Permeability (spatial and transport planning)
Encyclopedia
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.
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 Department for Transport guidance Manual for Streets says:
. Although the issue is contested, there is some research evidence to suggest that permeability may be positively correlated with crimes such as burglary.
New research has expanded the discussion on this disputed issue. A recent study did extensive spatial analysis and correlated several building, site plan and social factors with crime frequencies and identified subtle 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. Steve Melia gave the terms "unfiltered permeability" and "filtered permeability" to the opposing views.
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".
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.
, 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". 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 some 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 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.
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 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.
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 (ref 8) shows that, differentiating between normal paved streets and non-vehicular paths, yields clear and positive results about the influence of the latter.
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.
Permeability is a central principle of New Urbanism
New urbanism
New Urbanism is an urban design movement, which promotes walkable neighborhoods that contain a range of housing and job types. It arose in the United States in the early 1980s, and has gradually continued to reform many aspects of real estate development, urban planning, and municipal land-use...
, 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 Department for Transport guidance Manual for Streets says:
Reservations
There are two principal reservations concerning permeability. The first relates to property crimeProperty crime
Property crime is a category of crime that includes, among other crimes, burglary, larceny, theft, motor vehicle theft, arson, shoplifting, and vandalism. Property crime only involves the taking of money or property, and does not involve force or threat of force against a victim...
. Although the issue is contested, there is some research evidence to suggest that permeability may be positively correlated with crimes such as burglary.
New research has expanded the discussion on this disputed issue. A recent study did extensive spatial analysis and correlated several building, site plan and social factors with crime frequencies and identified subtle 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. Steve Melia gave the terms "unfiltered permeability" and "filtered permeability" to the opposing views.
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".
There are also a range of arguments advanced by the proponents of Shared space
Shared space
Shared space is an urban design concept aimed at integrated use of public spaces. It encourages traffic engineers, urban planners and experts from other fields to consult with users of public space when planning and designing streets and squares in both built and non-built environments...
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 SustransSustrans
Sustrans is a British charity to promote sustainable transport. The charity is currently working on a number of practical projects to encourage people to walk, cycle and use public transport, to give people the choice of "travelling in ways that benefit their health and the environment"...
, 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
Freiburg
Freiburg im Breisgau is a city in Baden-Württemberg, Germany. In the extreme south-west of the country, it straddles the Dreisam river, at the foot of the Schlossberg. Historically, the city has acted as the hub of the Breisgau region on the western edge of the Black Forest in the Upper Rhine Plain...
, 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". 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
Shared space
Shared space is an urban design concept aimed at integrated use of public spaces. It encourages traffic engineers, urban planners and experts from other fields to consult with users of public space when planning and designing streets and squares in both built and non-built environments...
solutions, elsewhere in the same town or city. This is the case in some Dutch towns such as Drachten
Drachten
Drachten is a Dutch town, located in the municipality of Smallingerland in the province of Friesland.-Beginnings:Drachten began as a small community on the east side of the Drait River. There, early settlers started draining the land to use it for agriculture...
.
The principle of filtered permeability was endorsed for the first time in British Government guidance for the eco-towns programme in 2008 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.
A parallel debate has been occurring in North America, where researchers have proposed and applied the Fused Grid
Fused Grid
The Fused Grid is a street network pattern first proposed in 2002 and subsequently applied in Calgary, Alberta and in Stratford, Ontario . 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...
, 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 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
Fused Grid
The Fused Grid is a street network pattern first proposed in 2002 and subsequently applied in Calgary, Alberta and in Stratford, Ontario . 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...
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.
Permeability and connectivity
Stephen Marshall 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 (ref 8) shows that, differentiating between normal paved streets and non-vehicular paths, yields clear and positive results about the influence of the latter.
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
Grid plan
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...
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.
External links
- TDM Encyclopedia An Australian View on 'roadway connectivity' - contains many further links. Tends towards the conventional view of connectivity with little reference to the reservations outlined here