page_title: Permeability (spatial and transport planning)
section_title: Reservations
hierarchical_section_title: Permeability (spatial and transport planning) / Reservations
caption_attribution_description: English: An incidental, unplanned cul-de-sac that enhances traffic on a major commercial artery by limiting vehicular access to it.
context_page_description: Permeability or connectivity describes the extent to which urban forms permit movement of people or vehicles in different directions. The terms are often used interchangeably, although differentiated definitions also exist. 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’ street grid. New Urbanist thinking has also influenced Government policy in the United Kingdom, where the 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.
context_section_description: 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 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) 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".
  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.
caption: 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
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