Suburban Nation - Andres Duany [14]
WHY TRAFFIC IS CONGESTED
The first complaint one always hears about suburbia is the traffic congestion. More than any other factor, the perception of excessive traffic is what causes citizens to take up arms against growth in suburban communities. This perception is generally justified: in most American cities, the worst traffic is to be found not downtown but in the surrounding suburbs, where an “edge city” chokes highways that were originally built for lighter loads. In newer cities such as Phoenix and Atlanta, where there is not much of a downtown to speak of, traffic congestion is consistently cited as the single most frustrating aspect of daily life.
Why have suburban areas, with their height limits and low density of population, proved to be such a traffic nightmare? The first reason, and the obvious one, is that everyone is forced to drive. In modern suburbia, where pedestrians, bicycles, and public transportation are rarely an option, the average household currently generates thirteen car trips per day. Even if each trip is fairly short—and few are—that’s a lot of time spent on the road, contributing to congestion, especially when compared to life in traditional neighborhoods. Traffic engineer Rick Chellman, in his landmark study of Portsmouth, New Hampshire, applied standard suburban trip-generation rates to that town’s historic core, and found that they predicted twice as much traffic as actually existed there. Owing to its pedestrian-friendly plan—and in spite of its pedestrian-unfriendly weather—Portsmouth generates half the automobile trips of a modern-day suburb. k
But even if the suburbs were to generate no more trips than the city, they would still suffer from traffic to a much greater extent because of the way they are organized. The diagram shown here illustrates how a suburban road system, what engineers call a sparse hierarchy, differs from a traditional street network. The components of the suburban model are easy to spot in the top half of the diagram: the shopping mall in its sea of parking, the fast-food joints, the apartment complex, the looping cul-de-sacs of the housing subdivision.l Buffered from the others, each of these components has its own individual connection to a larger external road called the collector. Every single trip from one component to another, no matter how short, must enter the collector. Thus, the traffic of an entire community may rely on a single road, which, as a result, is generally congested during much of the day. If there is a major accident on the collector, the entire system is rendered useless until it is cleared.
Sprawl (above) versus the traditional neighborhood (below): in contrast to the traditional network of many walkable streets, the sprawl model not only eliminates pedestrian connections but focuses all traffic onto a single road
A typical neighborhood is shown in the bottom half of the diagram. It accommodates all the same components as the suburban model, but they are organized as a web, a densely interconnected system that reduces demand on the collector road. Unlike suburbia, the neighborhood presents the opportunity to walk or bicycle. But even if few do so, its gridded network is superior at handling automobile traffic, providing multiple routes between destinations.m Because the entire system is available for local travel, trips are dispersed, and traffic on most streets remains light. If there is an accident, drivers simply choose an alternate path. The efficiency of the traditional grid explains why Charleston, South Carolina, at 2,500 acres, handles an annual tourist load of 5.5 million people with little congestion, while Hilton Head Island, ten times larger, experiences severe backups at 1.5 million visitors. Hilton Head, for years the suburban planners’ exemplar, focuses all its traffic on a single collector road.
The suburban model does offer one advantage over the neighborhood model: it is much easier to analyze statistically. Because every single trip follows