Street hierarchy
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The street hierarchy is an
At the lowest level of the hierarchy,
In places where grid networks were laid out in the pre-automotive 19th century, such as in the
Since the 1960s, street hierarchy has been the dominant network configuration of
.Large subdivisions may have three- or even four-tiered hierarchies, feeding into one or two wide arterials, which can be as wide as the ten lane Champs-Élysées or Wilshire Boulevard. Arterials at this level of traffic volume generally require no 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
In the pre-automotive era of cities, traces of the concept of a hierarchy of streets in a network appear in Greek and subsequent Roman town plans. The main feature of their classification is their size. In Roman cities, such as
A clearer record of a stricter hierarchical order of streets appears in surviving and functioning Arabic-Islamic cities that originate in the late first millennium AD such as the Medina of Tunis, Marrakesh, Fez, and Damascus. In these cases there are four classes of streets starting with the cul-de-sac type (1.84-2.00 m wide) and moving up to the local (third order connector), then a collector that usually surrounds a residential quarter (second order connector) and, finally, to the first order connector (arterial). The latter connector usually crossed the city through its centre and led to the city gates (see drawing). These arterials were decreed to be at least wide enough for two crossing loaded animals, 3.23 to 3.5 m.[2] This tendency for hierarchical organization of streets was so pervasive in the Arab-Islamic tradition that even cities that were laid out on a uniform grid by Greeks or Romans, were transformed by their subsequent Islamic conquerors and residents, as in the case of Damascus.[3]
In the automotive 20th century, the street hierarchy concept was first elaborated by
Planners also began to modify the grid into a
This model prevailed between roughly 1930 and 1955, in "instant cities" such as
In the 1960s, when operations research and rational planning were the prevailing analytical tools, street hierarchy was seen as a major improvement over the regular, undifferentiated, "messy" grid system. It discouraged dangerous high-speed driving and street racing in residential areas. New master-planned suburbs often codified the street hierarchy into their zoning laws, restricting the use of grid layouts in residential districts.
Eventually, the street hierarchy was also adapted for
Criticisms and discussion
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 more miles of street to be laid than a grid plan to serve a much smaller population.
While housing unit density and, consequently, population density affects the per capita cost of infrastructure, it is not inextricably linked to the street network pattern whether hierarchical or uniform. Theoretically and historically a city block can be built at high or low density, depending on the urban context and land value; central locations command much higher land prices than suburban. The costs for street infrastructure depend largely on four variables: street width (or Right of Way), street length, block width, and pavement width. These variables affect the total street length of a neighbourhood and the proportion of land area it consumes. Street length increases costs proportionately while street area represents an opportunity cost of land unavailable for development. Studies show that regular, undifferentiated grid patterns generally incur infrastructure costs about 20 to 30 percent higher than the discontinuous hierarchical street patterns, reflecting an analogous street length increase.[citation needed]
In suburban areas subject to
Pedestrian degradation
Traffic issues
Congestion causes and remedies
Most traffic engineers consider the street hierarchy to be optimal, since it eliminates through traffic on all streets except arterials. However, some have contended that it actually exacerbates traffic congestion, leading to air pollution and other undesirable outcomes.[6] An alternative to street hierarchy, Traditional Neighborhood Development (TND) networks, recommended by the Institute of Traffic Engineers, implies that a type of hierarchy is desirable nonetheless. It suggests that "While TND street networks do not follow the same rigid functional classification of conventional neighborhoods with local, collector, arterial and other streets, TND streets are hierarchical to facilitate necessary movements."[7]
A more precise image of the prevalent thinking about structuring road networks can be found in the 2006 ITE/CNU recommended practice for the design of urban thoroughfares.[8] In it, the functional, traffic-engineering classifications of roads are replaced by three basic road types: boulevard, avenue and street with the addition of a second type of boulevard – the multi-way. These road types reflect familiar names and images of roads and also real conditions in an urban environment, where each type normally performs multiple functions but only up to a hierarchical limit. For example, a boulevard can function as a principal and minor arterial but not as a collector or local access street; an avenue, as principal/minor arterial and a collector but not as a street; while a street can serve as minor arterial, a collector and a local (access road) but not as a principal arterial. These exclusions of functional roles derive from the design intention to put an emphasis either on mobility or access; both cannot be accommodated concurrently in every case.
These hierarchical distinctions of road types become clearer when considering the recommended design specifications for the number of through lanes, design speed, intersection spacing and driveway access. As the number of lanes increase from two to four and then six and, correspondingly, the operating speed from 40 km/h to about 60 km/h, the intersection spacing increases from a 90–200 m range to its double (200–400 m). Similarly, the restriction on driveway access becomes more stringent and, in effect, impossible in the case of a required raised median for boulevards and multi-way boulevards. Thus a multi-way and simple boulevard (corresponding to the functional definition of arterial) are deemed to perform their mobility function better when access to them is limited to intervals between 200 and 400 m, that is every three to five normal, 80-m-wide city blocks.
A common practice in conventional subdivision design is a road pattern that limits access to the arterials (or boulevards) to few points of entry and exit. These choke points produce traffic congestion in large subdivisions at rush hour periods. Congestion also increases on the boulevard (regional arterial) if the access restrictions are not observed. Furthermore, congestion can be density-dependent in addition to being configuration-dependent. That is, the same geometric configuration ideally suited to improve traffic flow, roundabouts for example, fails to function adequately beyond a certain threshold of traffic volume. Increased traffic volume is a direct outcome of increased household density of a district.
These relationships of congestion to layout geometry and density have been tested in two studies using computer-based traffic modeling applied to large subdivisions. A 1990 study[9] compared the traffic performance in a 700-acre (2.8-km2) development that was laid out using two approaches, one with a hierarchical street layout that included cul-de-sac streets and the other a Traditional Neighborhood Design street layout. The study concluded that the non-hierarchical, traditional layout generally shows lower peak speed and shorter, more frequent intersection delays than the hierarchical pattern. The traditional pattern is not as friendly as the hierarchical to long trips but friendlier to short trips. Local trips in it are shorter in distance but about equivalent in time with the hierarchical layout.
A later more extensive comparative traffic study
In edge cities the number of cars exiting a large subdivision to an arterial that links to a highway can be extremely high, leading to miles-long queues to get on
Safety
Transportation planners and traffic engineers have expressed concerns over the traffic safety drawbacks presented by the street hierarchy. Recent studies 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.[11][12] 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.
An earlier study[13] found significant differences in recorded accidents between residential neighbourhoods that were laid out on an undifferentiated grid and those that included culs-de-sac and crescents in a hierarchical structure. The frequency of accidents was significantly higher in the grid neighbourhoods.
Two newer studies examined the frequency of collisions in two regional districts using the latest analytical tools. They investigated the potential correlation between street network patterns and frequency of collisions. In one study,[14] cul-de-sac hierarchical networks appeared to be much safer than the uniform grid networks, by nearly three to one. A second study[15] found the grid plan to be the least safe by a significant margin with respect to all other street patterns.
A 2009 study [16] suggests that land use patterns play a significant role in traffic safety and should be considered in conjunction with the network pattern. While all intersection types in general reduce the incidence of fatal crashes, four-way intersections, which occur regularly in a uniform grid, increase total and injurious crashes significantly. The study recommends hybrid street networks with dense concentrations of T-intersections and concludes that a return to the 19th century gridiron is undesirable.
Banning on-street parking
Banning on-street parking can provide social benefits if the car users and the general public pay for off-street parking.[17][18][19]
Future prospects
This section needs additional citations for verification. (November 2020) |
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
Europe
The 1967 design 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. The 2006 expansion plans for Milton Keynes will abandon this model in favour of "mixed-use traditional British city streets".[citation needed]
Developing countries
In countries such as 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 suburban-like cities in China are the aftermath of excessive implementing hierarchical street-layout and rapid urban development. With high-rise residential towers, over-engineered roads and public transportation systems, they are distinctively different from American suburbs. The street hierarchy theory forms the center of the Chinese planning system which was adapted from ex-soviet in 60's. Today, Chinese planning schools are continuing to teach the theory unaware of its effects on the suburbanization, congestion and wasteful road-engineering.[citation needed]
See also
- Complete streets
- Green transport hierarchy
- Road hierarchy
- Main Street
- Pedestrian zone – Urban car-free area reserved for pedestrian use
- Permeability (spatial and transport planning) – Freedom of movement of traffic
- Settlement hierarchy
- Side road
- Transit desert
- Transit mall
- Types of road
References
- ^ [1] An Oregon Guide for Reducing Street Widths | Neighborhood Street Design Guidelines
- ^ Besim Hakim 1986, Arabic-Islamic Cities – Building and Planning Principles KPI Ltd, London
- ^ Nezar Alsayyad, 1991 Cities and Caliphs: on the Genesis of Arab Muslim Urbanism, Greenwood Press
- ^ "Fresno May End Low-Fee Policy for Developers", Los Angeles Times, 23 August 2005
- ^ "America's Most Sedentary Cities - Forbes". Forbes. June 3, 2016. Archived from the original on 2016-06-03.
- ^ Budiansky, Stephen (December 1, 2000). "The Physics of Gridlock". The Atlantic.
- ^ "Archived copy" (PDF). Washington, DC. Archived from the original (PDF) on 2011-02-20. Retrieved 2017-05-23.
{{cite web}}
: CS1 maint: archived copy as title (link) - ^ Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities
- ^ Traditional Neighborhood Development: Will the Traffic Work? Presentation by Walter Kulash at the 11th Annual Pedestrian Conference in Bellevue WA, October 1990
- ^ Taming the Flow—Better Traffic and Safer Neighbourhoods. Canada Mortgage and Housing Corporation, July 2008
- ^ http://www.minority.unc.edu:9014/sph/minconf/2004/materials/ewing.etal.pdf [dead link]
- ^ "Archived copy". Archived from the original on 2006-09-03. Retrieved 2006-09-03.
{{cite web}}
: CS1 maint: archived copy as title (link) - ^ Eran Ben-Joseph, Livability and Safety of Suburban Street Patterns: A Comparative Study (Berkeley, CA: Institute of Urban and Regional Development, University of California, Working Paper 641, 1995)
- ^ Using Macrolevel Collision Prediction Models in Road SafetyPlanning Applications Gordon R. Lovegrove and Tarek Sayed Transportation Research Record: Journal of the Transportation Research Board, No. 1950, Transportation Research Board of the National Academies, Washington, D.C., 2006, pp. 73–82
- ^ Sun, J. & Lovegrove, G. (2009). Research Study on Evaluating the Level of Safety of the Fused Grid Road Pattern, External Research Project for CMHC, Ottawa, Ontario
- ^ Eric Dumbaugh and Robert Rae. Safe Urban Form: Revisiting the Relationship Between Community Design and Traffic Safety. Journal of the American Planning Association, Vol. 75, No. 3, Summer 2009
- ^ [2] Transportation Research Board | On-street versus off-street parking: an urban economic analysis | Created: Nov 12 2018
- ^ [3] Federal Highway Administration |On-Street Parking
- ^ [4] Shoup, Donald. "On-Street parking management v. Off-Street parking requirements." The access almanac 42 (2013): 38-40.
- General
- 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.