Street hierarchy: Difference between revisions

Revision as of 21:50, 30 March 2023

Revision as of 01:39, 14 July 2023

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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 2 to 4 and then 6 and, correspondingly, the operating speed from 40&nbsp;km/hrh to about 60&nbsp;km/hrh, 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.

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.