Street Hierarchy - History

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