Street hierarchy

The street hierarchy is an urban design technique for separating automobile through-traffic from developed areas. It can be seen as a hierarchy of roads that embeds the hierarchy (importance of different roads) in the network topology (the connectivity of the nodes to each other). The street hierarchy completely eliminates all straight-line connections between arterial roads, whereas arterials in a traditional grid plan are connected by dozens of through streets.

Instead, a series of cul-de-sacs feed into a primary or secondary "collector"—either a ring road, or a long, curvilinear "front-to-back" path—that in turn feeds the arterial. In the American Midwest, where many grid roads were already laid out, larger subdivisions may have a partial hierarchy, with two to five entrances off one or two main roads limiting through traffic.

Since the 1960s, it has been the dominant spatial arrangement of suburbs and exurbs in the United States, Canada, and Australia. It is also increasingly popular in Latin America, Western Europe, and China.

Large subdivisions may have three- or even four-tiered hierarchies, feeding into one or two massive collectors, that can be as wide as the Champs-Élysées or Wilshire Boulevard. Arterials at this level are rarely fewer than four lanes in width; and in large contemporary suburbs, such as Naperville, Illinois, or Irvine, California, are often eight or ten lanes wide. Adjacent street hierarchies are rarely connected to one another.

History

The street hierarchy was first elaborated by Ludwig Hilberseimer, in his "City Plan" of 1927. His major priorities were making it safe for primary school-age children to walk to school, and increasing the speed of traffic circulation.

Planners also began to modify the grid into a superblock system, where high traffic generators such as shops and apartments were banished to the arterial roads that formed the boundaries of the superblock. Schools, churches, and parks were concentrated at the center. Within the block, "T"-intersections and culs-de-sac slowed or prevented through traffic.

This model prevailed between roughly 1930 and 1955, in "instant cities" such as Lakewood, California, and the Los Angeles district of Panorama City. The street hierarchy has dominated the landscape of new suburbs from the Levittowns onward.

In the 1960s heyday of operations research and rational planning, the street hierarchy was seen as a major improvement over the "messy" grid system. It prevented drag racing and other forms of dangerous high-speed automobile travel from occurring in residential areas. New "master-planned" suburbs often codified the street hierarchy into their zoning laws, banning grids from all residential areas.

Eventually, the street hierarchy was also adapted for industrial parks and commercial developments. Use of the street hierarchy is a nearly universal characteristic of the "edge city", a roughly post-1970 form of urban development exemplified by places such as Tysons Corner, Virginia, and Schaumburg, Illinois.

Criticisms

Social commentators and urban planners have often pointed out that the street hierarchy arrangement has serious limitations. These criticisms are generally part of a broader indictment of mid-20th-century urban planning, with critics charging that planners have only considered the needs of young children and their working-age parents in creating the spatial arrangement of the late 20th and early 21st centuries.

Financial costs

Some planners and economists consider the street hierarchy to be financially wasteful, since it requires far more miles of street to be laid than a grid plan to serve a much smaller population. In suburban areas subject to property tax caps such as California's Prop 13, the enormous per-capita expenditures required to maintain streets mean that only houses costing over half a million dollars can provide enough property tax revenue to cover the cost of maintaining their street hierarchies. In areas with low developer impact fees, cities often fail to provide adequate maintenance of internal and arterial roads serving newly constructed subdivisions ("Fresno May End Low-Fee Policy for Developers," "Los Angeles Times", 23 August 2005).

Pedestrian denigration

New Urbanists decry the street hierarchy's deleterious effects on pedestrian travel, which is made easy and pleasant within the subdivision but is virtually impossible outside it. Residential subdivisions usually have no pedestrian connections between themselves and adjacent commercial areas, and are often separated from them by high masonry walls intended to block noise. New Urbanist writers like Andres Duany and James Howard Kunstler often point out the absurd nature of car trips forced by the street hierarchy: while a grocery store may be less than a quarter-mile distant "as the crow flies" from a given home in a subdivision, the barriers to pedestrian travel presented by the street hierarchy mean that getting a gallon of milk requires a car trip of a mile or more in each direction. Jane Jacobs, among other commentators, has gone so far as to say that modern suburban design—of which the street hierarchy is the key component—is a major factor in the sedentary lifestyle of today's children. [http://www.activeliving.org/index.php/Jane+Jacobs/19] Mass transit advocates contend that the street hierarchy's denigration of pedestrian traffic also reduces the viability of public transportation in areas where it prevails, sharply curtailing the mobility of those who do not own cars or cannot drive them, such as teenagers and the elderly.

Traffic issues

Congestion-encouraging characteristics

Most traffic engineers consider the street hierarchy to be optimal, since it eliminates through traffic on all streets except arterials. However, some have [http://www.theatlantic.com/issues/2000/12/budiansky.htm contended] that it actually exacerbates traffic congestion, leading to air pollution and other undesirable outcomes. Since especially large subdivisions often have hundreds of cars entering or exiting them at rush hour periods, the restriction of automobile traffic to a single collector-arterial intersection can create choke points that lead to significant traffic congestion. In edge cities, these numbers can be multiplied by a factor of ten or even one hundred, leading to miles-long queues to get on freeway ramps nearby. "See Rat running"

afety

Transportation planners and traffic engineers concerned with safety issues have expressed concern over the safety drawbacks presented by the street hierarchy. Recent studies ( [http://www.minority.unc.edu:9014/sph/minconf/2004/materials/ewing.et.al.pdf] , [http://www.virginia.edu/topnews/releases2002/lucy-april-30-2002.html] ) have found higher traffic fatality rates in outlying suburban areas than in central cities and inner suburbs with smaller blocks and more-connected street patterns. While some of this disparity is the result of distance from emergency medical facilities (hospitals are usually not built in a newly developed suburban area until a fairly late stage in its development), it is clear that the higher speeds engendered by the street hierarchy increase the severity of accidents occurring along arterial roads.

Future prospects

United States

While street hierarchies remain the default mode of suburban design in the United States, its 21st century usefulness depends on the prevalence of low density developments. To the degree that developable land becomes scarce in coastal urban areas and in geographically constrained inland cities such as Tucson, Las Vegas, and Salt Lake City, the street hierarchy's inability to handle any but the lowest population densities is a long-term liability. The street hierarchy is also unpopular in the coastal city of New Orleans because of its geographic barriers, and because like Philadelphia, New York, and Cleveland, New Orleans already had suburbs before the new design became popular. Grids were used in New Orleans to fit a population that had at one time reached over 700,000 into 180 square miles of land with over 20 percent of that number being dedicated to uninhabitable wetlands. There a street hierarchy took up too much space to be economical. Real estate developers in areas with high land prices, such as Southern California's Inland Empire, are finding that the relatively high population density of contemporary subdivisions is leading to severe traffic congestion on arterial roads that were country lanes a decade earlier. The street hierarchy is also becoming less attractive as awareness increases of the environmental consequences of the urban planning paradigm of which it is an integral part. The "smart growth" movement calls for street patterns with a high degree of connectivity, and with it a more balanced provision for various travel modes, both vehicular and non-vehicular.

Europe

Anecdotal reports from Germany, France, and the United Kingdom indicate that American-style street hierarchies are becoming increasingly popular as European cities suburbanize along the lines of post-1970 American cities. The of Milton Keynes, with its (national speed limit) grid roads at 1 km intervals containing 'organic' road lay-out grid-squares, was strongly founded on the 'street hierarchy' principle. It is interesting to note that the 2006 expansion plans for Milton Keynes will abandon this model in favour of "mixed-use traditional British city streets".

Developing countries

In countries such as China and India, where automobile ownership is increasing at double-digit annual rates, the street hierarchy is becoming increasingly popular as suburban development takes on forms strongly resembling those of American exurbs. However, the Chinese government has announced it that will give priority to the development of urban public transportation systems (which had stagnated in many cities) over the 2005-2010 period. The topology of Chinese suburban development will ultimately depend on the price of oil.

References

*Handy, Susan, Kent Butler and Robert G. Paterson (2003). "Planning for Street Connectivity (PAS 515)." Chicago: American Planning Association. ISBN 1-884829-86-4.
*Hise, Gregory (1997). "Magnetic Los Angeles: Planning the Twentieth-Century Metropolis." Baltimore: Johns Hopkins University Press. ISBN 0-8018-5543-8.
*Kunstler, James Howard (1993). "The Geography of Nowhere: The Rise and Decline of America's Man-Made Landscape." New York: Simon and Schuster. ISBN 0-671-70774-4.
*Nivola, Pietro (1999). "Laws of the Landscape: How Policies Shape Cities in Europe and America." Washington: Brookings Institution Press. ISBN 0-8157-6081-7.

ee also

*Hierarchy of roads
*Main street
*Side street