Street hierarchy: Difference between revisions

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{{more citations needed|date=July 2019}}

{{Short description|Urban planning restricting through traffic of automobiles}}

{{Multiple issues|

{{Original research|date=October 2023|reason=Several paragraphs contain unsourced information on a niche subject.}}

{{more citations needed|date=July 2019}}

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[[File:Radburn Cellular Street Pattern.jpg|thumb|300px|right|The network structure of [[Radburn, New Jersey]] exemplifies the concept of street hierarchy of contemporary districts. (The shaded area was not built.)]]

The '''street hierarchy''' is an [[urban planning]] technique for laying out road networks that exclude automobile through-traffic from developed areas. It is conceived as a [[hierarchy of roads]] that embeds the link importance of each road type in the [[network topology]] (the connectivity of the nodes to each other). Street hierarchy restricts or eliminates direct connections between certain types of links, for example residential streets and [[arterial road]]s, and allows connections between similar order streets (e.g. arterial to arterial) or between street types that are separated by one level in the hierarchy (e.g. arterial to highway and collector to arterial.). By contrast, in many regular, traditional [[grid plan]]s, as laid out, higher order roads (e.g. arterials) are connected by through streets of both lower order levels (e.g. local and collector.). An ordering of roads and their classification can include several levels and finer distinctions as, for example, major and minor arterials or collectors.

At the lowest level of the hierarchy, [[cul-de-sac]] streets,<ref>[https://www.oregon.gov/lcd/Publications/NeighborhoodStreetDesign_2000.pdf] An Oregon Guide for Reducing Street Widths | Neighborhood Street Design Guidelines</ref> by definition non-connecting, link with the next order street, a primary or secondary "collector"—either a ring road that surrounds a neighbourhood, or a curvilinear "front-to-back" path—which in turn links with the arterial. Arterials then link with the intercity highways at strictly specified intervals at intersections that are either signalized or grade separated.

, by definition non-connecting, link with the next order street, a primary or secondary "collector"—either a ring road that surrounds a neighbourhood, or a curvilinear "front-to-back" path—which in turn links with the arterial. Arterials then link with the intercity highways at strictly specified intervals at intersections that are either signalized or grade separated.

In places where grid networks were laid out in the pre-automotive 19th century, such as in the [[American Midwest]], larger subdivisions have adopted a partial hierarchy, with two to five entrances off one or two main roads (arterials) thus limiting the links between them and, consequently, traffic through the neighbourhood.

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==History==

[[File:MedinaTunisStreetNetwork.jpg|thumb|300px|Hierarchical street network in the [[Medina of Tunis]] includes culs-de-sac (green), local streets (yellow), collectors (orange), and arterials (red) linking the gates to the city centre.]]

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 [[Pompeii]], major thoroughfares (e.g. the [[Decumanus Maximus|decumanus]]) had a width of 12.2 m, secondary streets (e.g. the [[cardo]]) 6 m and tertiary streets (e.g. vicinae) measured 4.5 meters. The first allowed for two way cart traffic, the second generally only one, while the third only loaded animals. Narrower streets that could only accommodate pedestrians were also present in both Greek and Roman cities. Thus the restriction on connections between major streets on particular modes (carts and chariots) was the effect of the width of the street itself and not the lack of linkage. This method is akin to the contemporary concept of [[permeability (spatial and transport planning)|filtered permeability]].

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]], [[Fes el Bali|Fez]], and [[Damascus]]. In theseThese cases there arefeature 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.<ref>Besim Hakim 1986, Arabic-Islamic Cities – Building and Planning Principles KPI Ltd, London</ref> 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.<ref>Nezar Alsayyad, 1991 Cities and Caliphs: on the Genesis of Arab Muslim Urbanism, Greenwood Press</ref>

In the automotive 20th century, the street hierarchy concept was first elaborated by [[Ludwig Hilberseimer]], in his ''City Plan'' of 1927. His major priorities were increasing the safety of [[primary school]]-age children [[walk to school campaign|walking to school]], and increasing the speed of traffic.

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===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|reason= Referencing "studies" and making quantitative claims without citing anything|date=January 2016}}.

In suburban areas subject to [[property tax]] caps such as California's [[California Proposition 13 (1978)|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 fee]]s, cities often fail to provide adequate maintenance of internal and arterial roads serving newly constructed subdivisions.<ref>"Fresno May End Low-Fee Policy for Developers,", ''Los Angeles Times'', 23 August 2005</ref>

Municipal records show that street maintenance represents a large portion of a municipal budget, particularly in Northern climates where snow removal is added to the regular lifecycle upkeep. Two planning strategies have been suggested to deal with these costs in new developments: reduction of street length or increase in household density, or a combination of the two. Of the two strategies, reducing street length is the most effective and permanent; densities can vary over time and cannot be effectively controlled.

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Most [[traffic engineering (transportation)|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.<ref>{{Cite web|url=https://www.theatlantic.com/magazine/archive/2000/12/the-physics-of-gridlock/378457/|title=The Physics of Gridlock|first=Stephen|last=Budiansky|date=December 1, 2000|website=The Atlantic}}</ref> 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."<ref>{{cite web |url=http://www.cues.fau.edu/cnu/docs/Traditional_Neighborhood_Development_Street_Design_Guidelines-ITE.pdf |title=Archived copy |access-date=2017-05-23 |url-status=dead |archive-url=https://web.archive.org/web/20110220174833/http://www.cues.fau.edu/cnu/docs/Traditional_Neighborhood_Development_Street_Design_Guidelines-ITE.pdf |archive-date=2011-02-20 |___location=Washington, DC}}</ref>

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.<ref>[http://www.ite.org/bookstore/RP036.pdf -Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities]</ref> 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 2two to 4four and then 6six and, correspondingly, the operating speed from 40&nbsp;km/h to about 60&nbsp;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 3three to 5five 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 point]]s 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, [[roundabout]]s 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<ref>Traditional Neighborhood Development: Will the Traffic Work? Presentation by Walter Kulash at the 11th Annual Pedestrian Conference in Bellevue WA, October 1990</ref> 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<ref>Taming the Flow—Better Traffic and Safer Neighbourhoods. Canada Mortgage and Housing Corporation, July 2008</ref> of an 830-acre (3.4-km²) subdivision tested three types of layouts: conventional, TND, and [[Fused Grid]]. It also tested the resilience of all three layouts to an increased traffic load generated by increased residential densities. The study concluded that all types of layouts perform adequately in most low to moderate population density scenarios up to a certain threshold of 62 persons per hectare (ppha). As densities increased beyond the threshold so did travel time. At a 50% density increase to 90 ppha, the conventional hierarchical pattern showed the highest increase in travel time (20%), followed by the TND (13%) and the fused grid (5%). When the density increased further to include one local job per 2two residents, delays increased respectively by 139%, 90% and 71% for the conventional, traditional, and fused grid. This confirms the density influence on congestion levels and that a hierarchical pattern can improve flow if laid out following the access restrictions proposed in the ITE/CNU practice guide.

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 [[freeway]] ramps nearby. ''See [[Rat running]]''.

====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.<ref>http://www.minority.unc.edu:9014/sph/minconf/2004/materials/ewing.etal.pdf

{{deadlinkdead link|date=November 2020}}</ref><ref>{{cite web |url=http://www.virginia.edu/topnews/releases2002/lucy-april-30-2002.html |title=Archived copy |access-date=2006-09-03 |url-status=dead |archive-url=https://web.archive.org/web/20060903234004/http://www.virginia.edu/topnews/releases2002/lucy-april-30-2002.html |archive-date=2006-09-03 }}</ref> 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<ref>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)</ref> 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,<ref>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</ref> cul-de-sac hierarchical networks appeared to be much safer than the uniform grid networks, by nearly three to one.

A second study<ref>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</ref> found the grid plan to be the least safe by a significant margin with respect to all other street patterns.

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===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.<ref>[https://trid.trb.org/view/1567244] Transportation Research Board | On-street versus off-street parking: an urban economic analysis | Created: Nov 12 2018</ref><ref>[https://web.archive.org/web/20220105160458/https://safety.fhwa.dot.gov/saferjourney1/library/countermeasures/55.htm] Federal Highway Administration |On-Street Parking</ref><ref>[https://escholarship.org/content/qt3xj0q23z/qt3xj0q23z_noSplash_946f2e7b912e7cc9e71f98c81b3912d0.pdf] Shoup, Donald. "On-Street parking management v. Off-Street parking requirements." The access almanac 42 (2013): 38-40.</ref>

==Future prospects==

{{refimprovemore citations needed|section|date=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 [[Tucson, Arizona|Tucson]], [[Las Vegas, Nevada|Las Vegas]], and [[Salt Lake City, Utah|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, Louisiana|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 {{convert|180|sqmi|km2}} 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.

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*{{Annotated link|Pedestrian zone}}

*{{Annotated link|Permeability (spatial and transport planning)}}

*[[Radial route]]

*[[Ring road]]

*[[Settlement hierarchy]]

*[[Side road]]

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*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}}.

*Southworth Michael and Eran Ben-Joseph (2003). ''Streets and the Shaping of Towns and Cities.'' {{ISBN|9-781559-639163}}

{{Road types}}

[[Category:Hierarchy]]

[[Category:Road transport]]