In a nutshell, filtered permeability is enabling some modes of transport to continue through a street while keeping others out. In practice it is principally used to “filter out” car traffic while still letting pedestrians and those on bikes through.
The main reason why filtered permeability is good for non-car transportation is because it makes it easier to move about by foot or on bike instead of in a car.
I believe that filtered permeability should be integrated into many places of Chicago, and is a great idea in particular for grid street patterns, like those in Chicago. By enabling auto traffic on all streets in the Loop, Chicago drivers can easily get around downtown – too easily. You may argue that Loop traffic is too heavy to begin with, but in reality it is too easy to get through downtown in an automobile, especially for such a large city. The grid street network can be redesigned to allow pedestrians, people on bikes, and even public buses to “filter” through the Loop while drivers are forced to the perimeter streets and freeways, leaving the interior downtown free of the pollution, noise, and danger that drivers impose.
In particular, filtered permeability is best implemented in busier districts where street traffic is likely to always exist.
Examples of cities and neighborhoods that have filtered permeability
Groningen, the Netherlands
As the above Streetfilms video shows, Groningen city planners split the center city into 4 quadrants and made it so that you could not get from any one quadrant to another in a car. Drivers are forced to use the perimeter road instead. Of course, other transport modes are free to move about the city without such hassle.
The “bike capital of France,” the central part of Strasbourg (the island) is designed to discourage through automobile traffic by making it difficult to drive through the city, but pleasant for people walking and on bikes. And it is! The street network is easy to navigate on foot and bike, but raised bollards restrict auto traffic, making it easier to just go around or park on the perimeter and walk, bike, or take the tram like everyone else.
Smaller examples of filtered permeability
There is a street near my alma mater, the University of Wisconsin – Milwaukee, where car traffic is forced to turn but pedestrians can still walk through. It is a small example of filtered permeability (and perhaps not even intentional), but reduces through car traffic, forcing it onto arterial streets instead.
Vancouver, BC, Canada
On a trip to Vancouver I encountered several examples of filtered permeability. Read more about some on my post about the trip. You can also see one example below:
Car traffic may only go left, but bike traffic may proceed straight through.
I know there are more great examples, and this list will grow. Please let me know of some by tweeting at me or contacting me: transitized [at] gmail [dot] com.