Cul de Sacs, Grid Street Patterns Both Have a Role to Play
Modern urban planning literature is filled with references to grid street patterns and their alleged superiority to the cul de sac--a loop or “dead-end street” pattern typical of modern suburban developments. The new urbanists claim grids are needed to reduce traffic congestion and smooth the flow of traffic.
But advocates of grid street patterns contradict themselves. They claim local grid streets are needed to smooth traffic by easing the load on higher-capacity streets ... but then they also propose “traffic calming” measures, such as speed bumps and roundabouts, to make traffic move more slowly on the local streets. Such measures nullify any advantage local grids might be argued to have over cul de sacs.
Indeed, residents of neighborhoods in high-crime areas often seek to convert their grids into virtual cul de sacs, in which traffic barriers make travel along a local street impossible for more than a block or two. A key feature of cul de sacs is that they cannot handle through traffic, and thus they create a safer environment for children, pedestrians, and even the low-density automobile traffic that uses them.
Grid street patterns consume more land than do cul de sacs. That should concern urban planners and new urbanists, who seem to believe that the last undeveloped lot in the nation is about to be sold to a greedy developer. The Canada Housing and Mortgage Corporation reports grid patterns can require 50 percent more space for streets than cul de sac patterns. That means an area with 400 square miles--approximately the area of Portland--would require 200 more square miles if it were developed with a comprehensive regional grid of local streets.
Grid street patterns have a role to play, of course, but not necessarily through leafy neighborhoods or suburban residential developments. Grid-based networks are best for ensuring efficient traffic along arterials--major roadways.
For example, regional arterial grid systems in Los Angeles, Dallas-Fort Worth, Houston, and Phoenix provide high-capacity surface alternatives to freeways. They keep freeway traffic volumes lower and provide alternate routes when accidents, road construction, or other issues cause trouble on the freeways.
Traffic congestion in Los Angeles would be dramatically worse if the urban area did not have a well-designed grid system of arterials. It is the lack of such a grid that makes Atlanta traffic so bad.
Milton Keynes, the English new town 40 miles northwest of London, provides a good example. There, planners developed a terrain-constrained grid of high-capacity arterial streets at approximately one kilometer (0.6 mile) intervals. Inside of the grid, streets generally follow the modern suburban style of cul de sacs and loops. Milton Keynes has a very high population density for a new development--at 8,400 persons per square mile, it is 20 percent more dense than Los Angeles or Toronto. Yet, the Milton Keynes roadway design has kept traffic flows comparatively favorable.
Grid-based arterial systems serve modern automobile-oriented urban areas well because they facilitate mobility throughout the area. Notes the Milton Keynes Discovery Center, “The grid road system permits an infinite range of flexibility between any two points in the city.”
Trips in modern urban areas are somewhat random in their distribution. Trips occur from virtually any point in the urban area to any other. An effective way to accommodate this type of demand is an arterial grid system designed on the assumption that mobility and access must be facilitated throughout the urban area.
Wendell Cox (firstname.lastname@example.org) is a senior fellow of The Heartland Institute; a consultant to public and private public policy, planning and transportation organizations; and a visiting professor at a French national university.