Monday, October 18, 2010

TOD Galore

A new massive database has been assembled and released at large by the Center for Neighborhood Technology and the affiliated Center for Transit-Oriented Development (TOD). The direct link to Dallas is here, we'll see if that works for you, since you have to be a registered user to access it.

Below is a sample where they map all existing and proposed transit stations and with a click will return population, # of jobs, and median income within a 1/2 mile radius. Let's see if you can guess my one issue with it:

Get this far? Well, my one and only problem is inherent to such massive assemblies of data. Each of these TOD zones requires specific attention to detail. This gets at a broader problem preventing TOD planning from becoming smarter, and that is the ubiquitous 1/2-mile walk circle.

The issue with the 1/2-mile circle, intended to simulate a 10-minute walk or a popularly accepted distance to travel by foot to transit stations is that not all 10-minute walks are the same. What is the road network like within that 1/2-mile generic circle? How many roads must be crossed? How direct is the route (yielding a more radial pattern emanating from a center of gravity or attractor - in this case the transit station)? Are there highways to be traversed? How long is the wait to cross at crosswalks, etc. etc.?

This is the type of thing that can really only be field tested and I apologize for not doing that here. Instead, I did a quick estimate based on distances and first-hand experience of riding DART to the CityPlace station and walking to various destinations (Target/West Village) several times. Below is a quick map of that.

Click to get a closer look.

As you can see, what actually constitutes an acceptable walking distance is FAR, FAR smaller than the 1/2-mile circle, because it takes a similar amount of time. In the case of CityPlace station, being 20,000 leagues beneath the sea of traffic on North Central Expressway is its problem as surfacing from the subway station can take at least 5-minutes unless you're in the mood for 2-minute thighs and feel like running up the monumental stairwells that are only missing the monument.

Other areas of the city would actually come far closer to the actual perimeter of this circle. To do so, would require a tightly-knit grid of smaller, easily crossable, pedestrian-oriented streets, preferrably in a more radial pattern than perfectly squared gridiron. The reason for this is that it creates more direct routes based on desire lines (if the transit-station is the most desirable destination in the area). Obviously, this exists only in a perfect world which over time balances the competing desire lines to other destinations.

Example of a "pure" radial grid. Many of the garden cities, which were little more than purely theoretical exercises, exercised in exurban greenfield locations (much like the pop-up "sustainable cities" of the Middle East and China that are super awesome theoretically, just lack a reason for being in the first place besides 'can do?' not 'should do?'), display similar features. Both promised utopia.

While I would never espouse a pure geometric form since no place is perfectly flat or so singular in its hierarchy of desire lines, I do prefer an overlay of radial and gridded much like Paris or DC or Papal Roman Trivia or Broadway, because it creates natural points of convergence and responds to this hierarchy and instills a logical and predictable order unto the real estate market. I should add that I'm also not a rigid adherent to the importance of straight lines and the divergence of road centerline axes leads to decay or reduced value that the Space Syntax crowd preaches. But order created by building form certainly is necessary.

Here is an image of how desire lines and changing demands shape cities over time (Rome during the empire and Rome closer to today) creating a natural order of smaller blocks, system of streets and open spaces.

For more information, see my previous posts on convergence and intersection density analysis: