Choosing the right kind of road surface significantly reduces air temperatures and helps cut greenhouse gas emissions, a study has found.
Researchers at the Massachusetts Institute of Technology’s Concrete Sustainability Hub (CSHub) are examining the relationship between road surfaces and temperature in urban environments. Their model estimates that an increase in pavement ‘albedo’ (the proportion of light reflected by a surface) could reduce the frequency of heatwaves and offset enough CO2 to remove the equivalent of 4 million cars from the road each year.
Pavements alter the climate depending on the proportion of light, or albedo, they reflect. Darker surfaces with a lower albedo absorb more radiation and generally warm the environment, while higher-albedo lighter surfaces reduce air temperatures.
The lower-albedo surfaces commonly found in cities contribute to urban heat islands – a phenomenon characterised by more intense heatwaves and higher air temperatures in urban areas. A 2014 study found morning summer surface temperatures in Sydney’s treeless urban areas are on average 12.8 degrees higher than vegetated non-urban areas.
CSHub found a moderate increase in pavement reflectivity could decrease urban air temperatures by 1.4 degrees over 20 years. Across all US urban areas, the number of heatwaves would fall by 41 per cent. Houston, the nation’s fourth-largest city, would see a 59 per cent drop in heatwaves.
But as Hessam AzariJafari, a postdoctoral associate in engineering at MIT, and MIT CSHub co-director Randolph E. Kirchain explain, reflective surfaces must be used strategically. “The wrong placement can actually heat up nearby buildings instead of cooling things down,” they say.
What is a reflective pavement?
As CSHub paper Mitigating Climate Change with Reflective Pavements (2020) explains, a conventional pavement such as asphalt might have an albedo of 0.1 (it reflects 10 per cent of the radiation it receives). A reflective pavement has an average albedo of 0.3 or higher, which means it’s at least three times as reflective.
The use of reflective aggregates and lighter-coloured materials such as concrete can increase a pavement’s albedo, as can the application of reflective coatings. When materials such as light-coloured binders or aggregates are locally sourced, these roads can also save money.
Pavements with a higher albedo reflect radiation into the atmosphere, known as radiative forcing. However, some reflected light strikes buildings, which can increase the demand for cooling in summer. Trials of cool roads in Los Angeles show that while reflective surfaces can cool footpaths as much as six degrees, the reflected light increased air temperatures a metre off the ground, elevating pedestrians’ exposure to heat.
MIT’s AzariJafari and Kirchain say “the climate change impacts of reflective pavements are highly context-specific. Though they will always lower air temperatures, their net emissions will vary greatly depending on a city’s unique climate and urban morphology, which includes building heights and street layout.”
The CSHub paper uses the American cities of Boston and Phoenix as case studies. Reflective pavement would have little effect in downtown Boston, where narrow streets and tall buildings block light from hitting the pavement most of the day. On the city’s suburbs and freeways, however, the researchers found doubling the current albedo of the city’s roads could cut peak summer temperatures by 0.3 to 1.7 degrees.
In Phoenix, reflective pavement could reduce summer temperatures by 1.4 to 2.1 degrees. In some neighbourhoods, however, modelling shows that incident radiation caused by reflective surfaces could increase the demand for cooling.