A solution to the “density dilemma”?

Following on from posts made last week on both the Humantransit blog and on my blog about the complex relationship between urban density and public transport use – and how sometimes we end up with some rather bizarre relationships between the two – Jarrett at Humantransit has done a post that I really think finally cracks the issue of linking density and public transport use. As per the previous posts, it all comes down to what we mean by the word “density” and how we go about measuring it.

I concluded my previous post by stating the following:

So, returning to Mees’s question – is density destiny? I would certainly argue that average density has nothing to do with our public transport destiny: just compare New York and Los Angeles for your answer there. However, urban densities around public transport – particularly around rail – will determine the success or failure, the destiny, of that public transport system. Park and rides, feeder buses and so forth can all help in making public transport work better in low density areas, but ultimately if you want a situation where people don’t feel as though they have to own a car to live a meaningful life – I really do think you’re going to need some density.

The useful thing that Jarrett’s latest post does is offer an alternative method of measuring density – one that turns out to be far more useful than the “average” figure we tend to use at the moment. Jarrett’s concept, adapted from various comments on his blog and on other blogs, looks to measure density in terms of people rather than in terms of area. By that I mean instead of saying “the population of this city is x, the area is y and therefore the density is x/y”, a more useful measurement is to ask the question “what percentage of the population lives at or above x density, what percentage of the population lives at or above y density and what percentage lives at or above z density.”

This achieves two things. For one it somewhat removes the effects of outliers on the average total. For example, it would seem as though some parts of outer New York City must have extremely low densities in order for the city’s average density to be lower than Los Angeles’s. The idea is easiest to understand when presented in graph form – which has been done by Fedor Manin of the “We Alone on Earth” blog for various US cities: The graphs do require a bit of explaining. In the top graph, we’re talking about the percentage of the population that lives above a certain density. The faster the line drops, the greater the percentage of people living at relatively low densities. At 1000 people per square km, which seems to be a relatively medium-low density, we can see that 40% of Atlanta’s population lives above this level. Interestingly, Boston is the city with the next lowest percentage of people living above 1000/square km – suggesting that there are a fairly large number of very low density houses around the edges of Boston. In most other cities, over 80% of the population lives above 1000 per square kilometre. At 10,000 per square kilometre we can see the numbers have dropped dramatically. Basically nobody in Atlanta, Seattle or Las Vegas lives at densities higher than this, while for Boston, Chicago, the San Francisco Bay area and Los Angeles you have only around 15% of the population living at or above 10,000 per square kilometre. But for New York City, we can see that a good 40% of the population lives at higher densities than this – potentially creating a huge public transport market.

Turning now to the lower graph, you can see how New York and Los Angeles differ quite clearly. While New York has more people living at the very low densities, it also has masses more living at the very high densities – the kind of densities where anything but strong public transport use is basically impossible. On the other hand, Los Angeles has its densities very much squished in the middle.

So what does this mean for public transport, and in particular what does it mean for Auckland? For a start, it would be extremely interesting to see the graphs outlined above completed for Auckland, Wellington and various Australian cities. We know that Auckland’s average density is much higher than that of Brisbane and Perth, and instead is relatively similar to Sydney. This is shown below:

But having visited Sydney and seen the large number of terraced houses in its inner suburbs, I wonder whether – if we did undertake such an analysis – the result might be a kind of “Sydney is New York, Auckland is Los Angeles” outcome.

This of course doesn’t mean that Auckland should give up on improving its public transport system, just because its average density figures may be a bit misleading. What it does mean though is that we really do have to work on ensuring there is a good alignment between our land-use strategies and our transport plans. If we want to seriously boost public transport ridership, in order to give people alternatives to driving, then it would seem that we really do need to focus on seriously increasing densities within parts of the city that have good access to high quality public transport. In some places, like New Lynn, we are doing a good job at making that happen. In other areas, such as along the Northern Busway, the empty fields (next to Akoranga Station) and giant carparks (next to Smales Station) represent massive lost opportunities – or massive potential if you’re more optimistic – to locate the kind of densities that really seem to drive public transport use.

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