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.
The principle will presumably also apply to business-related trips. With a denser concentration of businesses it would be easier for someone to visit clients and suppliers using a choice of transport modes – including walking. Indeed there would be business advantage over competitors in locating to that part of the city that has that level of transport choice.
Carrying that thought through, the greater the area of the city with that level of transport choice, the greater the number of businesses that could gain competitive advantage – and possibly be in a better position to compete internationally?
They have higher densities in Phoenix, Arizona?
I once saw 3 people on the sidewalk on Mill St, Tempe. That’s a crowd by their standards.
But my best Phoenix story – A limo driver at Sky Harbour with a sign for who’s she’s meant to be giving a ride to. It read “Odour”. “Thanks for the warning” I said as I walked past. “No, It’s not like that” she assured me, but I wasn’t smelling the air.
I’m sceptical about the findings of this study because it does not appear to control (very well) for self-selection.
More specifically, the report notes that proving a causal relationship requires that the “cause” (i.e. compact urban form) precedes the “effect” (i.e. reduced vehicle travel). This in turn implies that panel data is required, which has both cross-sectional (i.e. geographical) and temporal (i.e. across time) dimensions.
From what I can tell (it’s not completely clear) the regression models used are cross-sectional, i.e. they compare differences between geographical areas. That means that a large part of the effects noted in the study are likely to be due to underlying differences in population preferences, rather than urban form variables.
The ideal scenario would be to have data on individuals travel patterns over time. But in the absence of that you could try a panel regression that allowed for cross-sectional fixed effects. Either way, a time dimension is necessary.
Stu may be right about the specifics of this study design (I haven’t looked at it), but having just come back from Tokyo, the example of density to end all examples of density, it rings true (if a little unlikely, in Phoenix…). The number of choices available to go anywhere to anywhere in and around Tokyo’s central city (by which I mean an area about as large as the isthmus in Auckland, but with a lot more people) was staggering.
I was staying at the U Tokyo Hong campus, and there were 4 subway stations on 4 different lines within 10 minutes walk of the office, meaning that the number of places you could reliably get to in 30 minutes or less was astonishing. It just took a bit of getting used to that level of complexity in a transport system. Extend the walking time to 15 minutes and there were 3 more subway stations and about 10 more options in terms of accessible rail/subway lines.
The rush hour aside (and that does get pretty crowded!) there was just a general air of busy-ness, rather than any serious delay-causing congestion. There was no sign of the sort of painful motorway crawl at 10ks that Auckland’s motorways produce every morning and night for the better part of 3-4 hours a day and the accompanying frustration of never knowing how long it’s going to take you to get anywhere.
Oh there is certainly no doubt in my mind that higher density does impact on travel demands, or more specifically that higher density increases the “utility” of non-car modes.
But the relevant question for transport planners is not just about the direction of the effect (which is relatively intuitive) but its size relative to other possible policy interventions. I suspect, but can’t be sure, that the impacts of urban form on transport have more to do with the types of people that certain urban forms attract, than they have to do with changes in individual behaviour.
But that’s more a word caution, than an outright rejection of the research. It just means that if you take the same group of people and change the urban form then you will end up with a smaller reduction in travel demands than if you are able to attract an entirely new subset of people.
And given that so many of NZ’s talented, creative young people head for European cities as soon as they can, there’s probably some truth to the suggestion that a more compact urban form will actually have an influence on whether these people stay in Auckland.
Just don’t expect to a Holden loving bogan from West Auckland to necessarily jump on a bicycle when you stick him in an apartment downtown. He will, I suspect, keep trying to angle park his Commodore on Queen Street.
I think that this might just be picking up on centrality of older higher density neighborhoods near the core of the city. Generally speaking higher density tends to be in the middle of an urban area, and if you live in the middle of an urban area you have the most stuff within the shortest distances of your home.
So I think there is another factor in the equation, on of “inner city living ness”.