Alleys should be between 10 and 20 feet in width and be present in most, if not all, blocks.
Alleys are the unsung heroes of good urbanism. They increase connectivity, access, efficiency, sanitation, light, and air. Take, for example, Commonwealth Avenue in Boston, Massachusetts, one of the most beautiful streets in the world. It is essentially a 100-foot linear park flanked on either side by streets, sidewalks, front yards, and townhouses. Such a beautiful, manicured scene is only possible because of the alleys dividing the blocks. Alleys are the ideal locations for the necessary unpleasantries of modern urban life including power lines, parking spaces, dumpsters, water meters, and fire escapes. Without alleys, all these objects would need to occupy the front of the lots, interfering with the very face of the city and causing not only aesthetic harm but functional drawbacks.
Alleys provide additional access. Without alleys, all access (including curb cuts) must occur at the front of the lot. With alleys, access can occur in the back of the lot, freeing the front of the lot for broader porches or wider storefronts. Additionally, these consolidated access points for vehicular traffic make for a more walkable urbanism as there are fewer curb cuts (i.e., conflict points) with pedestrians.
Manhattan provides a good lesson of what happens when alleys are not incorporated into the urban form of a city. As the image below shows, trash pick up is forced to occur on the sidewalk which is not an ideal experience anyone: worker, resident, or tourist.
Note by the author: Consider whether alleys should be a "rule" or a "suggestion" as they are not ubiquitous across the study areas. There are more discussions that need to be added to this paper including street network, parks, etc. This discussion on alleys could be moved there, leaving three rules and, say, seven suggestions.