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The international border between the United States and Mexico at the San Diego-Tijuana checkpoint is the most trafficked in the world. Approximately 60 million people cross annually, moving untold amounts of goods and services back and forth. Zooming into the particularities of this volatile territory, traveling back and forth between these two border cities, we can expose landscapes of contradiction where conditions of difference and sameness collide and overlap.

It’s an urban juncture like no other in the world, where some of the wealthiest real estate in suburban San Diego lies barely twenty minutes away from some of the poorest settlements in Latin America, on Tijuana’s southern fringes. A series of off-the-radar, two-way crossings–north-south and south-north–suggests that no matter how high and long the post-9/11 border wall becomes, it will never stop the migrating populations and the relentless flows of goods and services back and forth across the formidable barrier that seeks to exclude them.

These illegal flows are physically manifested, in one direction, by the informal land use patterns and economies produced by migrant workers flowing from Tijuana into San Diego, lured by the strong economy of Southern California. But while human flow mobilizes northbound in search of dollars, the urban waste of San Diego moves in the opposite direction, where it is used to construct emergency housing in the shantytowns of Tijuana.

As the Latin American diaspora travels north, it inevitably alters and transforms the fabric of San Diego’s subdivisions. In these neighborhoods, multigenerational households of extended families shape their own programs of use, taking charge of their own micro-economies in order to maintain a standard for the household. The result: nonconforming uses and high densities that reshape the fabric of the residential neighborhoods where they settle. Alternative social spaces spring up in large parking lots; informal economies such as flea markets and street vendors appear on vacant properties. Housing additions in the shape of illegal companion units are plugged in to existing suburban dwellings to provide affordable living.

The areas of San Diego that have been most impacted by this nonconforming urbanism are concentrated in its first ring of suburbanization. The mutation of these older bedroom communities–from rigid, monocultural and one-dimensional environments to informal, multicultural and cross-programmed communities–opens the question: how do we anticipate density? It may be that the future of Southern California urbanism will be determined by tactics of retrofit and adaptation, making the large small.

In addition to immigrants retrofitting a large section of San Diego’s older mid-city neighborhoods (the typical post-war Levittowns) with alternative nonconforming structures, other parts of this first ring of suburbanization have been replaced by larger versions of themselves. As new McMansion subdivisions update these older suburbs in San Diego, the first ring of suburbanization is being dismantled, piece by piece. Small bungalows are dismembered and their pieces given away to Mexican speculators. Thus the debris of Southern California’s middle-class suburbs is recycled to build the new periphery of Tijuana.

The leftover parts of San Diego’s older subdivisions–standard framing, joists, connectors, plywood, aluminum windows, garage doors–are being disassembled and recombined just across the border. A few miles south, in Tijuana, new informal suburbs–some call them slums–spring up from one day to another. This river of urban waste flows across the Tijuana-San Diego to make something dramatically new.

On the edges of Tijuana, rife with poverty, social upheaval and a severe housing shortage, the detritus of San Diego’s suburbs is reassembled into a fresh milieu, a city made of waste. But not only small, scattered debris is imported and recycled into makeshift housing in Tijuana. Entire pieces of one city travel southward as residential ready-made houses are directly plugged in to the other city’s fabric. This process begins when a Tijuana speculator travels to San Diego to buy up the little post-World War II bungalows that have been slated for demolition. The little houses are loaded onto trailers to travel to Tijuana, where they clear customs before making their journey south. On some days here, one can see houses, just like cars and pedestrians, waiting in line to cross the border.

Finally, the houses enter Tijuana and are mounted on one-story metal frames, leaving an empty space at the street level to accommodate future uses. These floating houses define a space of opportunity beneath them, one that will be filled over time with additions like a taco stand, a car repair shop, a garden.

One city profits from the dwellings that the other one discards–recycling the “leftovers” of the other into a sort of second-hand urbanism, creating countless new possibilities. This is how the border cities enact a strange mirroring effect. While the seemingly permanent housing stock in San Diego becomes disposable overnight, the ephemeral dwellings in Tijuana yearn to become permanent.

Tijuana’s informal settlements are shaped by these cross-border recycling dynamics and by organizational tactics of invasion, allowing settlers to claim underutilized territory. While San Diego’s vast sprawl is incrementally made of gigantic infrastructure (freeways and gated communities) to support loosely scattered units of housing, on Tijuana’s edges dense habitation happens first so that incremental small infrastructure can follow. Ultimately, this intensive, recycled urbanism is emblematic of how Tijuana’s informal communities are growing faster than the urban cores they surround, creating a different set of rules for development, and blurring the distinctions between the urban, the suburban and the rural.

While the site of our intervention in Tijuana is the informal urbanism of the favela-like settlements that dot its periphery, our process begins by engaging the conflict between emergency housing, labor and maquiladora factories. We have observed that as NAFTA maquiladoras position themselves strategically adjacent to Tijuana’s slums in order to have access to cheap labor, they do not give anything to these fragile communities in return. So our first site of intervention is the factory itself, utilizing its own systems of material production and prefabrication to produce surplus micro-infrastructure for housing.

We are currently negotiating a maquiladora-made prefabricated frame that can act as a hinge mechanism to coordinate the multiplicity of recycled materials and systems brought from San Diego and reassembled in Tijuana. This small piece is also the first step in the construction of a larger, interwoven and open-ended scaffold that helps strengthen an otherwise precarious terrain, without compromising the improvisational dynamics of these self-made environments. Conditions of social emergency demand the reorganization of resources and the triangulation of prefabrication industry, government subsidies and social organization.

What can we learn from these informal settlements? At a moment when the top-down forces of privatization unleashed in recent years because of our blind belief in the power of “free market economies” have failed, it is more pressing than ever to rethink our institutions of urban and economic development.

Can the lessons hidden beneath these unofficial and precarious settlements be translated into alternative urban policies to redefine the conventional recipes of development in the official city? Producing more inclusive and sustainable land uses, new markets and economies from the ground up and within communities? I believe it is time for our institutions of representation, government and development to critically observe and translate the meaning of these invisible forces that are incrementally shaping the contemporary city.

It may be that the informal sector will become the basis for a new paradigm of environmental, social and economic sustainability.