Does higher density mean lower car use?

Travel outcomes (Barnes)

Not necessarily – in fact in the US, not even usually!

It’s a truism that denser, more concentrated cities tend to have higher public transport use. Various studies have confirmed this intuition but what is usually left unexamined is the implicit assumption that such cities consequently have lower car use.

This study of 31 of the largest cities in the US found that assumption is not correct. Higher public transport mode share does not translate on average to lower kilometres of travel by car, shorter commutes by car, or lower levels of traffic congestion.

The primary finding “is that land use, at least at the aggregate level studied here, is not a major leverage point in the determination of overall population travel choices”.

Undertaken by Gary Barnes from the Centre for Transportation Studies at the University of Minnesota, the research found that, if anything, “the higher densities that increase transit share tend to increase commute times and congestion levels”.

The main objective of the project was to identify the effect of land use on travel behaviour. Most studies concentrate on the effect of average density on one or two variables, usually transit share and sometimes total kilometres of car travel.

Barnes’ approach was much more extensive. For each urbanised area, he defined 15 descriptors of travel behaviour, 11 of land use and 15 other, mainly demographic, factors. Moreover, he employed the concept of ‘weighted density’ (he calls it ‘perceived density’) to more accurately describe the distribution of both population and employment in each city.

Barnes confirms that residential concentration increases transit’s share of travel, but he notes the effect is not large. Contrary to the underlying assumptions of much urban policy:

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.

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/mile2 (1,931/km2). Similarly, a 14% increase in density would only yield a 0.5% decrease in in-car travel time per person. Read the rest of this entry »