Logic says that higher densities will lead to more congestion. Concentrating many more trips within a smaller area may be more efficient (less need for so many roads and a greater role for public transport, walking and cycling) and as more people catch the train or ride the bus along exclusive bus lanes, congestion becomes irrelevant to an increasing number of traveller, but typically we’ve often thought that even taking all that into account it’s inevitable that you’ll see more congestion as you increase densities.
A recent study by the Arizona Department of Transport, highlighted by Todd Littman in this excellent Planetizen piece, questions that assumption. The first part of the results – that people in higher density neighbourhoods drive less than those in lower density neighbourhoods, is not that surprising:
[The study] found that residents of higher-density neighborhoods in Phoenix, Arizona drive substantially less than otherwise similar residents located in lower-density, automobile-dependent suburban neighborhoods. For example, the average work trip was a little longer than seven miles for higher-density neighborhoods compared with almost 11 miles in more suburban neighborhoods, and the average shopping trip was less than three miles compared with over four miles in suburban areas. These differences result in urban dwellers driving about a third fewer daily miles than their suburban counterparts.
But where things get interesting is the follow on finding from the study:
However, the study made an important additional discovery. It found that roadways in more compact, mixed, multi-modal communities tend to be less congested. This results from the lower vehicle trip generation, particularly for local errands, more walking and public transit travel, and because the more connected street networks offer more route options so traffic is less concentrated on a few urban arterials. This contradicts our earlier assumptions.
Littman’s article argues that the key difference higher densities make relates to the availabilities of transport alternatives, which means that as congestion gets higher people are able to make different transport choices. In lower density areas such alternatives simply aren’t available so everyone has not choice but to continue suffering in congestion. Put higher densities together with other key elements of smart growth – like mixing land-use types, improving transport options and having a highly connected street network, and you have the ingredients for lower congestion than you’d think:
Not only does smart growth significantly reduce automobile trips, by offering better accessibility options it allows people to respond to congestion by shifting mode and route. For example, when congestion is a problem you walk or bike to local stores rather than driving to a more distant shopping center, some commuters shift to alternative modes, and motorists can shift to less congested routes for some trips. These solutions are not possible on newer suburban communities where destinations are dispersed; walking, cycling and public transport inferior; and hierarchical road networks channel all traffic onto major arterials.
This has important implications for transport and land use planning. It indicates that smart growth development policies have smaller costs and greater benefits than usually recognized, including local and regional traffic congestion reductions, but it also indicates that these benefits are contingent; they require an integrated set of policies including increased density, mix, connectivity and transport options. As a result, the best response to smart growth criticism is more smart growth, for example, more density and mix, additional pedestrian and public transit improvements, more connected transport networks, more parking management, and additional incentives to shift travel mode.
Critics often assume that smart growth consists only of increased development density. If that were true then some of their criticisms could have merit, but it is inaccurate, as discussed in a previous column, An Inaccurate Attack On Smart Growth. Smart growth involves a combination of increased development density and mix, more connected paths and roads, and improved transport options. Together, these land use reforms can provide a host of direct and indirect benefits.
All the more reason to contain Auckland’s urban sprawl I guess.