Allegheny County, Pennsylvania, has more structurally deficient bridges than any other county in the United States—201 of 1,250, or 16 percent. Improvements would cost at least $150 million.

Structurally deficient bridges are characterized by deteriorating conditions and reduced load-carrying capacity. Although this classification does not imply that a bridge is unsafe, it does indicate that the bridge likely requires significant repair and maintenance to remain in service and eventually may need major rehabilitation or replacement.

View the PDF version of this map for more detail and a key.

Source: The Place Database. www.lincolninst.edu/research-data/data/place-database

Today an indigenous Bolivian produce vendor glides through the air on an aerial cable car to reach the market in La Paz. A student in Bogotá, Colombia, knows she will arrive on time for class because the city’s bus rapid transit (BRT) network never gets stuck in traffic. A car owner in São Paulo, Brazil, leaves the keys at home because the city’s ban on rush hour driving in the city center applies to his license plate number that day. A young middle-class family lives comfortably without a car in downtown Santiago, Chile, thanks to new sidewalks and bike lanes for neighborhood trips and a clean, safe combination of subway and BRT to navigate the rest of the city. And a day laborer in Rio de Janeiro’s favelas can count on a shared van that serves his neighborhood when the city’s official bus system does not.

These slices of life in Latin America’s big cities are not unusual. Bus rapid transit (BRT) lines zip through the heart of 54 cities in the region. Aerial cable cars connect steep hillside neighborhoods with the rest of town in over a half-dozen cities. Pedestrians and cyclists of all social classes are increasingly finding their way on busy urban streets. Informal transit options abound, although their safety and reliability vary widely, as does tolerance from public officials. Subway systems are being built out, albeit slowly. Car ownership remains well below averages in the developed world.

Altogether, Latin America has earned a reputation as a global innovator in urban transit. Latin American cities have garnered 9 of the 16 annual Sustainable Transport Awards given by the Institute for Transportation and Development Policy (ITDP), and they regularly place as finalists in the C40 Cities4Mobility Awards.

There have been some impressive successes. Public-private partnerships (P3s) are now de rigueur across the region: national governments are funding infrastructure and overseeing long-term plans while private firms bid to operate routes. The World Bank estimates that Latin America invested $361.3 billion for energy and transport infrastructure in more than a thousand P3s over the last decade, with the lion’s share in Brazil, Colombia, and Mexico. Meanwhile, Brazil and Colombia have deployed land value capture in order to finance the expansion of BRT networks and the construction of new rail lines (Smolka 2013).

Amid the Latin American transport boom, however, there have also been busts. Overcrowding on Bogotá’s TransMilenio system, the region’s largest BRT network, has led to periodic riots. Rio staked its Olympic legacy on enhanced mobility with a citywide build-out of BRT and three aerial cable cars to serve favela communities, but endemic corruption and top-down planning resulted in unkept promises. The zeal to implement new transit corridors in places like Quito, Ecuador, has come at the expense of informal operators serving the poorest urban dwellers.

“Latin America is innovating, but we still don’t know if that innovation brings a virtuous cycle to generate resources for the city,” said Clarisse Linke, director of ITDP’s Brazil office. “Are we benefitting the poor so they don’t have to travel 50 kilometers each way to work?”

Such questions are at the heart of Latin America’s transportation innovation paradox. The region may have invented creative ways to move people around crowded urban centers, but can it deliver on a broader need to reduce crushing inequality? When it comes to that level of innovation, the awards jury is still undecided.

BRT Boom

Transportation innovation has flourished in Latin American cities, primarily due to two factors: rapid urbanization and extreme inequality. Despite improvements in recent decades, 8 of the 20 least equal countries in the world, as measured by the Gini index, are in Latin America. And demographers consider the region the most urban in the world. Eighty percent of its population resides in cities, and rates are even higher in Argentina, Brazil, and Chile. Those teeming cities emerged during a postwar economic boom that sparked massive rural-to-urban migration. As peasants, farmers, and indigenous people came down from the Andes or left the arid hinterlands of northeastern Brazil, they did not encounter a ready supply of inner-city housing. Instead, they were shunted to the edges of cities or onto steep hillsides unsuitable for construction.

Seas of poor people surrounding islands of affluence became the socioeconomic norm in the region (Gutman and Patel 2018). New arrivals to a city often find centrally located jobs as maids, janitors, construction workers, or cooks. This creates a need to move large numbers of low-wage workers relatively long distances, to where they can afford to live.

Plenty of enterprising options have sprung up to meet the demand. Shared vans or taxis, known as colectivos (Spanish) or kombis (Portuguese), began plying routes to serve new neighborhoods that overburdened municipal governments couldn’t reach or intentionally neglected. Privately operated bus fleets popped up, offering frequent but uncoordinated service that saw companies competing against each other and drivers competing against the clock in ways that left gaps, duplication, and unsafe conditions.

In the late 1960s and early ’70s, the cash-strapped public sector used its limited resources to invest in rail networks in only the biggest cities. Subways in Mexico City, São Paulo, and Santiago are prime examples from this period. Although they serve millions of passengers daily, they don’t compare well to the comprehensive rail networks of similarly sized megacities like London and Tokyo.

Enter bus rapid transit. While the idea is credited to British urban planner Peter Midgley, a retired World Bank consultant who devised the first dedicated bus lanes in French and Belgian cities in the late 1960s, it was Curitiba, Brazil, that evolved the first BRT system. The 20-kilometer line that opened in 1974 featured not just dedicated bus lanes, but also enclosed stations, pre-boarding payment, and all-door boarding—features that make subways swift and convenient.

Curitiba’s then mayor, architect Jaime Lerner, who became famous for his urban design interventions in the southern Brazilian city, had federal funding for a metro line. But he realized that the city could produce a much longer dedicated bus system for the same price as a much shorter subway line. With bus stops that had the look and feel of subway stations, and zoning that allowed taller buildings on major corridors near the stations, Curitiba gained most of the benefits of a subway line with a limited budget.

That basic approach appealed to Latin American cities. “We didn’t have the resources or the time to implement rail-based transport,” said Linke. “It was an urgent situation because our cities were already heavily populated, and we needed more transit coverage.”

The model evolved in Bogotá under Mayor Enrique Peñalosa—who is back in office after a 14-year hiatus. The city of 8 million is conspicuously absent from the list of Latin American metropolises with subway systems, because Peñalosa, like Lerner, invested heavily in BRT instead in the late 1990s. Bogotá’s TransMilenio system grew to become one of the largest BRT networks in the world. With 210 kilometers of routes and over 2 million passengers daily, the TransMilenio rivals many underground networks.

Curitiba and Bogotá represent something of the golden era for Latin American BRT, as these two cities proved, at least for a time, that they could transport a critical mass of residents for a fraction of the cost of heavy rail, sparking a worldwide trend. Meanwhile, cities like Santiago, São Paulo, Rio, Mexico City, and Quito moved to implement BRT lines as a complement to trains, mostly filling in gaps rather than building out rail networks.

BRT, in turn, became identified with Latin America in transportation and policy circles. Think tanks like the Brookings Institute held seminars on what US public transportation could learn from the Latin American BRT boom. The World Resources Institute (WRI) championed BRT as a Latin American innovation and identified Latin America as home to the bulk of the world’s BRT passengers, nearly 20 million people daily.

Middle of the Road

While the global fervor around BRT continues, unabashed boosterism has been tempered by growing criticism, and Bogotá’s TransMilenio has been the main lightning rod. The system’s approval rating has plummeted from 90 percent to around 20 percent, with chronic overcrowding the main complaint. Like Tokyo’s infamously overcrowded mass transit system, TransMilenio is designed for 6 people per square meter—compared to Sweden’s transit design standard of 2 per square meter or New York City’s average of 2.7 per square meter. This means passengers are squeezed so tightly they may not be able to disembark at their stop. And the system routinely carries as many as 8 or 9 people per square meter, so at peak times it can take 45 minutes just to find a bus with room to board.

While the city continues its efforts to shore up TransMilenio, most recently announcing $8 million to enlarge 49 of 138 stations so they can accommodate more passengers, the system’s flaws have driven more Bogotanos to alternate modes of transportation. The increased reliance on private cars and taxis has produced the sixth-worst traffic congestion on the planet, according to the INRIX 2017 Global Traffic Scorecard. And after 60 years, Bogotá is finally poised to invest in a metro line.

“Today we celebrate that we reached a point of no return with the Bogotá Metro,” said Peñalosa last September when Colombian President Juan Manuel Santos approved national funding for the project. Transportation officials determined that a 30-kilometer subway system powered by renewable hydroelectric energy was preferable to more BRT, which has been slow to convert to clean electric buses from dirty diesels.

Some still favor the cost benefits of BRT. Colombian transportation economist Juan Pablo Bocajero at the University of the Andes estimates that the city loses $800 million annually (0.5 percent of its GDP) to traffic congestion. “If I had to decide between a 30-kilometer subway and a 200-kilometer BRT, I would probably choose the BRT,” he told Public Radio International’s The World in 2015. But even TransMilenio diehards like the system’s former deputy general manager, Dário Hidalgo, who now coordinates WRI’s Observatory of the BRT Center for Excellence, have publicly supported the Bogotá metro.

The BRT versus metro debate also played out in Brazil, where both Porto Alegre and Curitiba considered subway lines after receiving a huge injection of capital from the federal government’s public infrastructure spending campaign, much of it funneled into 2014 World Cup host cities. While on paper both opted for a subway, favoring higher capacity and ribbon-cutting potential over the cost-benefit efficiency of BRT, Brazil’s political and economic crisis over the last few years has led both cities to suspend their projects. Curitiba has petitioned the federal government for permission to redirect its roughly $500 million federal grant back into the city’s flagship BRT system.

Nonetheless, transit investment is not a zero-sum competition, notes Daniel Rodriguez, a University of California, Berkeley scholar and Lincoln Institute fellow, citing research on US metropolitan areas (Levine 2013). Overall, spending on different modes of transit tends to rise and fall together, and spending on one mode has a neutral or complementary effect on another.

Aerial Cable Cars

A more recent innovation reflects a willingness to invest in poorer neighborhoods shaped by the unique topography of Latin American cities, where informal settlements often cling to hillsides. As Curitiba inspired a BRT boom, the aerial cable car inaugurated in Medellín in 2004 likewise inspired a half-dozen other Latin American cities. At a cost of $5 to 10 million per kilometer, it compares favorably with rail transport that couldn’t necessarily navigate the formidable terrain above Medellín’s valleys or between high-altitude El Alto and La Paz. Cable cars have slashed travel times in complicated areas previously navigable only by motorbikes, pedestrians, and small vehicles. But there are notable exceptions: Rio’s two cable car lines have been shuttered for over two years after corruption probes discovered that construction firms colluded with public officials to overcharge for the projects by tens of millions of dollars.

While the public sector debates the merits of BRT, private bus fleets continue to serve every Latin American city, and local governments have tried with mixed success to rein in the chaotic overlapping networks of buses. In 2007, Santiago’s publicly subsidized, privately run Transantiago introduced smart cards, scrapped old modified trucks in favor of new buses, and brought the entire system under the authority of one agency. But commuters felt frustrated that the radical reform—considered the most ambitious in the transport sector of a developing country—was imposed on them too rapidly. Although Santiago’s system was more reliable than many Latin American cities’ overall transport networks, in 2017 the think tank Espacio Público called it the worst public policy decision since Chile’s return to democracy, in large part because of the billions of subsidies the government pays to private bus operators to keep the system running.

The inadequacies of Santiago’s BRT stemmed in part from an initial lack of public subsidies for the private bus companies, according to Rodriguez. “This translated into operators attempting to carry as many passengers as possible,” he said. The city also eliminated many existing routes and failed to inform riders of the changes (McCarthy 2007).

Such questionable public policy decisions could be a contributing factor to Latin America’s rising car ownership rates (Roque and Masoumi 2016). Still, a recent study showed car ownership rates below the averages in wealthier countries, from a low of 71 per 1,000 residents in Ecuador to a high of 314 per 1,000 residents in Argentina. Those relatively low numbers mean that a large constituency favors an increase in bus lanes at the expense of private car lanes.

But the annual growth rate of car ownership—up to 6.1 percent in Chile—far outpaced the 1 to 2 percent range in developed nations. These figures suggest that despite Latin America’s advances in mass transit, the upper class and upwardly mobile are still opting for private automobiles, regardless of traffic congestion. (Nine Latin American cities feature in the INRIX 100 cities with the worst traffic, more than in Asia and Africa combined.)

On the other end of the economic spectrum, the proliferation of BRT may be having other consequences. “BRT is the flavor of the decade in transportation and it is supplanting, in some cases problematically so, existing transport systems that are problematic in their own right,” said University of California, Berkeley scholar Daniel Chatman, who has studied the impact of new BRT routes in several cities, including Quito and Barranquilla, Colombia.

Preliminary research suggests that BRT in high-volume corridors tends to best serve those working in traditional office settings, moving them from dense, formal residential areas to job centers. That can leave the poor behind as ancillary routes through poorer parts of the city are cut off by transit planners aiming to formalize the existing transportation network, even though it underserves the 30 percent of the region’s residents who live in informal housing.

“BRT ends up serving the dominant traffic pattern in a city and doesn’t necessarily deal as well with other travel patterns that are not part of this main trunk system,” Chatman said.

BRT’s ability to move people over long distances has also facilitated worsening socio-spatial segregation. After creating access to land on the urban periphery, housing officials and private-sector developers in Brazil, Colombia, and Mexico moved to build social housing ever farther from the city center in order to take advantage of lower land prices.

“We now know this was a mistake, leading to social exclusion, higher fares, and travel burden,” said University of California, Berkeley’s Rodriguez.

The prevailing spatial structure of Latin American cities, with low-income residents located predominantly in the outskirts, means that BRT projects have largely benefitted middle-income residents. This is true in Bogotá (Combs 2017) and Lima (Scholl et al. 2017), where BRT serves concentrations of middle-income residents, connecting them to formal employment clusters. Residents of social housing in Brazil pay over 50 percent of their income on housing and transportation combined, while occupants of more centrally located housing pay 39 percent, according to Linke.

Urban Transportation in Latin America

In May 2017, the Lincoln Institute and the University of California, Berkeley’s department of city and regional planning hosted a symposium on urban transportation in Latin America. It focused on the influence of innovative transit schemes on real estate, urban development, and the lives of city residents. The aim of the symposium was to examine the evidence to date and discuss ways to apply recent scholarship to public policy.

Symposium papers paint a complex picture of experiences and impacts. Research was inconclusive about whether BRT investments can have distinct impacts on real estate markets, although most of the studies have focused on just a few cities in Colombia, Ecuador, and Mexico. Aerial cable cars have been empirically studied only in Medellín, which showed increased real estate activity. Both types of transport have led to increased building permit activity and population density. Land use trends shifted from residential to commercial in Bogotá and Quito but not in León, Monterrey, Guadalajara, and Puebla. Inconsistencies regarding estimated impacts point to differences in local conditions. Urban land markets are subject to a variety of forces—from planning institutions and development activity to the availability of land—that are likely to influence the price of land, making it difficult to generalize price impacts within corridors, across corridors, and over time. 

Opportunities for further research abound, including studies of the importance of these innovations relative to established urban transportation modes, how to target the benefits towards the poorest residents, and how to better coordinate with land development.

The high cost and inconvenience also reflect poor coordination between housing and transit planning. As a result, housing is often located without consideration for transit access, notes Enrique Silva, associate director of the Lincoln Institute’s Program on Latin America and the Caribbean. BRT’s failure to reach more underserved communities is the result of discrete choices of “how you plan your routes and how accessible the stops are to people,” he said. Planners decided to work on existing major routes and decided not to extend or consider routes that penetrated more effectively into poor neighborhoods, Silva explained.

Latin America’s advances are nevertheless impressive, and moving around cities in the region has improved demonstrably in recent decades. But until the region reduces the vast gulf between rich and poor—a division that manifests itself in where people can live—high-speed transit can serve at best as a salve on a deeper wound.

Gregory Scruggs, AICP, writes about cities and culture in the Americas. He is a correspondent with the Thomson Reuters Foundation, and his work has appeared in the New York Times, Washington Post, Atlantic CityLab, and The Guardian.

Photograph: Gwen Kash

References

BRTData. www.brtdata.org.

Combs, Tabitha. 2017. “Improving Equity in the Distribution of Public Transit Benefits.” Paper presented at the Symposium on Transportation Innovations and Urban Land in Latin America, Berkeley, CA, May 2017.

Gutman, Jeffrey, and Nirav Patel. 2018. Addressing Spatial Inequality in Latin American Cities. Washington, DC: Brookings Institution. www.brookings.edu/research/addressing-spatial-inequity-in-latin-american-cities.

Levine, Jonathan. 2013. “Is Bus Versus Rail Investment a Zero-Sum Game? The Misuse of the Opportunity-Cost Concept.” Journal of the American Planning Association 79:1, 5–15. DOI: 10.1080/01944363.2013.785285.

Linke, Clarisse, and Luc Nadal. 2017. “Housing, Transport and Access: A Case for Transit-Oriented Low-Income Housing in Rio de Janeiro.” Paper presented at the Symposium on Transportation Innovations and Urban Land in Latin America, Berkeley, CA, May 2017.

McCarthy, Julie. 2007. “In Chile, Commuters Sue City over Transit System.” Broadcast on All Things Considered, National Public Radio, October 8, 2007. www.npr.org/templates/story/story.php?storyId=15100976.

Rodriguez, Daniel A., Erik Vergel-Tovar, and William F. Camargo. 2016. “Land Development Impacts of BRT in a Sample of Stops in Quito and Bogotá.” Transport Policy 51: 4–14. DOI: 10.1016/j.tranpol.2015.10.002.

Roque, Daniela, and Houshmand E. Masoumi. 2016. “An Analysis of Car Ownership in Latin American Cities: A Perspective for Future Research.” Periodica Polytechnica Transportation Engineering 44(1): 5–12. DOI: 10.3311/PPtr.8307.

Scholl, Lynn, Daniel R. Oviedo, Marco Innao, and Lauramaria Pedraza. 2017. “BRT Systems and Social Inclusion: Impacts on Access to Jobs—The Case of Lima, Peru.” Paper presented at the Symposium on Transportation Innovations and Urban Land in Latin America, Berkeley, CA, May 2017.

Smolka, Martim O. 2013. Implementing Value Capture in Latin America: Policies and Tools for Urban Development. Policy Focus Report. Cambridge, MA: Lincoln Institute of Land Policy.

Velandia Naranjo, Durfari Janive. 2017. “The Impact of Bus Rapid Transit System on Land Prices in Mexico City.” Paper presented at the Symposium on Transportation Innovations and Urban Land in Latin America, Berkeley, CA, May 2017.

Vergel-Tovar, Erik. 2017. “The Impacts of Bus Rapid Transit on Land Use and Real Estate Activity in Bogotá, Colombia.” Paper presented at the Symposium on Transportation Innovations and Urban Land in Latin America, Berkeley, CA, May 2017.

En 2012, el Servicio Forestal de los Estados Unidos realizó un estudio de la cobertura arbórea urbana. en él, se estimó que las ciudades del país perdían alrededor de cuatro millones de árboles al año. Según el Fondo Mundial para la Naturaleza, todos los años, a nivel mundial se eliminan más de 75 000 kilómetros cuadrados de bosques por la agricultura, la explotación forestal y otros factores. Sin embargo, es difícil cuantificar el costo de esta pérdida. Es de amplio conocimiento que las plantas absorben dióxido de carbono y, así, ayudan a atenuar los efectos del cambio climático, pero los planificadores de ciudades podrían beneficiarse si se hiciera una evaluación más precisa y basada en datos del valor del manto verde urbano. Esta evaluación serviría como guía sobre el modo en que los árboles y otras plantas pueden presentarse en el diseño y la planificación de la ciudad contemporánea de la manera más razonable.

Después de todo, así es cómo evaluamos e instalamos infraestructura gris: contamos cada uno de los postes de luz y espacios de estacionamiento, para poder pensar cómo funcionan estos elementos en el diseño de una ciudad. Según David Nowak, científico experto del Servicio Forestal de los EE.UU., históricamente, no hemos sido tan considerados ni exigentes con la cuantificación y, por lo tanto, con la gestión, de la infraestructura verde.

Como regla general, las ciudades compilan los detalles de la infraestructura construida y hacen un seguimiento de estos, pero no hacen lo mismo con los árboles. Así, resulta más difícil planificar e incluso debatir los diversos impactos potenciales que implica mantener, aumentar o reducir la vegetación urbana.

Pero esto ha empezado a cambiar. Nowak encabeza un trabajo pionero manifestado en un proyecto del Servicio Forestal llamado i-Tree, un conjunto de herramientas web que, en parte, funciona con datos de sistemas de información geográfica (SIG). I-Tree combina imágenes satelitales con otros datos para ayudar a ciudadanos, investigadores y funcionarios a comprender los mantos urbanos y otros elementos de la infraestructura verde, en general, en términos económicos.

Por ejemplo, en un análisis de i-Tree de Austin, Texas, se descubrió que los árboles le ahorraron a la ciudad unos USD 19 millones al año en consumo de energía residencial, USD 11,6 millones en captura de carbono y casi USD 3 millones en eliminación de contaminación. Por ejemplo, la infraestructura arbórea de la ciudad produce oxígeno y consume dióxido de carbono, lo cual reduce aún más las emisiones de carbono, que i-Tree estima en USD 5 millones al año. Los árboles aportan otras ventajas; algunas de ellas están cuantificadas, otras no. Entre ellas, absorben la radiación ultravioleta, ayudan a absorber el agua de lluvia y reducen la contaminación acústica.

En otro análisis de i-Tree, realizado en 2017, investigadores de los Estados Unidos e Italia concluyeron que, en todo el mundo, las ciudades con más de 10 millones de habitantes alcanzan una mediana de ahorros anuales de USD 505 millones en reducción de contaminación atmosférica, atenuación de los efectos de la “isla de calor” y otros beneficios que aporta su manto urbano.

Gracias a este tipo de análisis, las ciudades pueden implementar recursos verdes para lograr un máximo impacto y comprender el balance entre las ventajas y las desventajas al momento de tomar muchas decisiones de planificación. Nowak destacó que derribar árboles para hacer estacionamientos genera una pérdida y no solo la ganancia asociada a la mayor cantidad de lugares de estacionamiento.

En el pasado, los árboles eran más bien una preocupación relacionada con los parques y el departamento de silvicultura. Ahora, tienen un papel cada vez más primordial en la respuesta de las ciudades al cambio climático. “Le puedo decir con absoluta certeza que, ya sea que se pueda hablar del cambio climático a nivel político o no, las ciudades y los pueblos de todo el país están muy interesados en descifrar la pregunta: ¿qué haremos hoy al respecto, exactamente?”, dijo Jim Levitt, director asociado de los programas de conservación territorial en el Instituto Lincoln y director de innovación en conservación en Harvard Forest. Agregó que eso es cierto desde Nueva Inglaterra hasta Miami, y Newport News, Virginia, y Phoenix, aunque los motivos específicos varíen, ya sea por problemas de inundaciones, los efectos de la isla de calor u otros.

La última tecnología relacionada con la infraestructura de los árboles responde de forma directa a este interés que tienen las ciudades. A fines de 2016, Senseable City Lab, de MIT, en colaboración con el Foro Económico Mundial, lanzó una herramienta llamada Treepedia y, desde entonces, ha publicado análisis de cobertura arbórea en 27 ciudades de todo el mundo. Con una interesante vuelta de tuerca, no obtiene los datos de los satélites, como muchos proyectos SIG, sino de imágenes seleccionadas de Google Street View. Ofrece otro enfoque a la información de los árboles, dado que, por ejemplo, representa en menor escala los parques urbanos grandes. Pero esta es una decisión de diseño. Los creadores de la herramienta creen que, al detallar el “verdor de la calle” que los ciudadanos experimentan de verdad, el proceso de planificación contará con más información. Según indica Carlo Ratti, director de Senseable City Lab de MIT y fundador de la empresa de diseño Carlo Ratti Associati, el laboratorio seguirá agregando ciudades y tiene un listado de pedidos de distritos, académicos y otros.

“Las ciudades intentan adquirir mejor información y comprender el estado actual del manto urbano”, explica. “La mayoría de ellas no posee los recursos para evaluar toda la ciudad de forma manual. Los datos de Treepedia pueden ofrecerles un punto de referencia firme” y se pueden concentrar en donde más se necesitan. “Para otros, como planificadores y diseñadores, es útil como agente para medir la percepción que tienen los ciudadanos sobre el espacio verde y los árboles”, indica, porque captura una especie de perspectiva compartida “desde el suelo”. Pronto, el laboratorio lanzará una versión de código abierto de su software para que las ciudades, las organizaciones no gubernamentales y los grupos comunitarios puedan recopilar sus propios datos. Con esto esperan que las ONG y los grupos locales utilicen Treepedia “como herramienta para determinar dónde se necesita plantar y presionar a los gobiernos locales con campañas basadas en evidencia”, explica Ratti.

Esto se condice con un interés más amplio entre los ciudadanos y los planificadores en las iniciativas de ciudades ecológicas, como los proyectos a gran escala de Nueva York y Atlanta, entre otros. Nowak, del programa i-Tree, dijo que sus herramientas ayudaron a guiar a los organizadores de Million Trees NYC, una iniciativa pública y privada que aumentó en alrededor de un 20 por ciento el total del bosque urbano de Nueva York. El proyecto London i-Tree Eco Project, según su informe de 2015, utilizó i-Tree para cuantificar “la estructura del bosque urbano (los atributos físicos, como densidad y salud de los árboles, área de las hojas y biomasa)”, con miras específicas a la captura de su valor “en términos monetarios”. Según el informe, se registraron ahorros en captura de carbono por GBP 4,79 millones (unos USD 6,75 millones) al año. “Esperamos ofrecer números derivados de forma local, para ayudar a las personas a tomar decisiones informadas, ya sea a favor o en contra de los árboles”, dice Nowak.

Landscape, una aplicación de i-Tree, está pensada para los planificadores en particular. Los usuarios pueden explorar el manto de árboles, cruzado con información demográfica básica que llega al nivel del censo por manzana, y ofrece datos relacionados con la atenuación de contaminación, impactos en la temperatura y otros factores. Por ejemplo, los usuarios pueden identificar con facilidad las zonas con alta densidad de población, pero baja cobertura arbórea. Durante el año que viene, el proyecto i-Tree agregará datos sobre especies de árboles y está intentando obtener comentarios para modificar la herramienta de forma que ayude más en la planificación, indica Nowak.

A grandes rasgos, la idea es la misma que dio forma a i-Tree desde el inicio: un enfoque basado en datos para pensar en la infraestructura verde. “Queremos ayudar a responder la siguiente pregunta: si puedo plantar un solo árbol o hacer un solo cambio en el entorno verde de la ciudad, ¿dónde debería hacerlo?”, dijo Nowak.

Rob Walker (robwalker.net) es columnista de la sección Sunday Business del New York Times.

Credito de imagen: Servicio Forestal de los EE.UU.

A 2012 United States Forest Service Study of urban tree cover estimated that American cities were losing around four million trees per year. Worldwide, agriculture, logging, and other factors eliminate 18.7 million acres of forest annually, according to the World Wildlife Fund. Yet the cost of that loss is hard to quantify. It’s widely recognized that plants absorb carbon dioxide, helping to mitigate the effects of climate change, but city planners could benefit from a more precise, data-driven assessment of the urban canopy’s value to guide how trees and other vegetation can most sensibly figure in the design and planning of the contemporary city.

After all, that’s how we evaluate and install gray infrastructure, counting every light pole and parking lot to help us think about how these elements work in a city’s design. Historically, we haven’t been as thoughtful or demanding about quantifying, and thus managing, green infrastructure, according to David Nowak, a senior scientist with the U.S. Forest Service.

As a rule, cities compile and track the details of the built infrastructure, but not trees. This makes it harder to plan for, or even debate, the various potential impacts of maintaining, increasing, or reducing urban vegetation.

But that has been changing. Nowak leads a pioneering effort in the form of a Forest Service project called i-Tree, a suite of Web tools drawing in part on geographic information system (GIS) data. I-Tree combines satellite imagery and other data to help citizens, researchers, and officials understand urban canopies and other green infrastructure elements, often in economic terms.

For example, an i-Tree analysis of Austin, Texas, found that trees save the city about $19 million a year in residential energy use, $11.6 million in carbon capture, and almost $3 million in pollution removal. The city’s arboreal infrastructure produces oxygen and consumes carbon dioxide, for instance, adding up to a reduction in carbon emissions that i-Tree values at $5 million annually. Other tree payoffs—some quantified, others not—include absorbing ultraviolet radiation, helping absorb rainwater, and reducing noise pollution.

In another i-Tree analysis, conducted in 2017, researchers in the United States and Italy concluded that, worldwide, cities with populations over 10 million realize median annual savings of $505 million from reduced air pollution, mitigated “heat island” effects, and other benefits derived from their urban canopies.

This type of analysis can help cities deploy green resources for maximum impact and understand the tradeoffs involved in many planning decisions. Clearing trees to make way for a parking lot entails a loss, not just the gain associated with increased parking, Nowak noted.

In the past, trees were more likely a concern for the parks or forestry department. Increasingly, they’re central to cities’ responses to climate change. “I can tell you definitively that cities and towns across the nation are very interested in figuring out, whether or not you can talk about climate change politically, ‘What exactly are we going to do about it today?’,” said Jim Levitt, associate director of land conservation programs at the Lincoln Institute, and director of conservation innovation at the Harvard Forest. That’s true from New England to Miami to Newport News, Virginia, and Phoenix, he added, even if the specific reasons vary, whether flood issues, heat island effects, or others.

Recent arboreal infrastructure-related technology responds directly to this city-level interest. In late 2016, MIT’s Senseable City Lab, in collaboration with the World Economic Forum, launched a tool called Treepedia and has since published analyses of tree coverage in 27 cities around the world. In an interesting twist, it draws not on the satellite data behind many GIS projects, but on imagery culled from Google Street View. It offers a different skew on tree data, since, for example, it underrepresents large urban parks. But this is by design. The tool’s creators believe that detailing the “street greenery” citizens actually experience can inform the planning process. The lab will continue to add cities and has a backlog of requests from municipalities, academics, and others, according to Carlo Ratti, director of the MIT Senseable City Lab and founder of the design firm Carlo Ratti Associati.

“Cities are trying to acquire better information and understand the current state of the urban canopy,” Ratti said. “Most of them do not have the resources to manually survey the entire city. Treepedia data can give them a solid baseline” and focus efforts where they may be needed most. “Others, like planners and designers, find it useful as a proxy for measuring the perception of green space and trees by citizens,” he said, because it captures a kind of shared perspective “from the ground.” The lab will soon release an open-source version of its software to let cities, nongovernmental organizations, and community groups compile their own data. The hope is that NGOs and local groups will use Treepedia “as a tool to both determine where planting is needed and lobby their local governments with evidence-based campaigns,” Ratti explained.

This is consistent with a broader interest among citizens and planners in green city initiatives, including high-profile projects from New York to Atlanta and beyond. Nowak, of the i-Tree program, said that its tools helped guide the organizers of Million Trees NYC, a public-private initiative that increased New York’s aggregate urban forest by an estimated 20 percent. The London i-Tree Eco Project, according to its 2015 report, used i-Tree to quantify “the structure of the urban forest (the physical attributes such as tree density, tree health, leaf area, and biomass),” with a specific eye toward capturing its value “in monetary terms.” Carbon sequestration savings logged in at £4.79 million (roughly $6.75 million) annually, according to the report. “Our hope is to provide numbers that are locally derived, to help people make informed decisions—whether it’s pro or against trees,” Nowak said.

One i-Tree Web application, Landscape, is intended for planners in particular. Users can explore tree canopy, cross-matched with basic demographic information down to the census-block level, offering data related to pollution mitigation, temperature impacts, and other factors. For example, users can easily identify areas with high population density but low tree cover. The i-Tree project is adding data on tree species over the next year and is seeking feedback to modify the tool in ways that make most sense for planning, according to Nowak.

The broad idea is the same one that has shaped i-Tree from the start—a data-driven approach to thinking about green infrastructure. “We want to help answer the question: If I can plant only one tree or make one change to the city’s green landscape, where should I do it?” Nowak said.

Rob Walker (robwalker.net) is a columnist for the Sunday Business section of the New York Times.

Image credit: U.S. Forest Service