Permeability (spatial and transport planning)

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 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:

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".[3]

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.

History of permeability of different types of traffic over time, in industrialised cities

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, 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]

Stephen Marshall[1] 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.

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.

• Bicycle-friendly – Urban planning prioritising cycling
• Cyclability – Degree of the ease of cycling
• Dead end (street) – Street with only one inlet/outlet
• Fifteen minute city – Urban accessibility conceptPages displaying short descriptions of redirect targets
• Fused grid – Type of urban planning design
• Low Traffic Neighbourhood – Urban planning to restrict through traffic
• Neighborhood effect averaging problem – Source of statistical bias
• Street hierarchy – Urban planning restricting through traffic of automobiles
• Urban vitality – Use intensity of a city space
• Walkability – How accessible a space is to walking
• Walking audit – Assessment of pedestrian accessibility

• Pafka, E and Dovey, K (2017) "Permeability and interface catchment: measuring and mapping walkable access", Journal of Urbanism 10(2): 150-162

• 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