|A traditional American main street, St. Cloud, Minnesota|
|The ground floor has near 100% lot coverage, the upper floors, which are residential, have about 50% due to the need for windows in order to have bedrooms, leading to an L-shape building|
|These buildings are another example, the upper floors are sitting on commercial pedestals that have near 100% lot coverage|
|Older residential area near St. Cloud's downtown area, mostly single-family houses with a density of about 10 000 people per square mile (4 000 per square kilometer), so about 40/hectare|
|Another example in the same city, but with a bit more low-rise apartments, resulting in a density of 15 000 to 20 000 people per square mile (6 0000 to 8 000 per square kilometer), about 60 to 80 per hectare|
Big boxes, mom-'n-pop stores: the logic of commercial uses
A modelized thought experiment
|Scenario 1 is a traditional North American town with a dense downtown and a relatively small population (10 000) Scenario 2 is a 20 000-people city with a typical sprawl-type commercial area in the center (low density commercial)|
|Scenario 3 is the opposite of scenario 2: the commercial center is still dense, but residential density is reduced by 33%. Scenario 4 is what is often seen, with low-density commercial areas largely located on the periphery|
Scenario 2 is a scenario where the small town has grown to 20 000 people, allowing for a more diversified commercial choice. However, there are two main differences:
1- Segregated uses: there is no residential upper stories in the commercial downtown
2- Low-density commercial developments that reflect typical strip mall commercial developments with commercial areas surrounded by huge parking lots
|A South Burlington, VT mall (thanks to Bill Morris in the comments for pointing out it's not actually in Burlington)...|
|...which has 8 600 square meters of floor area on a lot of 41 000 square meters, for a FAR of roughly 20%|
|Levittown, NY is a good example of that design, however the commercial area is not radial but linear|
Comparing Scenario 2 to Scenario 3 should demonstrate the relative importance of commercial vs residential density.
Finally, Scenario 4 is a much too frequent example of a small town which traditional downtown has been converted to strip malls (20% FAR commercial), but it has not been allowed to increase in size, so the downtown only has one fifth of the commercial uses it could hold before. The rest of the commercial buildings have instead been built at the periphery, near a theoretical highway, because that was the only place available for it. So we have a commercial area at the periphery, with a small commercial node at the center of the town.
Distance to center of commercial area
So the old American town is a very walkable town, with most people living within 10 minutes of the downtown area, but with only 10 000 people, the commercial area will not offer a lot of variety and choice. This is a town of small stores with relatively high prices and poor product selection.
Scenario 4 shows how the habit of building low-density commercial areas on the periphery by lack of space zoned commercial in the city is disastrous. There's very, very few people who live within walking distance of the commercial zone's center.
Scenarios 2 and 3 are the most interesting results. Here, we can see that though scenario 3 has a lower residential density and the median distance to the center is higher than Scenario 2, it still remains that significantly more people are within walking distance of the commercial zone. This makes sense as a more compact commercial zone allows the residential zone to begin much closer to the center. It's important to remember that walkability is a very, very local matter.
Distance to farthest commercial useThis is a variant of the first analysis. Instead of looking at the distance to an arbitrary center of the commercial zone, I look at the distance to the farthest commercial use, basically the opposite side of the commercial zone. This is an interesting data because it shows how much any one person can walk to the ENTIRETY of the commercial offerings rather than to a "center" where there might be nothing at all.
|The center (red dot) of this commercial zone is not actually close to any commercial building except a small fast-food restaurant, every commercial building is 1,5 to 2 minutes away on foot.|
The traditional small town is still exemplary, but the small advantage of the scenario with a compact commercial zone but lower residential density (scenario 3) over that of the scenario with traditional residential density but a low density commercial zone (scenario 2) has become much more significant. Nearly 10% of the population in scenario 3 lives within very walkable distance from all commercial uses, but 0% do in scenario 2 (normal, as the commercial zone itself has a diameter of over 600 meters).
Note also that in reality, the patterns today are often more linear than radial, which makes things even worse for walkability. Also, that people may demand access to certain commercial uses that a 20 000-people city or neighborhood cannot support on its own (major mall, Wal-Marts or the like), which increases the need for proper transit so these uses can still be accessed without cars.
ConclusionAt least in the case of our recent development patterns, it would seem that achieving commercial density is far more important to re-establishing walkable cities than densifying residential areas. Concentrating commercial uses and economic activities creates an area where residents can access most economic activities on foot. Even if there are few people residing there currently, this is still an opportunity for redevelopment to bring people there, a way to urbanise a city or a neighborhood without even needing to affect any residential area beyond the walkable distance of the commercial core.
Currently, commercial areas tend to be both extremely low-density and far from residential areas...
|Above, a map of residential density of Levittown, NY, below, the WalkScore map of the same area, note how the highly walkable areas have no people in them, almost like a mirror image, due to use separation|
With a dense commercial core which is eminently walkable by itself, transit can become quite efficient as people living too far from it to walk only need to go there by transit once, when they get down from the stop or station, they have access to everything, needing transit only to go back home once they have finished their activities for the day.