Urban areas that straddle the U.S.-Mexico border have grown together, but in very different ways.

Every day, students living in Juarez cross the border to go to school in El Paso. Travelers flying into the Tijuana airport can walk over to San Diego on a pedestrian bridge. Folks living in Mexico work on the American side of the border every day to earn a living in dollars. Around 350 million such people cross the border every year, many through ports of entry designed to welcome rather than ward off. Among other reasons, that’s because the U.S.-Mexico border is a boundary separating several sister cities that are, essentially, one urban and economic unit.

But while these border cities have grown together in recent decades, that growth has manifested differently for a variety of economic and cultural reasons. The difference in their urban footprints is evident in a new map, created by cartography enthusiast Sasha Trubetskoy.

Using land use data, Trubetskoy, who’s studying statistics at the University of Chicago, has arranged 14 border cities (each pair with at least 15,000 residents) side-by-side:

(Sasha Trubetskoy)

What’s immediately apparent is that, by and large, the Mexican cities seem more densely urbanized. In an accompanying table, Trubetskoy also notes that these cities are generally more populous, and calls this clustering of folks on the dense, Mexican side of the border the “urban pileup effect.” On the American side, the cities tend to sprawl.

Among the 14 urban areas he maps, San Diego is an exception with more people (around 3.2 million) than its Mexican counterpart (1.9 million). That said, its residents occupy a larger area*:

(Sasha Trubetskoy)

The two other border city pairs in the California/Baja California region are more built out on the Mexico side:

(Sasha Trubetskoy)

Here are close-ups of the cities in Arizona, where the density differential is particularly stark:

(Sasha Trubetskoy)

And here are the sister cities of Texas:

Caption
(Sasha Trubetskoy)

At the beginning of the 20th century, most of these Mexican cities were small frontier towns, writes Jesús Ángel Enríquez Acosta, a sociologist at the University of Sonora. After World War II, that started to change. As cities in the American Southwest grew, so did their Mexican equivalents, often in response to economic demands from across the border. In Tijuana, dive bars, taverns, and restaurants popped up to cater to American clientele. Nogales became a cross-border transit town, and Juarez became a nerve-center for the cross-border drug trade (although that’s changing now). Catalyzed by changes in immigration and trade policy, maquiladoras—manufacturing plants usually owned by U.S. companies—mushroomed along the border in the past few decades of the 20th century.

Within Mexico, these border cities became beacons of opportunity, of jobs and potential avenues—legal and illegal—to America. As migrants from other parts of the country poured in, these cities grew dense—with mixed-use buildings packed around plazas and parks. But not all of this growth was positive, Acosta writes in a 2009 article in the Journal of the Southwest:

The growth of maquiladora plants drastically altered the border cities’ physiognomy and social dynamics. The industry provided an extreme shock to the regional (as well as national) economy, producing a huge boom in spin-off businesses that serve the maquiladoras. Heavy and rapid population growth brought with it enormous challenges for urban planning. The pressure on city governments to maintain (much less expand) basic services, infrastructure, and equipment increased dramatically, while their capacity remained more or less stagnant. Insecurity and violence came to characterize large portions of these cities, as so-called irregular settlements grew seemingly overnight. Competition for urban land grew fierce, as the real estate market fueling “the city” boomed.

On the American side of the border, the effect of North American Free Trade Agreement has been mixed across border towns. Still, some are booming, in large part, because of their Mexican equivalents. San Diego/Tijuana together have built an economy worth $230 billion—on more than just manufacturing—Bloomberg reports. And El Paso has felt a lift in recent years since conditions in Juarez have improved. From a 2011 article in the The New York Times Magazine:

“Sometimes I wonder what El Paso lives off of,” says Tony Payan, a professor of political science at UTEP. To a large extent, the answer is that it subsists off of Juárez. There’s no real agriculture in its arid climate, and much of the city’s once-significant industrial sector has closed down or moved away.

These American border cities can do with more compact development. But, as El Paso is learning, changing a culture of sprawl is not an easy feat.

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