We have frequently raised concerns about projected future traffic growth, given that in recent years there has been an extended flat-lining of traffic growth. What’s perhaps most concerning about these projections is how they ignore what has actually happened in the recent past and how those producing the projections don’t seem to learn from past mistakes.
This isn’t just an Auckland problem. An article I came across recently looks at projections for a bridge across Lake Washington in Seattle highlights how stubborn the projections of growth are despite evidence to the contrary:
What’s crazy about this graph is how persistently wrong the projections have been – yet without any change to reflect the reality of declining traffic volumes over a 15 year period between 1996 and 2011.
Yet it’s not just a specific example of a bridge in one American city where we see these persistently wrong projections coming through. Let’s look at a comparison of official traffic projections across the UK over the past 20 years and compare those with what actually happened:
Our own local example of this ignorance is in the traffic projections being used for the stupid Additional Harbour Crossing Project, where modelled traffic growth rates completely ignored recent trends and therefore were calculated from a base that was significantly too high:
This graph was from a year ago and in the past when I’ve posed it, there have been some that say “look it’s starting to rise again” but the reality is it isn’t. The most recent monthly data shows traffic have flat-lined and volumes are still less than it was a decade ago (monthly figures only started in late 2007).
Similarly another frequent comment we see when this is discussed is to the effect hat the downturn is only due to the current state of the economy. However many economic indicators are pointing to the economy being much healthier today than it was a few years ago. Other indicators highlight that while on a per capita or percentage basis we might not be doing as well as in the past, on a total basis we are doing well. For example despite the percentage of people who are unemployed being higher than it was in 2007/08, in total there are actually significantly more people employed at the moment.
I suspect traffic projections keep making these mistakes because they are calculated using models with fundamental problems in them. They are generally designed to predict the future based on extrapolating our behaviour from some point in the past. That may have worked in the 90′s (and earlier) but it doesn’t work now and one of the key reasons is that we are seeing generational changes occurring with young people choosing not to drive as much as older generations. Yet while road models might be well over estimating vehicle trips, PT models have been doing the opposite. One of the best examples is Britomart where we exceeded the 2021 projected daily patronage in 2011.
And even the Ministry of Transport in their response to the City Centre Future Access Study said that private vehicle trips were probably being overestimated.
When there are tens of billions of dollars of public money is riding on these faulty projections, it suggests we need a new approach starting with not believing the current projections.