The Problem With ‘Cool Pavements’: They Make People Hot

Workers apply CoolSeal to a street in Pacoima in June.

Workers apply CoolSeal to a street in Pacoima in June. Los Angeles Bureau of Street Services

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A tool to help solve the problem of urban heat islands could have an unwelcome side effect, new research in L.A. finds.

About two months ago, Ariane Middel walked the empty streets of Sun Valley, a suburban neighborhood in Los Angeles’ San Fernando Valley. The roads there had recently been coated with an asphalt mixture called CoolSeal, which lowers air temperatures by reflecting the energy from sunlight, rather than storing it and converting it into heat.

For hours, Middel and a team of researchers dragged an elaborate heat sensor, mounted to a garden cart, down the streets and sidewalks to grab meteorological data. A climate scientist at Arizona State University in Tempe—where air temperatures soar into triple digits—Middel didn’t feel all that hot. But after crunching the data, she discovered the reflected sunlight hadn’t disappeared: She had probably absorbed it.

According to her sensors, on a hot, dry day, a person could feel more than 7 degrees warmer on a “cool pavement,” as the reflective roads are called, as opposed to a normal blacktop.

That discovery is significant. Los Angeles, like many other cities, is trying to cool down, and city officials have been counting on cool pavements as part of the solution. But if they can make people feel that much warmer, they may actually undermine the city’s ambitious plans. Summer days won’t feel better for pedestrians. They could, in fact, feel worse.

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Scientists have been trying to lower urban temperatures with cool pavements for decades, since the identification of urban heat islands. Cities are hotter than rural areas, because they’re covered in dark, impermeable surfaces that have low albedo, or solar reflectance—meaning they absorb heat from sunlight. They get hot, and warm the air above. Light-colored surfaces, however, reflect more of that sunlight back into space, and keep cities cooler.

In the 1990s, scientists at Lawrence Berkeley National Laboratory calculated that if all pavements and roofs in downtown Los Angeles increased albedo by 25 and 35 percent, respectively, air temperatures would drop by nearly 3 degrees. If that happened throughout the city, and was combined with other efforts, the cooling effects could be even greater. In the years since, more studies have emerged, with varying predictions. They all agree: If done right, cool pavements can help cities move the needle on ending heat islands.

While reflective coatings are now fairly common on roofs, they haven’t taken off on roads. That’s due to concerns about wear and tear, and potential glare, says David Sailor, who directs urban climate research at Arizona State University. (He wasn’t involved with Middel’s study.) More often, he says, cool pavements are found in parking lots of buildings seeking LEED credits.

Los Angeles, however, has taken up the idea with zeal. Since 2015, the city has covered around 50 city blocks in various reflective coatings and seals. In April, Mayor Eric Garcetti announced a plan to cover 250 lane-miles of city roads by 2028.

Although city officials have observed cool pavements’ lower surface temperatures for years, they had not measured air temperatures and human comfort effects. In July, Middel and V. Kelly Turner, a UCLA urban planner, studied unshaded streets in Pacoima and Sun Valley, taking readings on reflective pavements and traditional asphalt. They measured air temperature, wind speed, humidity, and radiation for 10 hours, from morning to sundown.

The results were troubling. On a typical Los Angeles summer day, with a high of 88 degrees, they found the reflective roads could make people feel much hotter. Just before noon, the mean radiant temperature—a calculation of the amount of heat emitted by surrounding surfaces—was more than 7 degrees higher on the coated pavement. That temperature, which is another way of measuring human comfort, decreased as the day went on, but in the afternoon, the roads still felt more than 3 degrees warmer.

The warmer feeling, Middel says, is almost entirely attributable to solar radiation reflected off the roads. Normally, asphalt sucks it in, and it dissipates slowly into the air. These roads, however, reflected it back at a rate of 130 watts per square meter—akin to adding 10 percent more direct sunlight. That reflection was visible as glare, which was “really big” in the early evening, she says, just as people were getting home from work.

Middel and Turner stress that this is a small sample. They can’t draw conclusions about how cool pavements might affect human comfort in other circumstances, like on cloudy or windy days, or during Los Angeles’ wetter winter season, or even on streets built differently, with taller buildings or more trees. And, they point out, they didn’t study how the roads behaved at night, when they’re expected to have greater cooling impacts.

Nevertheless, Los Angeles has no shortage of dry, sunny days. And the city lacks shade. The streets Middel and Turner studied, in other words, are fairly typical. For those reasons, the measurements, which Turner describes as preliminary findings, are concerning to other scientists.

“That’s a negative effect on the human body, there’s no doubt about it,” says Larry Kalkstein, a bioclimatologist at the University of Miami who was not involved in Middel and Turner’s research. The extra heat implied by their findings, he says, would makes people sweat more, and force them to drink more water. For the most vulnerable populations, like the elderly, homeless, and obese, it could contribute to respiratory failure, heart attack, heat strokes, and under extreme conditions, death.

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The results, which were provided exclusively to CityLab in advance of their publication by the American Meteorological Society later this month, are likely the first real-world, empirical assessments of human comfort—or discomfort—caused by cool pavements in any American city. They’re not a complete surprise. Modeling studies have predicted that reflected energy could be absorbed by pedestrians, and in hot, dry cities, make people feel warmer. One such study, conducted for El Monte, an L.A. suburb, suggested pedestrians could avoid those heat effects if they were protected by shade trees, and more than 16 feet away from the road. George Ban-Weiss, one of the co-authors and an environmental engineer at USC, is also measuring L.A.’s cool pavements, separate from Middel and Turner.

Despite the implications for pedestrians, city officials are continuing on with the program. Cool pavements are an anchor of the mayor’s plan to reduce the urban heat island by 3 degrees by 2035. Supporters are excited about the potential cooling benefits. Moreover, L.A. is the archetypal American road city, blanketed in thousands and thousands of miles of asphalt roads. They’re a symbolically important place to make an impact on urban heat.  

“As a climate-change organization, what we want, with our streets in Los Angeles, is for them to look substantially different in the future,” says Jonathan Parfrey, director of Climate Resolve, a group that’s helping to test four cool-pavement solutions, including a cement composite, acrylic paint, and a ceramic spray. He says discontinuing the program because of these body-heat results would be a lost opportunity—a sentiment echoed by scientists, like Ban-Weiss, who say the city’s neighborhood-scale installations are a chance to take measurements and conduct studies that no one else has done before.

From a managerial perspective, cool pavements offer plenty of benefits. The material is relatively cheap—CoolSeal costs around 30 to 40 cents per square foot, and less in larger quantities. The maintenance, though significant due to traffic eroding their reflectiveness, is predictable.

“There’s an attractiveness and a neatness to creating a program like that,” says Edith de Guzman, the research director at TreePeople, a Los Angeles-based environmental nonprofit. Along with Sailor, Kalkstein, and Parfrey, she’s part of an interdisciplinary group called the Los Angeles Urban Cooling Collaborative that’s studying heat-related mortality. “It’s a product you can purchase and have an agreement with the manufacturer on, and you can quantify how large it is, and exactly how reflective it will be, and it has a specific per-unit cost.”

Cool roofs have the same environmental benefits with none of the effects on pedestrians—but they’re not visible to climate-conscious citizens. De Guzman says that’s key to engaging people, and getting them to buy in. “You want to show that you did something,” she says. “It creates an opportunity to talk about the issue, and think about it, and ask, how does it impact my neighborhood?”

No one’s disputing that Los Angeles needs to rethink its prodigious quantities of pavement, which hold in heat throughout the city. The question, however, is how to use cool pavements in such a way that allows the city to lower air temperatures without hurting people. They’re already being installed on streets that are being redesigned to encourage walking and biking, like a two-block thoroughfare in South Los Angeles and a protected bike lane downtown. The city has similar plans for a rapid bus lane in Canoga Park. But if those streets can feel up to 7 degrees warmer on a summer day, Kalkstein says people should avoid them altogether.

In some instances, the pavements will be paired with new street trees, which scientists suspect could counteract the body-heat effects. Turner, the UCLA planner, intends to study that. But if the trees block the roads, then there’s no sunlight to reflect, which could make the cool pavements ineffective.

Nor is it clear that tree planting will succeed. Greg Spotts, the sustainability director of the city’s streets division, has said that planting shade trees at scale is hard because they take years to grow, while the alternative of planting mature trees is expensive. Trees have to be watered to maturity, which is problematic in dry environments. A spokesperson for Spotts’ department said it is working with university researchers to refine the roads program, using “emerging research” on the connections to human health and comfort.

If widespread shade-tree planting fails, the new study shows that cool pavements could end up making those same streets even hotter, and more punishing, while the city continues its pursuit of citywide air temperature cooling. In fact, it could disproportionately affect the city’s brown and black populations, because darker skins absorb more solar radiation than white skins, Sailor says. (The same is true of darker and lighter clothes.)

The mayor’s climate plan calls for prioritizing neighborhoods that have the most severe urban heat—covered in concrete, bereft of canopy, and dense with people. Rather than installing cool pavements here, scientists wonder whether they might be more effective in neighborhoods where there aren’t many pedestrians around. Like, for instance, the Dodger Stadium parking lot—a huge bowl of asphalt that’s typically empty during the day, when Middel and Turner observed the heat effects. Going private, however, would take away the city’s big lever: the ability to install these surfaces on public land, without negotiating with landowners.

“There’s not going to be a silver bullet,” says de Guzman. “We need to do what we can, and collectively, we need to be smart about it.”

Sam Bloch is a staff writer at The New Food Economy

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