Permeability (spatial and transport planning)

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:

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

This cul-de-sac retrofit exmplifies 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. 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.

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". 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.

Permeability and connectivity

Stephen Marshall has sought to differentiate the concepts of "connectivity" and "permeability". 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. Thus widening the roads or paths leading from a place would increase its permeability, but leave its connectivity unchanged. Similarly, 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 the connectivity intact but limits its permeability to pedetrians and bicycles only while it "filters" out motorized transport.

See also

• Bicycle-friendly
• Fused Grid
• Walkability
• Walking audit

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