As outlined in The Public Purpose #57, there is a strong relationship between higher population density and more intense traffic congestion. This conclusion is strongly supported by international and US evidence.

However, traffic intensity (vehicle miles per square mile) is not, in itself, a determinant of traffic congestion. For example, an urban area with a modest traffic intensity, but with a substandard road network can have worse traffic congestion than an area with much greater traffic intensity.

Atlanta is an example of an urban area with a substandard road network (the virtual lack of a quality arterial street system), relatively low traffic intensity and highly congested traffic. But, all things being equal, greater traffic intensity leads to worse traffic congestion. This is a critical factor as urban areas are implementing plans to increase population densities, which are associated with greater traffic intensities.

But holding the roadway system constant, higher densities will make traffic congestion worse. Of course, the roadway network makes a difference in how well varying traffic volumes are accommodated. But the essential fact is that, as densities increase, so too does traffic. This means a more dense area needs a better roadway system than a less dense area --- Better, more high capacity roadway systems are required.

This creates a public policy dilemma in an age when the conventional urban planning wisdom calls for higher densities. There is generally not the political will to provide the correspondingly higher roadway capacity that would keep traffic congestion from growing. To the contrary, such urban areas are usually seeking to minimize highway investment, so they do not develop the higher capacity infrastructure required to keep traffic congestion from growing. Portland, Oregon exemplifies this approach, where plans for higher density are not only not associated with correspondingly greater highway capacity, but traffic standards have been weakened to permit greater traffic congestion. To keep traffic congestion and air pollution from increasing, Portland would need to expand its roadway system by a factor of 80 percent of any density increase.

Thus, while greater traffic intensity is not always associated with greater traffic congestion when comparisons are made between urban areas with differing roadway patterns, greater traffic intensity (from higher density) is virtually always associated with greater traffic congestion in the same urban areas, because of the lack of correspondingly robust highway expansions to accommodate the new demand.

Smart growth proponents make the point that higher density areas have lower levels of auto travel per capita, which is true. It is on the basis of this argument that proponents of smart growth and overly restrictive land use regulations suggest that their policies lead to less traffic in an urban area.

But the issue for people is not the whether an urban area, such as Portland, has a total of 32 million daily miles of travel at a lower density and 31 million daily miles at a higher density. People do not experience traffic congestion at the urban area level --- they experience it at the local area. The same is true of air pollution, which tends to become more intense as traffic densities increase, as speeds slow and as traffic flows become less stable (an inevitable consequence of allowing traffic congestion standards to be retarded, as noted above).

Thus, an area with double the density of another, will tend to have, according to the federal research cited in The Public Purpose #57, 180 percent of the traffic level of the lower density area. The slower speeds and less stable traffic will make the air pollution even worse. For the child with asthma living in the more dense area with more intense air pollution emissions, it would be little comfort that overall urban area emissions might be lower, while their lungs have a pollution intake perhaps double that of the less dense area (1). Some of the worst traffic congestion and vehicle emitted air pollution in the world is in urban areas with comparatively low auto usage per capita (Seoul is a good example).

Yes, it is possible to have smart-growth's densification without increasing traffic congestion. The answer is simply to build enough roadway capacity to accommodate the new traffic. Thus, Portland, which seeks to increase population density by more than 50 percent over the next 40 years, would need to increase roadway capacity by 40 percent --- not just freeways and arterials, but also some local streets. For Portland, this would mean some double-decking of freeways and arterial streets (on the assumption that massive widenings would not be politically popular). Short of a commitment to a much more robust, higher capacity roadway system, smart growth's densification strategies must inevitably lead to more intense traffic congestion.

Note

(1) It should also be noted that substantial progress has been made in reducing vehicle related air pollution, which at the national level has fallen dramatically over the past 30 years, while traffic volumes have increased. This progress will continue and air pollution is likely, as a result, to command less attention in the future.

Credentials:

Author Wendell Cox is a transportation and demographics consultant and a public policy commentator. He is principal of Wendell Cox Consultancy in the St. Louis area. He also serves as a visiting professor at the Conservatoire National des Arts et Metiers in Paris. Mayor Tom Bradley appointed him to three terms on the Los Angeles County Transportation Commission (1977-1985), during which time he was elected to chair the American Public Transit Association Policy and Planning and Governing Boards Committees. In 1999, Speaker of the House of Representatives Newt Gingrich appointed him to the Amtrak Reform Council, to fill the unexpired term of New Jersey governor Christine Todd Whitman. He has testified by invitation to committees of the US Senate and US House of Representatives and to 35 state and provincial legislative committees.