More than 85% of Chelsea is covered by roads, parking lots or buildings. So finding ways to cool city-owned surfaces is a vital piece of Chelsea’s heat mitigation plan. Roughly 20% of Chelsea residents live below the poverty line, and many others can’t afford air conditioning.
The National Asphalt Pavement Association says shot blasting to cool pavement is being used in Europe but not in the U.S., as far as staff know. For this experiment, Chelsea worked with the asphalt manufacturer Aggregate Industries to produce a special mix with lighter stones.
Chelsea could have asked Aggregate to add tan, gray or another light color to the asphalt, but that would have been four times more expensive.
As it is, the special mix with lighter colored stones — and shot blasting this section of road — cost about $55,000. Multiplying that across city streets would add up quickly.
The big player when it comes to roadways in Massachusetts is the state. Some city planners say the Massachusetts Department of Transportation, MassDOT, isn’t changing its pavement practices quickly enough.
Kevin Beuttell doesn’t like the scuff marks. No one expects a parking lot to be pristine, but the black marks seem to stand out on this once uniformly gray surface.
Beuttell points across a lot Cambridge painted in June 2021. It was the city’s first test of this approach that Phoenix and Los Angeles are using to try to reduce heat. Beuttell, the city’s supervising landscape architect, says Cambridge will likely paint more city-owned lots even though the results are “not quite as good as the marketing material would have you believe.”
A study Cambridge commissioned found the painted lot reflected about the same amount of sun as did concrete sidewalks. Dirt and those scuff marks likely reduced the paint’s reflectivity. The upshot: the lot is retaining and emitting more heat than expected but still less than asphalt.
Some municipal leaders say they are cautious about using paint to cool pavement, because the sunshine that bounces off a beige or gray lot may hit buildings. That could lower heating bills in the winter but increase cooling costs in the summer.
And, the light that bounces off lighter, reflective pavement may get absorbed by pedestrians. It is more like walking on concrete than asphalt. Cambridge hasn’t had any complaints, but Beuttell gets the concern.
“It can actually be quite a bit less comfortable on a lighter surface than a darker surface, because you’re getting hit not only by the sun, but also by that heat reflecting off the pavement,” Beuttell says.
Still, Beuttell says paint may be a fine option for parking lots where people don’t typically hang out. Researchers who’ve studied the effects of painted streets in Los Angeles reached a similar conclusion: make parking lots or streets that don’t get much foot traffic lighter and more reflective — or plant trees to shade these surfaces.
“If you want to cool people, plant trees. If you’re trying to reduce the urban heat island effect, especially at night, then cool pavement might be a good strategy,” says Ariane Middel, lead author on the L.A. street painting study. “If you smartly combine strategies you’ll get the most benefit.”
The Cambridge lot includes a small strip of trees. And there are lots of new trees on Chelsea’s cool block with the shot blasted asphalt. Trees have many benefits, but when it comes to cooling pavement, there are limits.
Ziter’s research shows trees can only significantly counteract the heat pavement radiates if at least 40% of a paved area is shaded. That’s more tree cover than many city streets and parking lots have room for. And trees aren’t effective at night.
More trees and cooler pavement are some of the more manageable steps municipal leaders can take to reduce temperatures at night and prevent more heat-related health harm.
“[Pavement] is under the control of city authorities,” says Hessam AzariJafari, deputy director of MIT’s Concrete Sustainability Hub. They “can use it to achieve their carbon neutrality goals, but it hasn’t been well discussed yet.”
Research out of MIT shows that a moderate increase in reflectivity would lower air temperatures in on average in U.S. cities by 2.5 degrees. The cumulative effect would be 41% fewer days over 105 degrees.
And lighter colored, more reflective pavement could help offset the carbon dioxide cars send into the atmosphere, says Randy Kirchain, who co-directs the Concrete Sustainability Hub. Kirchain would like to see a lot of reflective streets and parking lots bouncing light back, and possibly through, the blanket of CO2 that is warming the planet.
“If we reflect more energy back out, it’s the same as emitting less CO2,” Kirchain says “and making the blanket thinner.”
Another approach: get rid of or install less pavement.
Smaller parking lots means less pavement
Vast parking lots at malls and shopping centers contribute to extreme heat in many Massachusetts communities, including Natick, Burlington, Norwood, Framingham and Fitchburg, where Nick Erickson is the public works commissioner. He and other municipal leaders are starting to ask — do we need all this parking?
On a recent fall day, Erickson stands in front of a strip mall with many empty storefronts, looking across a nearly vacant lot.
“The parking lot that you see here is just a sea of asphalt, made to hold lots of cars because that’s what it was designed to do back in the day,” Erickson says.
Erickson envisions ripping out a strip of pavement through the middle of the lot and restoring a stream that used to run through it.
“We’d basically create a new stream channel through the parking lot,” he says, “with green space on either side of the channel, a bridge or two to allow vehicular access or pedestrian access to the stores.”
Erickson’s goal is to stop flooding. The stream diverted to build this strip mall often jumps its banks during heavy rain and pours into the lot. Flooding and stormwater runoff, not heat, are the main reasons municipal leaders like Erickson talk about removing pavement or replacing it with permeable pavement, trees, bushes or grass.
The idea that communities also need these changes to cope with rising temperatures is just starting to take hold. State grants through the Municipal Vulnerability Preparedness program are helping fund new pavement strategies and other projects related to climate change. The state has awarded $100 million to 341 communities since 2017.
Mendon reduced pavement around municipal buildings by 36%, making room for a pocket park and rain garden. Shrewsbury is using grant money to analyze existing parking needs and look for ways to reduce pavement.
Other communities are spending the funds to replace pavement with greenspace. Fitchburg plans to get rid of the middle lane on a major thoroughfare and plant trees along the road. Millbury built green buffers at street corners with landscaping, not more pavement.
Several communities received money to map how much pavement they have and assess alternatives.
The mindset in Boston and neighboring cities has been “never enough” when it comes to parking. Some residents are joining communities ready to buck that tradition.
A hands-on approach to pavement removal
“One of the selling points was that you could park six cars here,” says Amanda Rychel, a homeowner in one of the state’s hottest cities, Somerville. “It wasn’t a selling point for us. We have one car and six bikes.”
Rychel didn’t need the parking, but buying a home with pavement she could remove was enticing.
“There’s just too much pavement in Somerville,” Rychel says. “It’s bad for our rivers because rain just runs off pavement into the sewers. And it makes the city hotter.”
So Rychel and her husband, Darren Begley, are hosting an asphalt smashing party. About 15 volunteers with the local group, DePave the Way, wield sledge hammers, crowbars and shovels to break up asphalt that covers much of the Rychel Begley yard. Rychel says she thinks the former owners of her home paved for easier maintenance. Paved yards are a trend reported in cities with lots of rental property.
“Are we halfway done yet?” asks Begley as he inches up to a straight back. Begley can almost see the grass and garden he and Rychel hope to plant soon.
“I have a dog,” Begley explains. “We want him to have a place to run around and use the bathroom — so I don’t have to put my shoes on. Basically, this is all for the dog.”
In dense cities, any significant effort to replace gray surfaces with more green may not be possible without major redesigns.
Rethinking pavement in city design
Jordan Zimmermann has a suggestion: build up, not out.
Parking, stores and some residential units would be stacked in a new tower. A white roof on top of the tower could dramatically reduce the heat this parcel and many other strip malls emit on hot days. The changes would be expensive. Zimmermann says the project might be attractive if the city granted developers zoning rights for the taller building with more space.
But for parking, developers may not have a choice.
