In a new paper, Urban form and driving: Evidence from US cities, Gilles Duranton of the Wharton School and Mathew A Turner of Browns University find it is “more costly to manipulate driving behaviour through densification policies than through congestion pricing or gasoline taxes”.
Professor Duranton and Professor Turner wrote the influential 2011 paper, The fundamental law of traffic congestion: evidence from US cities; they concluded that “both road capacity expansions and extensions to public transit are not appropriate policies with which to combat traffic congestion”.
In their new paper, they map data from the US National Household Travel Survey on a national one square kilometre grid describing urban form, population density, demographics and sectoral employment.
Our estimates of the relationship of driving to urban form allow us to assess the cost effectiveness of densification as a policy response to excessive driving. These estimates suggest that urban form is not cost effective compared to explicit pricing programs.
In particular, even concentrating the population residing in 83% of the area of the continental US into an area of about 1,500 square kilometers would result in only about a 5% decrease in aggregate driving, and this policy appears to describe the upper envelope of what densification policies can accomplish.
On the other hand, existing estimates of the gasoline price elasticity of driving suggest that a similar decrease in driving would be accomplished with a gas tax that is no larger than gasoline price fluctuations observed over the past five to ten years. Congestion pricing programs appear to have even larger effects.
They also consider a more modest policy that illustrates the likely limits of density as a tool of policy. They find that if 1% of US population and employment were moved from the area inhabited by the fifth population decile of density to the ninth decile, aggregate driving would decrease by less than 1%.
Higher density is desirable for other reasons like lifestyle and productivity, they say, but it isn’t a cost-effective way to reduce car use in the US.
While the US isn’t Australia, the implications of this argument are similar to the points I made recently about the limited pay-off in mode share from investing in mega public transport projects in Australian cities (see Will simply building more public transport seriously suppress car use?).
There are good reasons to promote higher densities in established areas as well as increase investment in public transport, but their combined impact on car use in a car-oriented country like Australia is likely to be modest and protracted. They’re not enough.
Managing the negative impacts of driving – like congestion, pollution, emissions and reduced street amenity – can be addressed more speedily, efficiently and effectively by pricing, taxing and regulatory measures. They might be poison fruit in political terms but they’re the low-hanging fruit in policy terms.