Filtered permeability

Filtered permeability describes design of our streets and urban realm that allows through journeys for selected modes of transport, typically walking and cycling (but sometimes also buses), but removes it as a through route for motor traffic.

Filtered permeability can be achieved on existing streets either by a straightforward physical point closure, called a modal filter (e.g. a bollard), or by the use of opposed one-way streets (with exemptions for cycling), or simply by signs. Once a road or street has been 'filtered' in this way, it remains accessible to motor vehicles, but is no longer usable as a through-route. Filters can be used in isolation, but are most powerful when used strategically to create a low traffic neighbourhood, in which all of the residential streets within an area bounded by main roads become unusable or unattractive as "rat run" short-cuts for through journeys.

Filtered permeability can also describe new-build neighbourhoods which are designed from scratch with complete permeability for walking and cycling, e.g. using cycle paths and traffic-free plazas, while only allowing point access for motor traffic from a distributor road.

Filtered permeability is most commonly discussed in urban and residential contexts, but it is also useful in rural areas to prevent motorists using narrow lanes as short-cuts on through journeys, ensuring they remain attractive for walking and cycling for local journeys.

The term was coined by transport researcher Dr Steve Melia in response to prevailing attitudes of transport campaigners and practitioners in the 1990s and 2000s which were then in favour of complete permeability of streets to all traffic. That attitude was in turn a reaction to the earlier trend towards impermeable urban design, such as suburban cul-de-sacs and Radburn layout housing estates, which are usually impermeable to cycle traffic.

Public Health England specifically call for filtering on residential streets, to encourage active travel, and improve public health. (p.15).

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