Street hierarchy: Difference between revisions

Revision as of 01:25, 10 January 2024

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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====