- 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. [HANDY, S., CAO, X. and MOKHTARIAN, P.L., 2005. Correlation or causality between the built environment and travel behavior? Evidence from Northern California. Transportation Research Part D: Transport and Environment, 10(6), pp. 427-444.]
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 [http://www.dft.gov.uk/pgr/sustainable/manforstreets/ Manual for Streets] [http://www.dft.gov.uk/pgr/sustainable/manforstreets/ DFT, 2007. Manual for Streets. London: Thomas Telford Publishing] . Paragraph 4.2.3] , says:
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. [WHITE, GARLAND F. (1990). "Neighborhood Permeability and Burglary Rates". Justice Quarterly 7(1). ]
The second reservation concerns the effects of permeability for private motor vehicles. Steve Melia [http://www.stevemelia.co.uk/articles.htm MELIA, S.] , 2007. Eco Town Mobility. Town and Country Planning, November. and MELIA, S. 2008. Neigbourhoods Should be Made Permeable for Walking and Cycling But Not Cars. Local Transport Today, Jan 23rd 2008 ] 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:
There are also a range of arguments advanced by the proponents of
Shared spacethat where speeds are low, road users should be mixed rather than segregated.
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 Freiburgand 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 spacesolutions, 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 [http://www.tcpa.org.uk/news.asp] , 2008. Town & Country Planning Association and Department of Communities and Local Government. Ecotowns Transport Worksheet] 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" [http://www.adph.org.uk/news.php?PHPSESSID=0gcir94fu4rg4h0gm0sjumr4e6] Take Action on Active Travel, published for the working group by Sustrans, Bristol, 2008] .
A parallel debate has been occurring in North America, where some commentators have proposed the
Fused Grid, an urban form 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 StateFrank, L and Hawkins C, 2008, Giving Pedestrians an Edge—Using Street Layout to influence transportation choice, Ottawa, Canada Mortgage and Housing Corporation ] found that the fused grid was associated with significantly higher levels of walking than the other two alternatives.
Permeability and Connectivity
Stephen Marshall [MARSHALL, S., 2005. Streets and Patterns. Spon Press. ] 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.
* [http://www.vtpi.org/tdm/tdm116.htm 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
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