Is Auckland abnormally congested?
I occasionally hear people bemoaning that Auckland is one of the most congested cities in the world, or at least one of the most congested cities for its size.
I’ve previously taken a look at this from a few angles – looking at trends in traffic delay in Auckland and average commuting time in large cities around the world. Auckland looks pretty good on the latter measure, which kind of belies the “most congested city” rhetoric:
For another look at the same issue, we can take a look at data on traffic delay in cities across New Zealand and Australia.
Helpfully, the Australian Bureau of Infrastructure, Transport and Regional Economics (BITRE) publishes data on the cost of congestion in large Australian cities. They measure congestion as the amount of traffic delay that could feasibly be avoided without reducing the overall value that people get out of travelling.
Unfortunately, comparable figures aren’t available for New Zealand cities. The Ministry of Transport tracks delays in traffic, but doesn’t make monetised estimates of avoidable congestion costs. However, it isn’t that hard to make a reasonable estimate, if you combine MoT’s traffic delay statistics (average delay of 0.52 minutes per kilometre in March 2014, but less in the November survey) with their data on the total amount of vehicle travel in Auckland (12.7 billion vehicle-kilometres driven in 2014).
Following BITRE’s approach, I’ve assumed that avoidable congestion is about 55% of total delay – reflecting the fact that many people prefer to travel even on congested roads. I then monetised the total delay-hours using NZTA’s standard figures for the value of travel time (around $23.40 in 2015 dollars) and converted between Australian and New Zealand money. After mixing in some data on city population in Australia and New Zealand, I got the following chart:
Each individual point is an observation from a single city in a single year – so it’s possible to see how congestion has evolved over time in each city. We can immediately see three important things from this chart.
The first is that Auckland is right on the trend-line. We have the congestion levels that you’d expect to see in an Australasian city with 1.5 million residents – right between Adelaide and Perth. So once again, there are no signs that Auckland is particularly exceptional in the traffic congestion area.
The second is that congestion is a nonlinear phenomenon – it increases faster than city size. You can see that in the upward-sloping trendline fitted through the points on the graph. On average, in this sample of cities, a 10% increase in population is associated with around a 13% increase in congestion levels.
What that means is that new residents entering the city experience the average congestion levels in the city – 10% of that 13% increase – and also have a (relatively small) negative impact on congestion for existing residents – the remaining 3% increase.
The third is that, setting aside the average relationships across all of the cities, individual cities appear to follow slightly different trends. For instance, while Perth and Melbourne have followed the trend-line pretty closely, there seems to be a steeper relationship between congestion and population size in Adelaide and Brisbane.
That suggests that urban policies – land use, transport investments, etc – can enable some cities to grow in more or less efficient ways. Which specific urban policies are better or worse is a bit of a vexing question – but there does appear to be something there.
What do you make of the data on congestion costs in Australasian cities?