justification is that – supposedly – traffic volumes will continue to rise dramatically across the harbour bridge into the future, and therefore we will need more lanes in the form of another crossing. This assumption is shown in the information below (taken from the business case): According to the traffic models, supposedly over the next 15 years we’re going to see nearly a 20% increase in vehicles travelling across the bridge. I wonder how they work out it, one would imagine not by looking at recent historical data. In fact, if we look at that data we see a quite different trend: The 2010 and 2011 data is “monthly average daily traffic” whereas previous years are “annualised average daily traffic” (July 2010 data from here, Feb 2010 data from here, earlier data from here). So take the last two columns with with a bit more of a grain of salt than the others. The important thing to note is the trend, which I’ll graph to show a bit clearer (and also take back a few more years thanks to data from here): Up until 2007/2008 the world made sense for traffic engineers, traffic modellers and NZTA. As population grew, traffic numbers grew – almost like a golden rule of physics. Except something strange has happened in the past few years that just doesn’t make sense to engineers, modellers and NZTA: volumes have started falling.

Now let me think, what has happened to the Northern Motorway in the past few years? Oh yeah that’s right – we built a busway! And it has been rather successful. Here’s another graph (this time courtesy of ARTA) showing changes in peak time traffic volumes over the past few years, but this time comparing people going by car with those by bus

So in 2004 around 22,000 people crossed the bridge heading into the city – 17,000 by car and around 5,000 by bus during the 7-9am period. By 2010 this had changed quite interestingly, with an increase in total people to 24,000 but a decrease in those travelling by car down to around 16,000. In the meanwhile bus patronage has gone up from 5,000 to 8,000. Therefore, not only has the busway enabled a couple of thousand more people to get across the bridge during peak times (surely good for the economy), it has actually taken around 1000 cars off the motorway over the two hour period – surely speeding up traffic for everyone else.

The assumption that “traffic will always grow” is not only misplaced for the Harbour Bridge, but also more generally all over New Zealand. You can clearly see how the last few years has broken away from long-term growth trends in NZTA’s analysis of state highway volumes: I’ve added in what seem to me to be the trend lines: for all traffic this follows the general rate of increase between 1989 and 2005. For heavy vehicle traffic it follows the general trends from around 1997 to 2004. You can see how volumes have fallen far below these ‘anticipated’ levels in the past few years.

The world is clearly changing. As fuel prices increase and as alternatives to driving slowly but surely are improved, it seems as though there’s a clear tailing off in the level of traffic increase on New Zealand’s roads over the past few years. This cuts to the heart of justifying new roading projects of course – and therefore it’s not really a surprise to see traffic modellers, engineers and most particularly NZTA still in absolute denial over this issue. After all, if traffic levels aren’t going up it becomes pretty difficult to justify new spending (just like it would be difficult to argue for more spending on public transport if patronage was plummeting).

A world of never-ending traffic growth is a false assumption. It’s time those in charge of transport planning started to realise this.

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