Yesterday I tried taking Jane Jacobs' four tenants of vibrancy and applying them to the car-based city, describing the concept of the mobility/draw zone. It can be roughly summarized in this excerpt:
"So the four tenets of vibrancy transformed for the car-based city get reduced to two:
These two principles maximize the population within the largest possible mobility/draw zone, which gives vibrancy its best chance of reaching critical mass and flourishing."
The I promised these two topics in a future post. This is that post.
Density is a big focus of debate in today’s urban planning. Again, if your assumed mobility mode is 3mph walking, or walking plus mass transit, you need a lot of people in a small area to create vibrancy within the mobility zone. In Jacob’s world, mobility is basically fixed and density is variable. In the car-based world, density is relatively fixed (well below Jacob’s standard of >100 dwellings/acre because of the need to accommodate cars and parking plus the majority desire for single-family residential living or mid-density apartments), but mobility is variable depending on the road network and traffic congestion – which can substantially affect the size of the mobility zone. Since what really counts is the population within the 10-20 minute mobility zone – as a proxy for easily accessible diversity and vibrancy – lets take a look at some estimated mobility zones in Manhattan and Houston:
15 min off-peak trip in 5 min intervals, speed in mph1st 5m2nd 5m3rd 5mDist (mi)Area (pi*r^2)Population in zoneManhattan scenarios All walking3330.751.8116,255Walk/wait + subway + walk33033.0028.31,860,078Walk/wait + taxi*312122.2515.91,046,294All taxi*1212123.0028.31,860,078Houston scenarios Arterial drive3030307.50176.6619,737Artery, freeway, artery30653010.42340.71,195,480Artery, then all freeway30656513.33558.21,958,674
* Average Manhattan taxi covers 1.9 miles in 10 minutes, ~12 mph (source)
(note that some Manhattan scenarios actually show a mobility zone population larger than the actual population of Manhattan, due to the circular nature of the model vs. Manhattan’s actual long, thin-island geography – but it still serves its illustrative purpose)
Several interesting observations come out of this table:
Comments welcome and encouraged.
[This article was originally published by the blog Houston Strategies.]
Founding Senior Fellow with the Center for Opportunity Urbanism and Editor of the Houston Strategies blog.
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