Permeability (spatial and transport planning)

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.

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.

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.

[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.

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.

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.

Stephen Marshall differentiates connectivity as the number of connections, and permeability as their capacity . From light gray to dark, streets become less permeable to an increasing number of transport means such as trucks, buses, cars and bikes until only pedestrians have access to the narrowest, steep or stepped streets. Connectivity remains unchanged. [ 1 ]
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
History of permeability of different types of traffic over time, in industrialised cities