Filtered permeability is the concept, supported by organizations such as Sustrans, that walking and cycling networks should be more permeable than the road network for motor vehicles. It is argued that this encourages walking and cycling by providing a more attractive traffic-free environment and a time and convenience advantage over car driving. Evidence for this view comes from European cities such as Freiburg, and its railway suburb Vauban "Vauban (Freiburg)"), and Groningen, which have achieved high levels of walking and cycling following similar principles, sometimes described as: "a coarse grain for cars and a fine grain for cyclists and pedestrians".[5] Filtered permeability requires that cyclists, pedestrians (and sometimes public transport) be separated from private motor vehicles in some places, although it can be combined with shared space solutions, in other places in the same town or city. This is the case in Dutch cities like Drachten.

The principle of filtered permeability was first endorsed in the British government's guidance for the eco-cities program in 2008 and later by an alliance of 70 organizations concerned with public health, planning and transport in their policy statement: Take Action on Active Travel.[7]​.

A parallel debate has occurred in North America, where researchers have proposed and applied the Fused Grid, an urban street network pattern that follows the principles of filtered permeability, to address the perceived deficiencies of both the 'traditional' grid and more recent suburban street designs. A study conducted in Washington State[8]​ found that the fused grid was associated with significantly higher levels of walking than the other two alternatives. [9]​

Stephen Marshall[2]​ has sought to differentiate the concepts of "connectivity" and "permeability" (see drawing). By Marshall's definition, "connectivity" refers only to the number of connections to and from a particular location, while "permeability" refers to the ability of those connections to transport people or vehicles.

Traffic studies on the influence of street patterns on travel generally overlook this distinction and instead use two metrics to characterize a street pattern for travel purposes: connectivity and intersection density, both of which refer to regular city streets. This omission is often the result of unavailability of data for connectors that do not appear on maps and cannot be geocoded. Consequently, the potential effect of connectors that are accessible only to pedestrians and bicycles on mode choice and trip length is overlooked. The aforementioned study shows that differentiating between normal paved streets and non-vehicular roads yields clear and positive results on the influence of the latter.[4]​.

The distinction between permeability and connectivity becomes clearer in practice. Expanding roads within a network leading to destinations would increase the permeability of the network, but would leave its connectivity unchanged. In contrast, transforming existing streets that are part of a grid plan into linked and permeable cul-de-sacs, as done in Berkeley, CA (Berkeley (California)) and Vancouver, BC, keeps their connectivity intact but limits their permeability to pedestrians and bicycles only, while "filtering" motorized transportation.

Filtered permeability is the concept, supported by organizations such as Sustrans, that walking and cycling networks should be more permeable than the road network for motor vehicles. It is argued that this encourages walking and cycling by providing a more attractive traffic-free environment and a time and convenience advantage over car driving. Evidence for this view comes from European cities such as Freiburg, and its railway suburb Vauban "Vauban (Freiburg)"), and Groningen, which have achieved high levels of walking and cycling following similar principles, sometimes described as: "a coarse grain for cars and a fine grain for cyclists and pedestrians".[5] Filtered permeability requires that cyclists, pedestrians (and sometimes public transport) be separated from private motor vehicles in some places, although it can be combined with shared space solutions, in other places in the same town or city. This is the case in Dutch cities like Drachten.

The principle of filtered permeability was first endorsed in the British government's guidance for the eco-cities program in 2008 and later by an alliance of 70 organizations concerned with public health, planning and transport in their policy statement: Take Action on Active Travel.[7]​.

A parallel debate has occurred in North America, where researchers have proposed and applied the Fused Grid, an urban street network pattern that follows the principles of filtered permeability, to address the perceived deficiencies of both the 'traditional' grid and more recent suburban street designs. A study conducted in Washington State[8]​ found that the fused grid was associated with significantly higher levels of walking than the other two alternatives. [9]​

Stephen Marshall[2]​ has sought to differentiate the concepts of "connectivity" and "permeability" (see drawing). By Marshall's definition, "connectivity" refers only to the number of connections to and from a particular location, while "permeability" refers to the ability of those connections to transport people or vehicles.

Traffic studies on the influence of street patterns on travel generally overlook this distinction and instead use two metrics to characterize a street pattern for travel purposes: connectivity and intersection density, both of which refer to regular city streets. This omission is often the result of unavailability of data for connectors that do not appear on maps and cannot be geocoded. Consequently, the potential effect of connectors that are accessible only to pedestrians and bicycles on mode choice and trip length is overlooked. The aforementioned study shows that differentiating between normal paved streets and non-vehicular roads yields clear and positive results on the influence of the latter.[4]​.

The distinction between permeability and connectivity becomes clearer in practice. Expanding roads within a network leading to destinations would increase the permeability of the network, but would leave its connectivity unchanged. In contrast, transforming existing streets that are part of a grid plan into linked and permeable cul-de-sacs, as done in Berkeley, CA (Berkeley (California)) and Vancouver, BC, keeps their connectivity intact but limits their permeability to pedestrians and bicycles only, while "filtering" motorized transportation.