Sunday, October 4, 2015

Urban Density Maps

All maps use the same color scale, but not size. Scroll to the bottom for an interactive version with hover-over. The maps were originally built for this post on Atlanta public transit.

Interactive version:

MARTA Expansion: What does the data say?

There have been several recent discussions in Atlanta around major MARTA expansions. News agencies this summer reported MARTA has a not yet funded plan for an $8 billion expansion to Alpharetta, Emory, and East along I-20. A spring poll of Gwinnett voters shows their interest in MARTA, and the City of Atlanta has early plans for a streetcar expansion.

Would these expansions be good for the city? How do we think about this beyond, "I'm a liberal who likes public transit" or, "I'm a conservative who likes low taxes and my awesome SUV"?

Current Usage

For starters, how are residents currently utilizing MARTA? The graph below shows usage per station by cities with heavy rail. Atlanta ridership is not a total disaster like Cleveland, but is well behind the top five. If Atlanta residents aren't utilizing our current stations, we probably don't need more.

Riders per station is calculated by dividing total unlinked trips by total number of stations, and dividing by two to account for one entry and exit per trip.

Cost is another important component to current usage. Is it sustainable? Current MARTA CEO Keith Parker has been praised for his fiscal leadership (and several other accomplishments) but all public transit systems are publicly subsidized. The graph below shows how MARTA compares.

Source: National Transit Database

MARTA rails rides averaged a $2.05 cost to the city in 2014. Atlanta, again, is performing much better than Cleveland, but MARTA is much more expensive to run than leading cities.


Density is also a useful data point for understanding public transit. Guerra and Cervero of UC Berkeley argue that cities should set goals of at least 45 people per gross acre in the half mile surrounding heavy rail stations. (This is equivalent to about 29,000 people per square mile.)

The map below shows that metro Atlanta has no neighborhoods that are even close to this metric. Two census tracts in midtown have densities of about 19,000 per square mile.

Forty-five people per acre is only an estimate. It is also useful to compare Atlanta with other cities that have large heavy rail investments. In the visuals below, Atlanta's density is mapped along with eight other cities that have at least 50 million annual heavy rail passengers. A different color scale (0-100,000) is used from above to accommodate more dense cities. The transition from orange to blue is set at 30,000- to demonstrate Guerra and Cervero's suggested cut-off.

See this post for flat files of the same images.

Atlanta is clearly far less dense than other heavy-rail cities, and is far less dense than experts recommend for heavy rail. There are two possible takes on this: "we need more public transit to get more dense!" or, "additional public transit infrastructure is a bad investment" I tend to fall in the second camp- Atlanta already has a significant public transit investment, yet we're one of the least dense cities in the US. (Related- Atlanta ridership is high relative to our low density.) More public transit investment alone won't change that. 

Instead, metro Atlanta needs to increase density around its current stations. Midtown has an impressive number of current and projected projects, which will increase that neighborhood's density and serve as a draw for riders boarding other stops. MARTA's transit-oriented development is also a good step in that direction, but the plots ranging from two to ten acres owned by MARTA are not large enough to have a major impact on density. (Helpful math fact: there are 640 acres in a square mile.) Increasing density in a meaningful way will require partnerships and investments by local agencies beyond MARTA.