A paper published a few months ago in the prestigious Journal of the American Planning Association (JAPA) casts doubt on the strength of the environmental claims made on behalf of compact development. In what JAPA calls the key “takeaway for practice”:
“Urban form policies can have important impacts on local environmental quality, economy, crowding, and social equity, but their influence on energy consumption and land use is very modest; compact development should not automatically be associated with the preferred spatial growth strategy.”
The paper, Growing cities sustainably: does urban form really matter?, was written by four academics from Cambridge, Leeds and Newcastle universities (Marcial H. Echenique, Anthony J. Hargreaves, Gordon Mitchell, and Anil Namdeo). As I discuss below, it’s provoked a debate among US planners about the normative role of research. Using statistical models, the researchers simulated three possible 30-year spatial futures — compaction, expansion, dispersal — for three distinctly different English city-regions. These are Tyne and Wear in north-east England, the south-west centred on London, and Cambridge sub-region.
They used 26 environmental, social and economic indicators to compare explicitly the benefits and costs of each of the three options. They conclude that land use and transport policies promoting compact development:
“… have virtually no impact on the major long-term increases in resource and energy consumption. They generally tend to increase costs and reduce economic competitiveness. The relatively small differences between options are overwhelmed by the impacts of socioeconomic change and population growth.”
That’s not a novel finding for studies at the broader metropolitan level. For example, in a study of 31 of the largest US cities, Gary Barnes from the Centre for Transportation Studies at the University of Minnesota found:
“Even very large changes in land use have very little impact on travel behaviour, in good ways or in bad. Apparently the larger effects sometimes observed in neighborhood-scale studies are just that: neighbourhood-scale effects that do not extend their benefits to the larger urbanized area.”
As I noted when reviewing this study last year, his analysis implies that increasing residential density by 100% would increase transit share by only 5-6%.
To get a 1% increase in walking and cycling’s combined mode share would require an increase in residential density of 5,000 persons/sq mile (1,931/sq km). Similarly, a 14% increase in density would only yield a 0.5% decrease in in-car travel time per person.
Closer to home, Brunton and Brindle examined the effect of density and land use mix on travel behaviour in Melbourne and found only “superficial support for a relationship between density and trip-making”.
They found accessibility to activities has a stronger bearing on travel choice than density. However they concluded that after income is taken into account “urban form characteristics as a whole played a relatively insignificant role in determining car travel in Melbourne”.
I pointed out last month that Sydney is twice as dense as Brisbane (using population-weighted density). Yet public transport’s share of all work trips is only five and a half percentage points higher — Sydney’s public transport mode share is 14.8% compared to Brisbane’s 9.3%.
There are of course other reasons why as a society we might want to promote density and limit urban expansion. But when it comes to the specific objectives of reducing emissions, pollution and energy consumption, it appears plausible land use policy is likely in most cases to make only a modest contribution.
That moderate gain needs to be compared against the costs. Echinique et al argue that compact development reduces housing choice and exacerbates congestion. They could’ve added that it’s also very slow and politically difficult. Indeed, while they’re all difficult, there are arguably more politically feasible, more technically effective, and certainly quicker ways of promoting environmental objectives than changing land use.
A key one is converting power generation to non-carbon sources. In the urban sphere, another approach is to implement regulatory and/or taxing policies that lower the demand for driving and provide incentives for the use of more efficient vehicles.