Earlier this week, I took a look at the relationship between congestion and density. I was investigating geographer Phil McDermott’s claim, based on some dodgy data comparing between cities, that increasing density would increase congestion.
Economists know that it is difficult to make inferences about causality using cross-sectional analysis. Simply looking at variations between different cities doesn’t allow you to form robust conclusions about how those cities got to where they are.
One of the ways in which economists seek to strengthen their understanding of causality is to look at changes over time. For example, if you observe that increases in density tend to be followed by increases in congestion, then that is stronger (although not necessarily conclusive) evidence that there is a causal relationship.
With that in mind, it is worth asking: How have congestion and density changed in New Zealand cities over time? Unfortunately, we don’t have enough data points to conduct a robust econometric analysis, but we do have enough to start painting a picture of recent changes. We can draw upon two relevant sources:
- The dataset on population-weighted density in New Zealand and Australian cities that I recently compiled, which provides a meaningful estimate of density for 15 NZ cities from 2001 to 2013
- The Ministry of Transport’s Congestion Index, which estimates the average delay per vehicle-kilometre for five NZ cities over the 2003-2013 period.
I’m going to focus on NZ’s three largest cities – Auckland, Wellington, and Christchurch – as two of the three experienced big increases in density between 2001 and 2013. (MoT hasn’t collected data on Hamilton and Tauranga for as many years.) If these increases in density coincided with rising congestion, it may be an indication that intensification can lead to increased congestion.
Here’s the data. It shows that density has risen 33% in Auckland between the 2001 and 2013 Censuses, 17% in Wellington, and a mere 3% in Christchurch:
And here’s the Ministry of Transport’s Congestion Index, which measures the average minutes of delay per vehicle-kilometre, relative to totally free-flowing conditions. This is a bit of an unrealistic comparison, as a 2013 NZTA research report by Wallis and Lupton shows. The only way that you can totally avoid all queuing or stopping at traffic lights is if there are no other cars on the road. So it wouldn’t be realistic to say that we could speed up the average Auckland trip by half a minute per kilometre. However, this is still a useful indicator for changes from year to year.
While the Index bounces around a bit from year to year, the overall trends are clear. Levels of congestion are flat or falling in Auckland and Wellington, which experienced big increases in density over the last decade, and rising in Christchurch, which hasn’t gotten denser. In particular:
- Average delay for Auckland drivers was 25% lower in 2013 than it was in 2003
- Average delay for Wellington motorists fell 5% from 2004 to 2013
- Average delay for Christchurch drivers rose a staggering 31% between 2004 and 2010. Unfortunately, MoT’s monitoring seems to have been disrupted by the earthquake, but anecdotal evidence suggests that congestion has worsened since then.
In short, data on changes in density and congestion in New Zealand cities contradicts the notion that intensification will necessarily cause worse traffic congestion. If anything, it suggests that rising density may do the opposite, by making it more feasible for people to walk, cycle, or take public transport.
Do we need to treat this data with caution? Most certainly. As I noted earlier in the week, there are a number of omitted variables that influence congestion, such as such as changing consumer preferences, macroeconomic changes, and significant investments in both roads and public transport over the last decade. But it does suggest that wild claims about the negative traffic impacts of new apartment buildings should be taken with a significant grain of salt.
What do you make of this data?