Unfiltered permeability refers to road layouts which provide equal permeability for all modes. 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.
Following the New Urbanist movement, unfiltered permeability tends to be favoured in North America and the UK, whereas filtered permeability is more common in the cities of Northern and central Europe. This article contrasts the two approaches.
Melia, S. (2012). Filtered and unfiltered permeability: The European and Anglo-Saxon approaches