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Greener City Streets Aren't Just About Traffic. They're About Rainwater, Too.

As cities push to become more environmentally friendly, transportation planners are being asked to consider how both traffic and water flows through their streets.

A rain garden near downtown D.C.
(David Kidd)
When you think of city streets, chances are you think of them as a transportation network, carrying cars, buses, pedestrians and cyclists to their destinations.

But these vast stretches of pavement are also key components of cities’ stormwater networks. Streets cover about a third of the land in cities, and they account for half of the impervious surfaces in cities. Impervious surfaces don’t allow water to soak through them, which means they can alter the natural flow of rainwater. City streets collect, channel, pollute and sometimes even speed along water as it heads to the sewers.

These two functions streets are tightly linked -- the more pavement that’s put down to handle traffic, the faster rainwater will flow to sewers -- but the city departments that oversee those different functions often don’t coordinate their efforts.

The National Association of City Transportation Officials (NACTO) wants that to change. The group recently released an “Urban Street Stormwater Guide” that it hopes will encourage city transportation engineers and policymakers to think about incorporating green infrastructure, such as permeable pavements or rain gardens, into redesigned streetscapes.

NACTO is known in transportation circles for promoting ideas like protected bike lanes and complete streets, which accommodate users of all kinds instead of prioritizing vehicle traffic. So its stormwater guide tries to show how city planners can promote good transportation practices while improving stormwater infrastructure.

For example, bumped-out curbs at intersections can slow traffic and improve pedestrian safety by shortening the distance required to walk across traffic lanes. But using permeable pavement for those bump-outs can also help the ground absorb more water during storms. Likewise, rain gardens (rather than concrete medians) can be used to separate bike lanes from vehicle traffic.

While there are many technical guides for how to install green infrastructure, few of them focus on how to integrate those features with transportation components on a street, says Corinne Kisner, NACTO’s director of policy and special projects. She says the transportation group developed their guide after officials in several cities said that managing runoff had become a more important task for them. “This is becoming an increasingly urgent problem in cities nationwide and worldwide, frankly,” Kisner says.

Part of the reason for that is because many cities, particularly older cities in the Northeast and Midwest where sewage and runoff share the same underground pipes, are under federal consent decrees to cut back on or eliminate sewage flows into local bodies of water.

But cities are also increasingly looking for ways to address both the causes and effects of climate change. That means preparing for heavier downpours and flooding, which green infrastructure can help mitigate. It also means addressing “urban heat islands,” which occur because pavement and other manmade surfaces absorb heat from sunlight. Planting trees and other vegetation, plus using permeable pavements, can make the effect less intense.

NACTO's guide was developed with input from transportation and water experts in several cities. It includes examples of how different cities have already incorporated green infrastructure into their transportation projects.

One of the most striking of those is the Green Line light rail project connecting the downtowns of Minneapolis and St. Paul, which opened in 2014. During construction, the cities planted 1,000 trees and included gardens and other components to absorb rain along the route. Local leaders estimate those improvements reduced stormwater runoff by half. The new infrastructure also absorbs pollutants like phosphorous and traps 40,000 pounds of sediment that would otherwise end up in the Mississippi River.

But green infrastructure works best when it's used throughout a city, Kisner says. Most green infrastructure projects are small, so their effectiveness depends on widespread adoption.

That’s why Philadelphia’s streets and water departments launched a “Green Cities, Clean Waters” program in 2011. They’ve completed 124 green street projects since then. The overall effort has helped improve 550 acres of impervious surfaces that can now handle 1 to1.5 inches of stormwater using tree trenches, vegetated bump-outs and porous streets, says Joanne Dahme, a spokeswoman for the water department.

Installing green infrastructure can be a complex process, she adds, because it requires working with or around existing utility lines, street trees, curbs and wheelchair-accessible ramps. “The key seems to be advanced planning and coordination, as public projects can take years to design, bid and construct,” Dahme says. “Syncing up our needs with those of other public agencies and private agencies can be challenging. But with experience, we improve.”

Kisner says many cities also struggle with how to maintain the infrastructure once it’s installed. That duty could fall to a city agency or to local neighborhood or business improvement districts, but it is crucial that the responsibilities are clearly defined, she says. But Kisner says those efforts will pay off.

“What’s neat about stormwater infrastructure, though, is it’s one of the few assets that really appreciates over time,” she says. “As the plants establish themselves, they can absorb more stormwater, which can be really beneficial.”

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