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New Understanding of Traffic Congestion

Congratulations to this year's high school, college and university graduates! The current crop includes our son, who was recruited by a major corporation. The location of his new job will affect his travel patterns and therefore the transportation costs he bears and imposes for the next few years: until now he could get around fine by walking, cycling and public transport, but his new worksite is outside the city center, difficult to access except by automobile. As a result he will spend a significant portion of his new income to purchase and operate a car, and contribute to traffic congestion, parking costs and pollution. This is an example of how land use decisions, such as where corporations locate their offices, affects regional transport patterns and costs.

Todd Litman | June 3, 2012, 8pm PDT
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Congratulations to this year's high school, college and university graduates! The current crop includes our son, who was recruited by a major corporation. The location of his new job will affect his travel patterns and therefore the transportation costs he bears and imposes for the next few years: until now he could get around fine by walking, cycling and public transport, but his new worksite is outside the city center, difficult to access except by automobile. As a result he will spend a significant portion of his new income to purchase and operate a car, and contribute to traffic congestion, parking costs and pollution. This is an example of how land use decisions, such as where corporations locate their offices, affects regional transport patterns and costs. It illustrates research showing that where people work and shop has as much impact on their travel habits as where they live.

Other recent research offers additional insights. A report titled Land Use and Traffic Congestion, published by the Arizona Department of Transportation, is changing the way we think about congestion and solutions. It 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. 

That is unsurprising. There is plenty of evidence that land use factors such as density, mix and road connectivity affect the amount people travel. 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.

In the past experts often assumed that, although more compact development increases overall accessibility (it increases the number of goods and activities within an area and therefore reduces the travel costs required to reach them), it increases local roadway congestion intensity, a point used to criticize smart growth.

It turns out that this criticism is untrue. It fails to account for real world conditions. 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.

 

For more information

Md Aftabuzzaman, Graham Currie and Majid Sarvi (2011), "Exploring The Underlying Dimensions Of Elements Affecting Traffic Congestion Relief Impact Of Transit," Cities, Vol. 28, Is. 1 (www.sciencedirect.com/science/journal/02642751), February, Pages 36-44.

Wendell Cox (2003), How Higher Density Makes Traffic Worse, Public Purpose (www.publicpurpose.com/pp57-density.htm).

J. Richard Kuzmyak (2012), Land Use and Traffic Congestion, Report 618, Arizona Department of Transportation (www.azdot.gov); at www.azdot.gov/TPD/ATRC/publications/project_reports/PDF/AZ618.pdf.

J. Richard Kuzmyak, Jerry Walters, Hsi-hwa Hu, Jason Espie, and Dohyung Kim (2012), Travel Behavior and Built Environment: Exploring the Importance of Urban Design at the Non-Residential End of the Trip, Lincoln Institute of Land Policy (www.lincolninst.edu); at www.lincolninst.edu/pubs/dl/2057_1379_Kuzmyak%20WP12RK1.pdf. 

Todd Litman (2011), Smart Congestion Relief: Comprehensive Analysis Of Traffic Congestion Costs and Congestion Reduction Benefits, Victoria Transport Policy Institute (www.vtpi.org); at www.vtpi.org/cong_relief.pdf.

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