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Reflective roads paving the way to cooler cities

Researchers in MIT’s Concrete Sustainability Hub have been modelling lighter-coloured and more reflective roads in a bid to lower air temperatures.

Dark asphalt is known to raise the surrounding air temperature. Approximately 40 per cent of urban areas are paved, and by absorbing and reflecting heat it can increase the urban heat island effect.

The researchers believe using a more reflective surface can reduce temperatures by 1.4°C, although the placement of this type of paving will need to be carefully considered – it could potentially heat up nearby surroundings even more.

The reflectivity of a road is measured using albedo, which indicates how well a surface reflects solar energy. If a surface has a low albedo, it can absorb more light and heat.

A darker surface is able to absorb more light than a lighter one, but the benefit of using a lighter surface is that it can send the heat back into space.

According to an article by Architecture & Design, an MIT CSHub model estimated that an increase in pavement albedo on all US roads could lower energy use for cooling and reduce greenhouse gas emissions equivalent to 4 million cars driven for one year.

However, the lighter surfaces need to be placed strategically to stop surrounding buildings from heating up and increasing the need for air conditioning.

A study titled Solar reflective pavements—A policy panacea to heat mitigation? (Ariane Middel et al 2020 Environ. Res. Lett. 15 064016) also notes that factors such as building shape and sky view factor can affect albedo. It highlights that each city will have different considerations that will need to be taken into account if lighter pavement is used.

The paper recommends that individual municipalities consider all “environmental, social, and economic tradeoffs for long-term viability”.

There is also a need to consider the full life-cycle emissions of roads and the materials that are used to build them.

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