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:
Loose zoning/permitting constraints to enable both a wide diversity of businesses as well as population density where there is consumer demand (apartments, condos, townhomes)
Maximized mobility with a well-designed, high-capacity arterial and freeway network
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.
Rename "mobility/draw-zones" to "opportunity zones", since they represent the opportunity region for a consumer, explorer, job seeker, or business owner - and the larger it is and the more people it has, the larger the opportunity and the resulting vibrancy.
How Manhattan and Houston have very similar opportunity zones despite dramatic differences in urban form, and have the potential for similar levels of vibrancy in some respects.
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:
A car-based city with a strong freeway network has the potential to match the vibrancy and diversity of a high-density city like Manhattan.
This is not to say that Houston and New York are equivalent. This is an analysis of the diversity available in a typical, everyday 15-minute trip. Special occasion trips (museums, sporting events, concerts, theater, etc.) have a much higher acceptable commute time, and therefore draw on a larger area. New York is a much older and larger city that can draw on a regional metro population of 21 million, substantially more than Houston’s 5 million.
The classic “monotony of the suburban edge cities” phenomenon is explained by looking at the all-arterial drive scenario, which is common on the fringes. The fringes also drop population density rapidly as they get farther out, further reducing the mobility zone population and therefore diversity/vibrancy (ex: the mobility zone of interest for suburban Sugar Land in southwest Houston is to the north and east, not south or west).
Los Angeles was the first large-scale car-based city, and it is often not held in high regard. Why? LA has many arterials with overloaded, slow freeways and no frontage roads (although they do have higher density to somewhat make up for it). That drives LA towards the “all-arterial” scenario, or the middle scenario at best. Houston has a strong frontage-road network with substantial retail, office, and other commercial services – the car-based city equivalent of “vibrant street retail.” Even commercial/retail space not on the frontage roads is often within a couple minutes of a frontage road. This allows Houston to make the third scenario a relatively common one, with it’s attendant high access to diversity within the mobility zone.
Jacobs describes a “density dead zone” of greater than 12 dwellings per acre but less than 100 dwellings per acre – too dense to be suburban but too sparse to be really urban. These areas almost never achieve vibrancy or diversity. Arterial-driven car-based cities with weak freeway networks seem to be the car-based equivalent of this “dead zone” with low density and relatively low-to-moderate mobility.