During one of our eternal research and reading probes which had us looking at and weighing the advantages, etc., of the many diverse approaches to creating “Livable Streets” (my favorite that being the term of the great and much missed Donald Appleyard), “Complete Streets”, “Quiet Streets”, “Fused Grids” . . . (just to cite a few of their many names”), we tumbled onto a phrase “Filtered Permeability” which was altogether new to us. After a bit we identified the person who had coined it, Steve Melia of the University of the West of England, and asked him to fill us in:
Filtered and Unfiltered Permeability
- Dr Steve Melia, Senior Lecturer in Transport and Planning, University of the West of England
The term ‘filtered permeability’ was first coined in Steve Melia (2008) and subsequently defined in guidance prepared for the Department of Communities and Local Government in the UK as follows:
Filtered permeability means separating the sustainable modes from private motor traffic in order to give them an advantage in terms of speed, distance and convenience. There are many ways in which this can be done: separate cycle and walk ways, bus lanes, bus gates, bridges or tunnels solely for sustainable modes.
(TCPA and CLG, 2008)
The terminology varies, but in cities such as Freiburg, Münster and Groningen the principle of filtered permeability is a key element in their success in restraining car use and promoting alternatives. Through traffic is channelled onto a limited network of main roads. Suburban developments are often designed as giant culs de sac for cars, while bridges, tunnels, cycle paths, bus gates – a whole panoply of short cuts provides a far more permeable network for the sustainable modes.
People use these modes – particularly cycling – because of the time and convenience advantage compared to travelling by car. The removal of through traffic also creates opportunities for improvements to public open spaces.
The term ‘filtered permeability’ was originally coined to differentiate these types of layout from the ‘unfiltered permeability’ which is widely recommended by governments, planners and urban designers in the UK and North America. Unfiltered permeability refers to road layouts which provide equal permeability for all modes. In North America, the rectilinear grid – with streets open to all traffic – was the traditional street layout for settlements developed before the late twentieth century.
In recent years, encouraged by the New Urbanist movement, this layout has been widely advocated as the most sustainable street form, one which encourages walking and cycling. It is worth noting, however, that the New Urbanists’ charter (Congress for the New Urbanism., 2009) is less dogmatic on this principle than some practitioners who claim to follow their principles.
The view that unfiltered permeability promotes walking and cycling is based partly on a misunderstanding, and partly on a selective reading of flawed evidence. Several studies in North America and some in the UK have tried to compare travel behaviour in ‘traditional’ grid-based streets, with layouts based on culs-de-sac and distributor roads. In fact, these comparisons disguise two countervailing forces: the traditional grid reduces journey distances on foot, but also by car. Where all road users travel together, the car will generally emerge as the quickest and most convenient option. The other possibility – filtered permeability – is generally absent from those studies because are rare in North America (and, at the town or city-wide level, in Britain).
One rare exception to this (Frank and Hawkins, 2008) compared four areas in Washington State, similar except for their different street layouts. One of these follows what has been termed the ‘fused grid’, where the streets of a ‘traditional grid’ have been blocked to through traffic, but kept open for pedestrians and cyclists. Of the four areas, this one had the highest level of walking.
In a public transport context, the principle of filtered permeability is uncontroversial. Its assumptions are built into the models used by transport planners. If a guided busway provides a time and convenience advantage compared to the same journey made by car, we would expect bus use to rise. Building a new road alongside the busway would undermine the relative advantage offered to the bus, encouraging people to travel by car. Why then do so many planners and urban designers fail to grasp the same principle applied to walking and cycling?
UK Government guidance (DfT, 2007) argues that unfiltered permeability leads to “a more even spread of motor traffic throughout the area and so avoids the need for distributor roads”. This is probably closer to the crux of the issue. By multiplying opportunities for ‘rat-running’ unfiltered permeability can increase the capacity of a road network to carry traffic – and, course to emit CO2. In other words, it is a cheaper variation on the ‘build our way out of congestion’ theme.
Distributor roads, it is sometimes argued, are ‘anti urban’, although most masterplans, even those following the New Urbanist principles still include a hierarchy of roads. You won’t find many huge road junctions in Freiburg or Groningen, but you will see substantial areas closed to through traffic, and wide main roads, some with tramlines down the middle, and wide, well-designed cycle routes – separating cyclists from cars and pedestrians. Congestion is generally less of a problem because most journeys are not made by car.
Another contested issue in this debate relates to crime. Again, conflicting, ambiguous research findings have been oversimplified to suggest that cycle paths, footpaths, or streets closed to through traffic lead to more crime. More sophisticated studies have challenged these assumptions (e.g. Cozens 2008): the devil, as always, is in the design detail. The transport advantages of filtered permeability can be combined with passive surveillance, and every street or path blocked to police cars is also blocked to getaway vehicles.
The built form is, of course, only one of several influences on travel behaviour. The circumstances of every development will differ, but if we are serious about encouraging more sustainable patterns of movement, permeability must be seen as an opportunity to differentiate between those modes we want to encourage, and those we need to restrain.
Congress for the New Urbanism. (2009) Charter of the New Urbanism.
Cozens. (2008) New Urbanism, Crime and the Suburbs, Urban Policy and Research, 26 (4) 429–444.
DfT (2007) Manual for Streets. London: Thomas Telford Publishing.
Frank, L.D. and Hawkins, D., (2008) Giving Pedestrians an Edge—Using Street Layout to influence transportation choice. Ottawa: Canada Mortgage and Housing Corporation.
MELIA, S., 2008. Neighbourhoods Should be Made Permeable for Walking and Cycling But Not for Cars. Local Transport Today, January 23rd.
TCPA and CLG, (2008) Eco-Towns Transport Worksheet. Town and Country Planning Association.
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About the author:
Steve Melia is a Senior Lecturer in Transport and Planning at the University of the West of England. His PhD explored the Potential for Carfree Development in the UK. During the three summers of 2006 to 2008 he cycled over 5,000 miles across seven countries visiting and studying European carfree developments and cities which have been successful in reducing car dependency. Steve has advised the UK Government on the transport aspects of the Eco-towns programme, and the Olympic Park Legacy Company on sustainable transport solutions for the London Olympic site when the games are over.
Filtered and Unfiltered Permeability
Dr Steve Melia, Senior Lecturer in Transport and Planning, University of the West of England