Street hierarchy

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 roads, 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 plans, 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. 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 roads, 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 plans, 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—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. Since the 1960s, street hierarchy has been the dominant network configuration of suburbs and exurbs in the United States, Canada, Australia, and the UK. It is less 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 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. 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) 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, 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. 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. 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.