Thursday, May 11, 2006

Density, vibrancy, and opportunity zones

Last week 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:
  1. 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)
  2. 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 next day, I promised these two topics (among others) 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:

(this is my first attempt at pasting tables into Blogger, so I hope they come out reasonably in your browser)

Population Sq miles Pop/sq.Mile
Manhattan 1,487,536 22.6 65,820
Houston 2,000,000 570 3,509

15 min off-peak trip in 5 min intervals, speed in mph 1st 5m 2nd 5m 3rd 5m Dist (mi) Area (pi*r^2) Population in zone
Manhattan scenarios

All walking 3 3 3 0.75 1.8 116,255
Walk/wait + subway + walk 3 30 3 3.00 28.3 1,860,078
Walk/wait + taxi* 3 12 12 2.25 15.9 1,046,294
All taxi* 12 12 12 3.00 28.3 1,860,078
Houston scenarios

Arterial drive 30 30 30 7.50 176.6 619,737
Artery, freeway, artery 30 65 30 10.42 340.7 1,195,480
Artery, then all freeway 30 65 65 13.33 558.2 1,958,674

* 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.
Comments welcome and encouraged.

UPDATE 4/4/12: This has been reposted over at Urbanophile.

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At 7:41 PM, May 11, 2006, Anonymous Anonymous said...

All that "vibrancy" in a car-based city makes me wonder what kind of air quality we're likely to have---isn't there a practical limit to the amount any city can depend on the auto?

At 10:23 PM, May 11, 2006, Blogger Tory Gattis said...

Air quality is actually improving every year as older cars die off and newer ones with far, far better emissions technologies are sold (I recently saw a chart of the progress of many cities, but I can't remember where I saw it). New diesel rules are kicking in now, too. And, of course, we all hope future vehicles will have better mileage and fewer emissions - a good bet if gas continues going up.

At 11:21 AM, May 12, 2006, Anonymous Anonymous said...

Is 30mph really a fair estimate for how fast you can travel on an arterial in Houston? I drove to the Red Onion last night on an all-arterial route that was 5 miles long, and it took a good 15 minutes despite it being light traffic. I'm guessing that it's waiting at traffic lights that drops you down from 30mph to 20mph.

At 12:58 PM, May 12, 2006, Blogger Tory Gattis said...

I think there can be quite a bit of variability, yes. I easily net well over 30mph on Beechnut where the lights are timed - and have had similar experiences on Westheimer. I believe the 30 estimate is used by most agencies and trip estimators.

At 2:37 PM, May 12, 2006, Blogger John Whiteside said...

These are interesting numbers, but there's an assumption here - that vibrancy is caused by density. I don't think that's true.

Having spent a fair amount of time in cities that are typically called "vibrant" (living in DC and Boston and outside of NYC, frequent family visits to SF, and this year about one month so far in Paris) I found myself thinking about what makes those places vibrant.

Density is part of it (and perhaps a requirement) but it's also how you navigate those cities. I think the importance of having pedestrian areas just can't be overstated. I may be able to get to as many destinations within 15 minutes of my Houston home as I could from my DC home, but the experience is completely different. In DC my neighborhood trips (the grocery store, the hardware store, out to a coffee shop) all took place on foot. That meant that along the way, I ran into neighbors - ones I knew well enough to talk to or those I just knew well enough to smile and not at. I saw what was happening on each block and with each building in a way that you can't driving by in a car - oh look, someone's fixing up that house, there's a new shop opening in that building by the post office, etc. That just doesn't happen in car trips.

One of the things I have noticed to some degree in dense American cities and to a much larger degree in Paris (and London) is the multiple uses of so much of the city, something Jacobs talks about at length. Paris stuns me with the way that one tiny neighborhood can contain shops, bars, restaurants, offices, and lots of residential space (often tucked away in courtyards behind street level). Car-oriented development (including Houston's) has a much bigger problem with single-use districts - all retail, all residential, etc.

Obviously there are many people who like that, but it is a big drag on that ever-elusive "vibrancy."

Don't take this as outright disagreement; I'm just not sure that traditional urban vibrancy is really possible in a city where people don't walk more than they do here.

At 3:05 PM, May 12, 2006, Blogger Tory Gattis said...

I think we have different definitions of "vibrancy". You're referring to "the pedestrian experience" - which is valid - but I'm referring to the energy/bustle of lots and lots of exchanges/transactions/meetings. I believe that is directly proportional to diversity/variety: if there are more things to do, people will do them. But for there to be diversity/variety, there have to be enough people - and discretionary income - within a reasonable travel time to support them.

I think it's fair to say Houston doesn't offer much of "the pedestrian experience", but you can't say it isn't a vibrant, growing city.

At 7:25 PM, May 12, 2006, Anonymous Anonymous said...


Point of information:

Your source for the COH size is wrong. The 570 Sq. Mi. figure is about 10.4% under the official area.

According to the pro-urban rail Greater Houston Partnership's 2005 HOUSTON FACTS, page 2, AREA, Houston is 636.12 square miles.

The portion of Houston which lies in Harris County is calculated to be 622.82 Sq. Mi.; the portion which lies in Fort Bend County is referenced at 8.37 Sq. Mi.; and, the portion in Montgomery County is measured at 4.93 Sq. Mi.

The Houston ETJ covers 1,271.73 Sq. Mi.

At 10:14 PM, May 12, 2006, Blogger Tory Gattis said...

I did these calculations a few years ago (there have been limited annexations since then), but according to the Wikipedia, the land area of the city of Houston is 579 sq.miles - that subtracts out water like Lake Houston.

And, of course, our population has gone beyond 2m since 2000, possibly as high as 2.1m at this point. But none of it changes the math substantially - the illustration still holds.

At 7:47 AM, December 25, 2006, Anonymous Anonymous said...

As for "isn't there a practical limit to the amount any city can depend on the auto?"

Yes. It's called 100 percent and has been virtually achieved in a number of urban areas, such as Kansas City and Indianapolis and others. Because lower densities are associated with less intense air pollution, they have no serious air pollution problem. Besides, air pollution is improving and has been for 3 decades, principally due to improvements in vehicle technology.

At 9:03 AM, October 07, 2011, Anonymous CityBeautiful21 said...

Interesting concept. While BEA is still matching up their economic output counts from 2010 with new Census data, if you take the 2010 metro GDP figures and divide by the MSA populations in Wikipedia, New York produces $67,762 per capita and Houston produces $64,673 per capita.

Obviously both metro areas perform well compare with national averages. However, the last few years have seen some of the worst in memory for finance (a key NYC industry) and some of the best for oil (a key Houston industry).

I'd still say edge: New York on this one. And even using Houston's metro population of 5.95 million, even what is a likely understated delta of $3000 per capita is almost another $18 billion dollars in the regional economy. That' real money.

At 11:57 AM, October 07, 2011, Blogger Tory Gattis said...

Thanks, although as far as NYC coming out ahead: I think if you compare the cost of living in NYC vs. Houston (ACCRA), you'll find that $64k goes a *lot* farther here. You'd need well north of $100k in NYC to live an even moderately equivalent lifestyle.


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