Must Read 

Building codes:  The Next Frontier in Affordability.  For decades, it seems housing affordability advocates have been focused on zoning, and the limits it puts on housing construction.  But now reformers are looking under the roof to see what’s driving up costs.  And building codes, especially for multi-family construction, make housing more expensive.  Writing at Slate, Henry Grabar has an illuminating essay explaining how building codes have been captured by a self-interested group, and have thoroughly–and almost invisibly–dictated urban form and affordability.  Grabar quotes Seattle’s Mike Eliason, who’s pointed out the devastating effects of second stairway requirements:

 . . . American building codes are unusual in their requirement that even short, small apartment buildings contain two staircases. This fire-safety mandate produces apartment complexes with long central hallways (think: the Overlook Hotel) and apartments with windows on only one side. In his book Building for People, Eliason shows how this rule makes apartment construction on small lots impossible or impossibly expensive, prohibiting many designs that flourish in places like Germany.

A growing band of people and organizations are revisiting the deeply buried assumptions and practices that are codified in building codes, and that among other things, result in a paucity of elevators (which are vastly more expensive in the US than elsewhere), and other features of codes that drive up costs without providing demonstrable safety benefits.  It may be time for affordability advocates to look at regulations “inside the box” as well as those outside, like zoning.

Induced travel trumps wider roads and autonomous vehicles. Time and again, engineers have peddled wider roads and self-driving vehicles as solutions to urban traffic problems.  In reality, they’re both never going to solve traffic congestion, and are actually almost certain to make it worse.  Anything that makes driving easier, cheaper and more convenient simply leads to more driving–and more auto-dependent land use patterns. Two articles this week remind us of these basic facts.  The Scientific American has a nice, non-technical summary of the science of induced travel.

In a similar vein, writing at The Verge, David Zipper reminds us of the 150-year old Jevons Paradox, which pointed out that making some thing cheaper (originally coal) led to dramatically more coal-burning (and pollution).  The same applies to self-driving vehicles:  the more we lessen the “work” associated with operating a car, the more likely people are to take more and longer trips. 

The societal impact of self-driving cars looks even worse when considering second-order effects related to land use. Just as the ascent of car ownership fueled suburbanization in the 20th century, AVs could lead people to relocate to larger, less energy-efficient homes on the urban fringe, where car trips — now more tolerable — are longer.

And critically, the second-order effects of cheaper/easier travel show up in the landscape—development becomes more scattered, and by definition even more car-dependent.  Neither wider roads nor self-driving cars will alleviate congestion, and they’ll make our pollution, climate, and equity problems worse.

New Knowledge

How highways dissolve social ties. It is well known that urban highways were located to disrupt or destroy high poverty and minority neighborhoods, and that highways are formidable barriers to routine urban interaction.  A new paper uses data on social media connections to show how highways are barriers to social interaction even today.  The study maps the frequency of connections between people using data from Twitter (X).  In general, you are more likely to have more contacts nearby, but the data show that an intervening highway is likely to significantly decrease the likelihood of connections.  The study computes a “barrier score” for each of the 50 largest cities in the US by estimating how many more social connections one would expect in the absence of highways for a given geography.  This “barrier score” is positive for all large cities, meaning the presence of highways leads to less social interaction than one would expect given the overall relationship between space and connections.

The following map illustrates the methodology used.  Dots represent the locations of twitter users, lines show connections between users and “x” marks show where connections cross major highways.  The modeling approach uses overall data on the spatial characteristics of connections to estimate how many connections would cross the highway location if the highway weren’t there.  The difference between the two estimates represents the “barrier” effect of the highway.

This research confirms the intuition that urban highways act as significant barriers to social interaction.  

. . . by going beyond demographic approaches, observing social ties explicitly, we have shown that there is a quantifiable association between urban highways and reduced levels of social connectivity, especially at short distances. Our analysis adds a highly granular perspective to former work, corroborating and quantifying the intuition that urban highways are indeed barriers to social ties.

Interestingly, highways are more of a barrier in some cities than in others.  The authors estimate that highways are about five times as disruptive to social connections in Milwaukee, Cleveland and Orlando, than they are in New York, San Francisco and Portland.

L.M. Aiello, A. Vybornova, S. Juhász, M. Szell, & E. Bokányi, Urban highways are barriers to social ties, Proc. Natl. Acad. Sci. U.S.A. 122 (10) e2408937122, https://doi.org/10.1073/pnas.2408937122 (2025). 

In the news

In an article entitled “Growing Community Together” in the Stanford Social Innovation Review, Seth Kaplan quoted out City Report, “Lost in Place,” which charted change over four decades in the nation’s highest poverty neighborhoods.