![]() The results suggest such coatings might be better for roads than for sidewalks or playgrounds. When the coating was used in areas where people walk, the ground itself was as much as 11 F (6.1 C) cooler, but a few feet off the ground, the temperature rose as the sun’s rays were reflected. In Los Angeles, where the city has been experimenting with a cooler coating over asphalt, researchers found another effect to consider. In a few low, sparse downtown neighborhoods, we found that reflective pavement could raise the demand for cooling because of increased incident radiation on the buildings. Phoenix could reduce its summer temperatures even more – by 2.5 to 3.6 F (1.4 to 2.1 C) – but the effects in some parts of its downtown are complicated. Using models, we found that doubling the traditional albedo of the city’s roads could cut peak summer temperatures by 1 to 2.7 F (0.3 to 1.7 C). But Boston’s unobstructed freeways and its suburbs would see a net benefit from reflecting a large fraction of incoming sunlight to the top of the atmosphere. Reflective pavement won’t help or harm much there. MITĬonsider the differences between Boston and Phoenix.īoston’s dense downtown of narrow streets has tall buildings that block light from directly hitting the pavement most hours of the day. Reflective roads can have different effects in summer and winter depending on the surrounding buildings. That’s why attention to location matters. When brighter pavements reflect radiation onto buildings – called incident radiation – they can warm nearby buildings in the summer, actually increasing the demand for air conditioning. Within cities, and even within urban neighborhoods, the benefits differ. Location mattersīut not all paved areas are ideal for cool roads. And when materials are locally sourced, such as light-colored binders or aggregates, the crushed stone, gravel or other hard materials in concrete, these roads can also save money. roads could lower energy use for cooling and reduce greenhouse gas emissions equivalent to 4 million cars driven for one year. An MIT CSHub model estimated that an increase in pavement albedo on all U.S. Though the benefits of reflective pavements can vary across the nation’s 4 million miles of roads, they are, on the whole, immense. When pavements instead use brighter additives, reflective aggregates, light-reflective surface coatings or lighter paving materials like concrete, they can triple the albedo, sending more radiation back into space. Conventional pavements such as asphalt have a low albedo of around 0.05-0.1, meaning they reflect only 5% to 10% of the light they receive and absorb as much as 95%. Typically, the darker the surface, the lower the albedo. The lower a surface’s albedo, the more light it absorbs and, consequentially, the more heat it traps. Albedo refers to the proportion of light reflected by a surface. To estimate a pavement’s reflectivity, we use a measure called albedo. Reflective materials on pavement can prevent that heat from building up and help counteract climate change by reflecting solar radiation back to the top of the atmosphere. The urban heat island effect in Washington, D.C., on Aug. It’s part of the reason cities are regularly a few degrees warmer in summer than nearby rural areas and leafy suburbs. ![]() This can exacerbate urban heat islands and worsen the effects of heat waves. The absorbed heat in the pavement mass is released gradually, warming the surrounding environment. In urban areas, about 40% of the land is paved, and that pavement absorbs solar radiation. Why surfaces heat upĪll surfaces, depending on the amount of radiation they absorb or reflect, can affect air temperatures in cities. Here’s how reflective pavement works and what cities need to think about. But reflective surfaces have to be used strategically – the wrong placement can actually heat up nearby buildings instead of cooling things down.Īs researchers in MIT’s Concrete Sustainability Hub, we have been modeling these surfaces and determining the right balance for lowering the heat and helping cities reduce their greenhouse gas emissions. Research shows that building lighter-colored, more reflective roads has the potential to lower air temperatures by more than 2.5 degrees Fahrenheit (1.4 C) and, in the process, reduce the frequency of heat waves by 41% across U.S. A hot street isn’t just hot to touch – it also raises the surrounding air temperature. Think about how hot the soles of your shoes can get when you’re walking on dark pavement or asphalt. One solution is right beneath our feet: pavement. When heat waves hit, people start looking for anything that might lower the temperature.
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