Permeability (spatial and transport planning)

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A Short History of Traffic Engineering A Short History of Traffic Engineering.jpg
A Short History of Traffic Engineering

Permeability or connectivity describes the extent to which urban forms permit (or restrict) movement of people or vehicles in different directions. The terms are often used interchangeably, although differentiated definitions also exist (see below). Permeability is generally considered a positive attribute of an urban design, as it permits ease of movement and avoids severing neighbourhoods. Urban forms which lack permeability, e.g. those severed by arterial roads, or with many long culs-de-sac, are considered to discourage movement on foot and encourage longer journeys by car. There is some empirical research evidence to support this view. [1]

Contents

Permeability is a central principle of New Urbanism, which favours urban designs based upon the ‘traditional’ (particularly in a North American context) street grid. New Urbanist thinking has also influenced Government policy in the United Kingdom, where the Department for Transport Guidance Manual for Streets says: [2]

Street networks should in general be connected. Connected or ‘permeable’ networks encourage walking and cycling and make places easier to navigate through.

Reservations

A diagramatic depiction of the transportation network in Vauban, Freiburg, Germany. It shows the departure from the traditional simple grid and the adoption of a complex combination grid. This network exemplifies the distinction between connectivity and permeability. The drawing shows the three types of connectors: roads in red, local streets in orange and pedestrian bicycle paths in green VaubanTraficNetwork-Schematic.png
A diagramatic depiction of the transportation network in Vauban, Freiburg, Germany. It shows the departure from the traditional simple grid and the adoption of a complex combination grid. This network exemplifies the distinction between connectivity and permeability. The drawing shows the three types of connectors: roads in red, local streets in orange and pedestrian bicycle paths in green
A diagrammatic illustration of the distinction between connectivity and permeability: From light gray to dark, streets become less permeable to an increasing number of transport means such as trucks, busses, cars and bikes until only pedestrians have access to the narrowest, steep or stepped streets. Connectivity remains identical. (Drawing based on Stephen Marshall) Connectivity versus permeability.svg
A diagrammatic illustration of the distinction between connectivity and permeability: From light gray to dark, streets become less permeable to an increasing number of transport means such as trucks, busses, cars and bikes until only pedestrians have access to the narrowest, steep or stepped streets. Connectivity remains identical. (Drawing based on Stephen Marshall)
This cul-de-sac retrofit exemplifies the difference between connectivity and permeability in practice. It was created to improve traffic flow on a major commercial "Main Street" by "filtering" cars out at this junction. Bicycles and pedestrians retain full accessibility Unplanned Rideau Street Cul-de-sac.jpg
This cul-de-sac retrofit exemplifies the difference between connectivity and permeability in practice. It was created to improve traffic flow on a major commercial "Main Street" by "filtering" cars out at this junction. Bicycles and pedestrians retain full accessibility

There are two principal reservations concerning permeability. The first relates to property crime. Although the issue is contested, there is some research evidence to suggest that permeability may be positively correlated with crimes such as burglary. [4] New research has expanded the discussion on this disputed issue. A recent study [5] did extensive spatial analysis and correlated several building, site plan and social factors with crime frequencies and identified nuances to the contrasting positions. The study looked at, among others, a) dwelling types, b) unit density (site density) c) movement on the street, d) culs–de-sac or grids and e) the permeability of a residential area. Among its conclusions are, respectively, that a) flats are always safer than houses and the wealth of inhabitants matters; b) density is generally beneficial but more so at ground level; c) local movement is beneficial, larger scale movement not so; d) relative affluence and the number of neighbours has a greater effect than either being on a cul-de-sac or being on a through street. It also re-established that simple, linear culs-de-sac with good numbers of dwellings that are joined to through streets tend to be safe. As for permeability, it suggests that residential areas should be permeable enough to allow movement in all directions but no more. The over-provision of poorly used permeability is a crime hazard.

The second reservation concerns the effects of permeability for private motor vehicles. Melia (2012) [6] proposed the terms "unfiltered permeability" and "filtered permeability" to distinguish between the two approaches.

Unfiltered permeability is the view supported by the New Urbanists that urban designs should follow "traditional" or mixed use streets, where pedestrians, cyclists and motor vehicles follow the same routes. The principal advantage claimed for this approach is that it "leads to a more even spread of motor traffic throughout the area and so avoids the need for distributor roads". [2]

There are also a range of arguments advanced by the proponents of Shared space that where speeds are low, road users should be mixed rather than segregated.

Filtered permeability

Filtered permeability is the concept, supported by organisations such as Sustrans, that networks for walking and cycling should be more permeable than the road network for motor vehicles. This, it is argued will encourage walking and cycling by giving them a more attractive environment free from traffic and a time and convenience advantage over car driving. Evidence for this view comes from European cities such as Freiburg, and its rail suburb Vauban, and Groningen which have achieved high levels of walking and cycling by following similar principles, sometimes described as: "a coarse grain for cars and a fine grain for cyclists and pedestrians". [6] Filtered permeability requires cyclists, pedestrians (and sometimes public transport) to be separated from private motor vehicles in some places, although it can be combined with shared space solutions, elsewhere in the same town or city. This is the case in Dutch towns such as Drachten.

The principle of filtered permeability was endorsed for the first time in British Government guidance for the eco-towns programme in 2008 [7] and later that year by an alliance of 70 organisations concerned with public health, planning and transport in their policy declaration: Take Action on Active Travel. [8]

A parallel debate has been occurring in North America, where researchers have proposed and applied the Fused Grid, an urban street network pattern which follows the principles of filtered permeability, to address perceived shortcomings of both the 'traditional' grid and more recent suburban street layouts. A study conducted in Washington State [9] found that the fused grid was associated with significantly higher levels of walking than the other two alternatives. A recent comparison of seven neighbourhood layouts found a 43 and 32 percent increase in walking with respect to a conventional suburban and the traditional grid in a Fused Grid layout, which has greater permeability for pedestrians than for cars due to its inclusion of pedestrian-only paths (filtering). It also showed a 7 to 10 percent range of reduction in driving with respect to the remainder six neighbourhood layouts in the set. [10]

Permeability and connectivity

Stephen Marshall [3] has sought to differentiate the concepts of "connectivity" and "permeability" (see drawing). As defined by Marshall, "connectivity" refers solely to the number of connections to and from a particular place, whereas "permeability" refers to the capacity of those connections to carry people or vehicles.

Traffic studies on the influence of street patterns on travel generally overlook this distinction and two metrics are instead used to characterize a street pattern for trip purposes: connectivity and intersection density, both of which refer to regular city streets. This omission is often the result of unavailability of data for connectors that do not appear on maps and cannot be geo-coded. Consequently, the potential effect of connectors that are accessible only to pedestrians and bicycles on mode choice and extent of travel is missed. The aforementioned study shows that differentiating between normal paved streets and non-vehicular paths yields clear and positive results about the influence of the latter. [5]

The distinction between permeability and connectivity becomes clearer in practice. Widening roads within a network that lead to destinations would increase the network's permeability, but leave its connectivity unchanged. Conversely, transforming existing streets that are part of a grid plan into permeable, linked culs-de-sac, as was done in Berkeley, CA and Vancouver, BC, retains their connectivity intact but limits their permeability to pedestrians and bicycles only, while it "filters" out motorized transport.

See also

Related Research Articles

Grid plan Type of urban plan in which city streets form a grid

In urban planning, 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. The infrastructure cost for regular grid patterns is generally higher than for patterns with discontinuous streets.

Traffic calming

Traffic calming uses physical design and other measures to improve safety for motorists, pedestrians and cyclists. It has become a tool to combat speeding and other unsafe behaviours of drivers in the neighbourhoods. It aims to encourage safer, more responsible driving and potentially reduce traffic flow. Urban planners and traffic engineers have many strategies for traffic calming, including narrowed roads and speed humps. Such measures are common in Australia and Europe, but less so in North America. Traffic calming is a calque of the German word Verkehrsberuhigung – the term's first published use in English was in 1985 by Carmen Hass-Klau.

Bicycle-friendly Urban planning prioritising cycling

Bicycle-friendly policies and practices help some people feel more comfortable about traveling by bicycle with other traffic. The level of bicycle-friendliness of an environment can be influenced by many factors including town planning and cycling infrastructure decisions. A stigma towards people who ride bicycles and fear of cycling is a social construct that needs to be fully understood when promoting a bicycle friendly culture.

Utility cycling Bicycling as transportation

Utility cycling encompasses any cycling done simply as a means of transport rather than as a sport or leisure activity. It is the original and most common type of cycling in the world.

Dead end (street) Dead-end street with only one inlet/outlet

A dead end, also known as a cul-de-sac, no through road or no exit road, is a street with only one inlet or outlet.

Car-free movement Movement to reduce the use of private vehicles

The car-free movement is a broad, informal, emergent network of individuals and organizations, including social activists, urban planners, transportation engineers and others, brought together by a shared belief that large and/or high-speed motorized vehicles are too dominant in most modern cities. The goal of the movement is to create places where motorized vehicle use is greatly reduced or eliminated, by converting road and parking space to other public uses and rebuilding compact urban environments where most destinations are within easy reach by other means, including walking, cycling, public transport, personal transporters, and mobility as a service.

City block Smallest area that is surrounded by streets

A city block, residential block, urban block or simply block is a central element of urban planning and urban design.

Living street Traffic calming in spaces shared between road users

A living street is a street designed in the interests of pedestrians and cyclists. Living streets also act as social spaces, allowing children to play and encouraging social interactions on a human scale, safely and legally. These roads are still available for use by motor vehicles, however their design aims to reduce both the speed and dominance of motorised transport. This is often achieved using the shared space approach, with greatly reduced demarcations between vehicle traffic and pedestrians. Vehicle parking may also be restricted to designated bays. These street design principles first became popularized in the Netherlands during the 1970s, and the Dutch word woonerf is often used as a synonym for living street.

Street hierarchy Urban planning restricting through traffic of automobiles

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. 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 or between street types that are separated by one level in the hierarchy By contrast, in many regular, traditional grid plans, as laid out, higher order roads are connected by through streets of both lower order levels An ordering of roads and their classification can include several levels and finer distinctions as, for example, major and minor arterials or collectors.

Coving (urban planning)

Coving is a method of urban planning used in subdivision and redevelopment of cities characterized by non-uniform lot shapes and home placement. When combined with winding roads, lot area is increased and road area reduced. Coving is used as an alternative to conventional urban "grid" and suburban zoning-driven land development layouts in order to enhance curb appeal, eliminate monotony, reduce costs, such as road surfacing and street length, while increasing the amount of land available for construction.

Carfree city Urban area absent of cars

A carfree city is a population center that relies primarily on public transport, walking, or cycling for transport within the urban area. Districts where motorized vehicles are prohibited are referred to as carfree zones. Carfree city models have gained traction due to current issues with congestion and infrastructure, and proposed environmental and quality of life benefits. Currently in Asia, Europe and Africa, many cities continued to have carfree areas due to inception before the origin of the automobile. Many developing cities in Asia are currently using the proposed model to modernize its infrastructure.

Fused grid

The fused grid is a street network pattern first proposed in 2002 and subsequently applied in Calgary, Alberta (2006) and Stratford, Ontario (2004). 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 city suburbs. Both concepts were self-conscious attempts to organize urban space for habitation. The grid was conceived and applied in the pre-automotive era of cities starting circa 2000 BC and prevailed until about 1900 AD. The Radburn pattern emerged in 1929 about thirty years following the invention of the internal combustion engine powered automobile and in anticipation of its eventual dominance as a means for mobility and transport. Both these patterns appear throughout North America. "Fused" refers to a systematic recombination of the essential characteristics of each of these two network patterns.

Automobile dependency Concept that city layouts may favor automobiles over bicycles, public transit, and walking.

Automobile dependency or car dependency is the concept that some city layouts cause automobiles to be favoured over alternate forms of transportation, such as bicycles, public transit, and walking.

Walkability Measure of pedestrianism in an area

Walkability is a measure of how friendly an area is to walking. Walkability has health, environmental, and economic benefits. Factors influencing walkability include the presence or absence and quality of footpaths, sidewalks or other pedestrian rights-of-way, traffic and road conditions, land use patterns, building accessibility, and safety, among others. Walkability is an important concept in sustainable urban design. Project Drawdown describes making cities walkable as an important solution in the toolkit for adapting cities to climate change: it reduces carbon emissions, and improves quality of life.

In England and Wales, the Manual for Streets, published in March 2007, provides guidance for practitioners involved in the planning, design, provision and approval of new streets, and modifications to existing ones. It aims to increase the quality of life through good design which creates more people-oriented streets. Although the detailed guidance in the document applies mainly to residential streets, the overall design principles apply to all streets within urban areas.

Active mobility Unmotorised transport powered by activity

Active mobility, active travel, active transport or active transportation is the transport of people or goods, through non-motorized means, based around human physical activity. The best-known forms of active mobility are walking and cycling, though other modes include running, rowing, skateboarding, kick scooters and roller skates. Due to its prevalence, cycling is sometimes considered separately from the other forms of active mobility.

Honeycomb housing

Honeycomb housing is an urban planning model pertaining to residential subdivision design.

Vauban, Freiburg

Vauban is a neighbourhood to the south of the town centre in Freiburg, Germany. It was built as "a sustainable model district" on the site of a former French military base named after Sébastien Le Prestre de Vauban, the 17th century French Marshal who built fortifications in Freiburg while the region was under French rule. Construction began in 1998, and the first two residents arrived in 2001.

The sustainable urban neighbourhood (SUN) is an urban design model which is part of 21st-century urban reform theory, moving away from the typical suburban development of the UK and US towards more continental city styles. It emerged in the UK in the 1990s, specifically from pioneering work by URBED, an urban regeneration consultancy and research centre in Manchester.

Modal filter

A modal filter, sometimes referred to as a point closure, is a road design that restricts the passage of certain types of vehicle. Modal filtering is often used to help create a low traffic neighbourhood (LTN), where motor traffic is diverted away from residential streets and instead toward feeder roads. Modal filters can be used to achieve filtered permeability within a transport network, and can encourage walking and cycling through more pleasant environments and improved safety.

References

  1. HANDY, S., CAO, X. and MOKHTARIAN, P.L., 2005. Correlation or causality between the built environment and travel behavior? Evidence from Northern California. Transportation Research Part D: Transport and Environment, 10(6), pp. 427-444.
  2. 1 2 DFT, 2007. Manual for Streets. London: Thomas Telford Publishing. Paragraph 4.2.3
  3. 1 2 MARSHALL, S., 2005. Streets and Patterns. Spon Press.
  4. WHITE, GARLAND F. (1990). "Neighborhood Permeability and Burglary Rates". Justice Quarterly 7(1).
  5. 1 2 An evidence based approach to crime and urban design Or, can we have vitality, sustainability and security all at once? Bill Hillier, Ozlem Sahbaz March 2008 Bartlett School of Graduate Studies University College London
  6. 1 2 Melia, S. (2012) Filtered and unfiltered permeability: The European and Anglo-Saxon approaches. Project, 4. pp. 6-9.
  7. Eco-towns Transport Worksheet Archived 2011-07-27 at the Wayback Machine , 2008. Town & Country Planning Association and Department of Communities and Local Government.
  8. Take Action on Active Travel, published for the working group by Sustrans, Bristol, 2008
  9. Frank, L and Hawkins C, 2008, Giving Pedestrians an Edge—Using Street Layout to influence transportation choice, Ottawa, Canada Mortgage and Housing Corporation
  10. Xiongbing Jin, 2010 Modeling the Influence of Neighbourhood Design on Daily Trip Patterns in Urban Neighbourhoods, Memorial University of Newfoundland

Further reading