Street hierarchy
Encyclopedia
The street hierarchy is an urban design
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, 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—that 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.
Since the 1960s, street hierarchy has been the dominant network configuration of suburb
s 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 wide arterials, which can be as wide as the Champs-Élysées
with ten lanes 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.
, major thoroughfares (e.g. the 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 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, Marrakech
, 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. 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.
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 walking 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 placed on arterial roads that formed the boundaries of the superblock. Schools, churches, and parks were located at the center, and houses filled the residential blocks. Within the superblock, T-intersections and culs-de-sac acted as traffic calming
devices, slowing or preventing 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 been the dominant model for network layout in new suburbs from the Levittown
s onward.
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, undiferentiated and "messy" grid system. It discouraged drag racing
and dangerous high-speed driving 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 industrial park
s 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
.
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 hiererchical street patterns, reflecting an analogous street length increase.
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 fee
s, cities often fail to provide adequate maintenance of internal and arterial roads serving newly constructed subdivisions.
Municipal records show that street maintenance represents a large portion of a municipal budget, particularly in Northern climates were 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.
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 physically 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. 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.
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. An alternative to street hierarchy, 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”.
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. 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 limit, 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 2 to 4 and then 6 and, correspondingly, the operating speed from 40 km/hr to about 60 km/hr, 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 3 to 5 normal, 80 m-wide city blocks.
Large subdivisions that have many cars entering or exiting them at rush hour
periods, create choke points that lead to traffic congestion, if the road pattern limits access to the arterials (or boulevards) to few points of entry-exit only, a frequently observed practice in conventional subdivisions. Congestion would also increase on the boulevard (regional arterial) if the access restrictions are not observed. Furthermore, congestion can be density-dependent in addition to network configuration. The same geometric configuration that is ideally suited to improve traffic flow, roundabouts for example, fails to perform its 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 that was applied to large subdivisions. A study, reported in 1990 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 to the long trips as the hierarchical 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 of a subdivision about 830 acres (3.4 km2) tested three types of layouts, a conventional, a TND and a Fused Grid
. It also tested the resilience of all tree 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 percent 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 2 residents, delays increased by 139%, 90% and 71% for the conventional, traditional and fused grid respectively. This outcome confirms the density influence on congestion levels and that a hierarchical pattern, if laid out following the access restrictions proposed in the ITE/CNU practice guide, can improve flow.
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
An earlier study found significant differences in recorded accidents between residential neighbourhoods that were laid out on an undifferentiated grid and those that included cul-de-sacs 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, cul-de-sac hierarchical networks appeared to be much safer than the uniform grid networks, by nearly three to one.
A second study found the grid plan to be the least safe by a significant margin with respect to all other street patterns.
A 2009 study 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.
, 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 (466.2 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.
, 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".
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 that it 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.
Urban design
Urban design concerns the arrangement, appearance and functionality of towns and cities, and in particular the shaping and uses of urban public space. It has traditionally been regarded as a disciplinary subset of urban planning, landscape architecture, or architecture and in more recent times has...
technique for laying out road networks that exclude automobile through-traffic from developed areas. It is conceived as a hierarchy of roads
Hierarchy of roads
The hierarchy of roads categorizes roads according to their functions and capacities. While sources differ on the exact nomenclature, the basic hierarchy comprises freeways, arterials, collectors, and local roads....
that embeds the link importance of each road type in the network topology
Network topology
Network topology is the layout pattern of interconnections of the various elements of a computer or biological network....
(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
Arterial road
An arterial road, or arterial thoroughfare, is a high-capacity urban road. The primary function of an arterial road is to deliver traffic from collector roads to freeways, and between urban centres at the highest level of service possible. As such, many arteries are limited-access roads, or feature...
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
Grid plan
The grid plan, grid street plan or gridiron plan is a type of city plan in which streets run at right angles to each other, forming a grid...
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
Cul-de-sac
A cul-de-sac is a word of French origin referring to a dead end, close, no through road or court meaning dead-end street with only one inlet/outlet...
streets, 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—that 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.
Since the 1960s, street hierarchy has been the dominant network configuration of suburb
Suburb
The word suburb mostly refers to a residential area, either existing as part of a city or as a separate residential community within commuting distance of a city . Some suburbs have a degree of administrative autonomy, and most have lower population density than inner city neighborhoods...
s and exurbs
Commuter town
A commuter town is an urban community that is primarily residential, from which most of the workforce commutes out to earn their livelihood. Many commuter towns act as suburbs of a nearby metropolis that workers travel to daily, and many suburbs are commuter towns...
in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. It is also increasingly popular in Latin America
Latin America
Latin America is a region of the Americas where Romance languages – particularly Spanish and Portuguese, and variably French – are primarily spoken. Latin America has an area of approximately 21,069,500 km² , almost 3.9% of the Earth's surface or 14.1% of its land surface area...
, Western Europe
Western Europe
Western Europe is a loose term for the collection of countries in the western most region of the European continents, though this definition is context-dependent and carries cultural and political connotations. One definition describes Western Europe as a geographic entity—the region lying in the...
, and China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
.
Large subdivisions may have three- or even four-tiered hierarchies, feeding into one or two wide arterials, which can be as wide as the Champs-Élysées
Champs-Élysées
The Avenue des Champs-Élysées is a prestigious avenue in Paris, France. With its cinemas, cafés, luxury specialty shops and clipped horse-chestnut trees, the Avenue des Champs-Élysées is one of the most famous streets and one of the most expensive strip of real estate in the world. The name is...
with ten lanes or Wilshire Boulevard
Wilshire Boulevard
Wilshire Boulevard is one of the principal east-west arterial roads in Los Angeles, California, United States. It was named for Henry Gaylord Wilshire , an Ohio native who made and lost fortunes in real estate, farming, and gold mining. Henry Wilshire initiated what was to become Wilshire...
. 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
Naperville, Illinois
Naperville is a city in DuPage and Will Counties in Illinois in the United States, voted the second best place to live in the United States by Money Magazine in 2006. As of the 2010 census, the city had a total population of 141,853. It is the fifth largest city in the state, behind Chicago,...
, or Irvine, California
Irvine, California
Irvine is a suburban incorporated city in Orange County, California, United States. It is a planned city, mainly developed by the Irvine Company since the 1960s. Formally incorporated on December 28, 1971, the city has a population of 212,375 as of the 2010 census. However, the 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 PompeiiPompeii
The city of Pompeii is a partially buried Roman town-city near modern Naples in the Italian region of Campania, in the territory of the comune of Pompei. Along with Herculaneum, Pompeii was destroyed and completely buried during a long catastrophic eruption of the volcano Mount Vesuvius spanning...
, major thoroughfares (e.g. the decumanus) had a width of 12.2 m, secondary streets (e.g. the cardo
Cardo
The cardo was a north-south oriented street in Roman cities, military camps, and coloniae. The cardo, an integral component of city planning, was lined with shops and vendors, and served as a hub of economic life. The main cardo was called cardo maximus.Most Roman cities also had a Decumanus...
) 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 filtered permeability
Permeability (spatial and transport planning)
Permeability or connectivity describes the extent to which urban forms permit movement of people or vehicles in different directions. The terms are often used interchangeably, although differentiated definitions also exist...
.
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, Marrakech
Marrakech
Marrakech or Marrakesh , known as the "Ochre city", is the most important former imperial city in Morocco's history...
, Fez
Fes el Bali
Fes el Bali is the oldest and walled part of Fes, Morocco. Fes el Bali was originally founded as the capital of the Idrisid Dynasty in between 789 and 808 AD...
and Damascus
Damascus
Damascus , commonly known in Syria as Al Sham , and as the City of Jasmine , is the capital and the second largest city of Syria after Aleppo, both are part of the country's 14 governorates. In addition to being one of the oldest continuously inhabited cities in the world, Damascus is a major...
. 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. 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.
In the automotive 20th century, the street hierarchy concept was first elaborated by Ludwig Hilberseimer
Ludwig Hilberseimer
Ludwig Karl Hilberseimer was a German architect and urban planner best known for his ties to the Bauhaus and to Mies van der Rohe, as well as for his work in urban planning at Armour Institute of Technology , in Chicago, Illinois.-Life:Hilberseimer studied architecture at the Karlsruhe Technical...
, in his City Plan of 1927. His major priorities were increasing the safety of primary school-age children walking to school
Walk to school campaign
The Walk to School Campaign is a British campaign promoting the benefits of walking to school. It is a founder member of the IWALK organisation....
, and increasing the speed of traffic circulation.
Planners also began to modify the grid into a superblock
City block
A city block, urban block or simply block is a central element of urban planning and urban design. A city block is the smallest area that is surrounded by streets. City blocks are the space for buildings within the street pattern of a city, they form the basic unit of a city's urban fabric...
system, where high traffic generators such as shops and apartments were placed on arterial roads that formed the boundaries of the superblock. Schools, churches, and parks were located at the center, and houses filled the residential blocks. Within the superblock, T-intersections and culs-de-sac acted as traffic calming
Traffic calming
Traffic calming is intended to slow or reduce motor-vehicle traffic in order to improve the living conditions for residents as well as to improve safety for pedestrians and cyclists. Urban planners and traffic engineers have many strategies for traffic calming...
devices, slowing or preventing through traffic.
This model prevailed between roughly 1930 and 1955, in "instant cities" such as Lakewood, California
Lakewood, California
Lakewood is a city in Los Angeles County, California, United States. The population was 80,048 at the 2010 census. It is bordered by Long Beach on the west and south, Bellflower on the north, Cerritos on the northeast, Cypress on the east, and Hawaiian Gardens on the southeast. Major thoroughfares...
, and the Los Angeles district of Panorama City
Panorama City, Los Angeles, California
Panorama City is a district in the San Fernando Valley region of the city of Los Angeles, California. It is known as the San Fernando Valley's first planned community.-Description:...
. The street hierarchy has been the dominant model for network layout in new suburbs from the Levittown
Levittown
Levittown is the name of four large suburban developments created in the United States of America by William Levitt and his company Levitt & Sons...
s onward.
In the 1960s, when operations research
Operations research
Operations research is an interdisciplinary mathematical science that focuses on the effective use of technology by organizations...
and rational planning
Rationalism
In epistemology and in its modern sense, rationalism is "any view appealing to reason as a source of knowledge or justification" . In more technical terms, it is a method or a theory "in which the criterion of the truth is not sensory but intellectual and deductive"...
, were the prevailing analytical tools, street hierarchy was seen as a major improvement over the regular, undiferentiated and "messy" grid system. It discouraged drag racing
Drag racing
Drag racing is a competition in which specially prepared automobiles or motorcycles compete two at a time to be the first to cross a set finish line, from a standing start, in a straight line, over a measured distance, most commonly a ¼-mile straight track....
and dangerous high-speed driving in residential areas. New master-planned suburbs often codified the street hierarchy into their zoning
Zoning
Zoning is a device of land use planning used by local governments in most developed countries. The word is derived from the practice of designating permitted uses of land based on mapped zones which separate one set of land uses from another...
laws, restricting the use of grid layouts in residential districts.
Eventually, the street hierarchy was also adapted for industrial park
Industrial park
An industrial park is an area zoned and planned for the purpose of industrial development...
s and commercial developments. Use of the street hierarchy is a nearly universal characteristic of the "edge city
Edge city
"Edge city" is an American term for a concentration of business, shopping, and entertainment outside a traditional urban area in what had recently been a residential suburb or semi-rural community...
", a roughly post-1970 form of urban development exemplified by places such as Tysons Corner, Virginia
Tysons Corner, Virginia
Tysons Corner is an unincorporated census-designated place in Fairfax County, Virginia, United States. Part of the Washington Metropolitan Area located in Northern Virginia, Tysons Corner lies between the community of McLean and the town of Vienna along the Capital Beltway . The population was...
, and Schaumburg, Illinois
Schaumburg, Illinois
Schaumburg is a city located in Cook County in northeastern Illinois. A common misspelling of the city name is Schaumberg, a spelling which persists on some modern maps. Schaumburg is located just under northwest of downtown Chicago and approximately northwest of O'Hare International Airport. As...
.
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 planGrid plan
The grid plan, grid street plan or gridiron plan is a type of city plan in which streets run at right angles to each other, forming a grid...
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
City block
A city block, urban block or simply block is a central element of urban planning and urban design. A city block is the smallest area that is surrounded by streets. City blocks are the space for buildings within the street pattern of a city, they form the basic unit of a city's urban fabric...
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 hiererchical street patterns, reflecting an analogous street length increase.
In suburban areas subject to property tax
Property tax
A property tax is an ad valorem levy on the value of property that the owner is required to pay. The tax is levied by the governing authority of the jurisdiction in which the property is located; it may be paid to a national government, a federated state or a municipality...
caps such as California's Prop 13
California Proposition 13
There are two different ballot proposition in California called Proposition 13:* California Proposition 13 , about property taxation.* California Proposition 13 , about state officer salary increases....
, 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
Impact fee
An impact fee is a fee that is implemented by a local government on a new or proposed development to help assist or pay for a portion of the costs that the new development may cause with public services to the new development within the United States...
s, cities often fail to provide adequate maintenance of internal and arterial roads serving newly constructed subdivisions.
Municipal records show that street maintenance represents a large portion of a municipal budget, particularly in Northern climates were 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.
Pedestrian denigration
New UrbanistsNew urbanism
New Urbanism is an urban design movement, which promotes walkable neighborhoods that contain a range of housing and job types. It arose in the United States in the early 1980s, and has gradually continued to reform many aspects of real estate development, urban planning, and municipal land-use...
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
Andrés Duany
Andrés Duany is an American architect and urban planner.Duany was born in New York City but grew up in Cuba until 1960. He attended The Choate School and received his undergraduate degree in architecture and urban planning from Princeton University...
and James Howard Kunstler
James Howard Kunstler
James Howard Kunstler is an American author, social critic, public speaker, and blogger. He is best known for his books The Geography of Nowhere , a history of American suburbia and urban development, and the more recent The Long Emergency , where he argues that declining oil production is likely...
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 physically 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
Jane Jacobs
Jane Jacobs, was an American-Canadian writer and activist with primary interest in communities and urban planning and decay. She is best known for The Death and Life of Great American Cities , a powerful critique of the urban renewal policies of the 1950s in the United States...
, 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. 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.
Congestion causes and remedies
Most traffic engineersTraffic engineering
Traffic engineering can mean:* traffic engineering , a branch of civil engineering* teletraffic engineering, a field of statistical techniques used in telecommunications...
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
Traffic congestion
Traffic congestion is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, and increased vehicular queueing. The most common example is the physical use of roads by vehicles. When traffic demand is great enough that the interaction...
, leading to air pollution
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....
and other undesirable outcomes. An alternative to street hierarchy, 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”.
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. 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 limit, 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 2 to 4 and then 6 and, correspondingly, the operating speed from 40 km/hr to about 60 km/hr, 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 3 to 5 normal, 80 m-wide city blocks.
Large subdivisions that have many cars entering or exiting them at rush hour
Rush hour
A rush hour or peak hour is a part of the day during which traffic congestion on roads and crowding on public transport is at its highest. Normally, this happens twice a day—once in the morning and once in the evening, the times during when the most people commute...
periods, create choke points that lead to traffic congestion, if the road pattern limits access to the arterials (or boulevards) to few points of entry-exit only, a frequently observed practice in conventional subdivisions. Congestion would also increase on the boulevard (regional arterial) if the access restrictions are not observed. Furthermore, congestion can be density-dependent in addition to network configuration. The same geometric configuration that is ideally suited to improve traffic flow, roundabouts for example, fails to perform its 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 that was applied to large subdivisions. A study, reported in 1990 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 to the long trips as the hierarchical 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 of a subdivision about 830 acres (3.4 km2) tested three types of layouts, a conventional, a TND and a Fused Grid
Fused Grid
The Fused Grid is a street network pattern first proposed in 2002 and subsequently applied in Calgary, Alberta and in Stratford, Ontario . It represents a synthesis of two well known and extensively used network concepts: the "grid" and the “Radburn” pattern, derivatives of which are found in most...
. It also tested the resilience of all tree 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 percent 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 2 residents, delays increased by 139%, 90% and 71% for the conventional, traditional and fused grid respectively. This outcome confirms the density influence on congestion levels and that a hierarchical pattern, if laid out following the access restrictions proposed in the ITE/CNU practice guide, can improve flow.
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. 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 found significant differences in recorded accidents between residential neighbourhoods that were laid out on an undifferentiated grid and those that included cul-de-sacs 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, cul-de-sac hierarchical networks appeared to be much safer than the uniform grid networks, by nearly three to one.
A second study found the grid plan to be the least safe by a significant margin with respect to all other street patterns.
A 2009 study 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.
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 TucsonTucson, Arizona
Tucson is a city in and the county seat of Pima County, Arizona, United States. The city is located 118 miles southeast of Phoenix and 60 miles north of the U.S.-Mexico border. The 2010 United States Census puts the city's population at 520,116 with a metropolitan area population at 1,020,200...
, Las Vegas
Las Vegas, Nevada
Las Vegas is the most populous city in the U.S. state of Nevada and is also the county seat of Clark County, Nevada. Las Vegas is an internationally renowned major resort city for gambling, shopping, and fine dining. The city bills itself as The Entertainment Capital of the World, and is famous...
, and Salt Lake City
Salt Lake City, Utah
Salt Lake City is the capital and the most populous city of the U.S. state of Utah. The name of the city is often shortened to Salt Lake or SLC. With a population of 186,440 as of the 2010 Census, the city lies in the Salt Lake City metropolitan area, which has a total population of 1,124,197...
, 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
New Orleans, Louisiana
New Orleans is a major United States port and the largest city and metropolitan area in the state of Louisiana. The New Orleans metropolitan area has a population of 1,235,650 as of 2009, the 46th largest in the USA. The New Orleans – Metairie – Bogalusa combined statistical area has a population...
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 (466.2 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
Inland Empire (California)
The Inland Empire is a region in Southern California. The region sits directly east of the Los Angeles metropolitan area. The Inland Empire most commonly is used in reference to the U.S. Census Bureau's federally-defined Riverside-San Bernardino-Ontario metropolitan area, which covers more than...
, 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
Smart growth
Smart growth is an urban planning and transportation theory that concentrates growth in compact walkable urban centers to avoid sprawl and advocates compact, transit-oriented, walkable, bicycle-friendly land use, including neighborhood schools, complete streets, and mixed-use development with a...
" 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 1967 design of Milton KeynesMilton Keynes
Milton Keynes , sometimes abbreviated MK, is a large town in Buckinghamshire, in the south east of England, about north-west of London. It is the administrative centre of the Borough 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
Expansion plans for Milton Keynes
In January 2004, Deputy Prime Minister John Prescott announced the United Kingdom government's Expansion plans for Milton KeynesHe proposed that the population of Milton Keynes should double in the subsequent 20 years...
will abandon this model in favour of "mixed-use traditional British city streets".
Developing countries
In countries such as ChinaChina
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
and India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, 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 that it 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.
See also
- Hierarchy of roadsHierarchy of roadsThe hierarchy of roads categorizes roads according to their functions and capacities. While sources differ on the exact nomenclature, the basic hierarchy comprises freeways, arterials, collectors, and local roads....
- Main streetMain StreetMain Street is the metonym for a generic street name of the primary retail street of a village, town, or small city in many parts of the world...
- Side street