Recently, I found a new, useful term that was introduced in the discussion about street networks: filtered permeability. Its author Steve Melia explains: “It is the concept… that networks for walking and cycling should be more permeable than the road network for motor vehicles”
This new term helps place many known street patterns into a spectrum, from unfiltered to filtered permeability, by degree of exclusion. For example a steep, stepped, narrow street in an island village is only permeable to pedestrians and it excludes ALL other means of transport. Narrow streets in Old Fez city are permeable to pedestrians and animals but impermeable to carts or cars. Some city streets, though in principle permeable to pedestrians have turned impermeable because of the dominance of cars. By contrast, a Highway is fully permeable to all motorized vehicles but not to bicycles or pedestrians (by decree).
For planning new neighbourhoods, this means we can consciously design a degree of permeability for each of the streets in a layout. See how this is accomplished here.