Lighter coloured roads could reduce temperatures in hot urban areas

Urban areas are considered heat islands due to land use changes (materials that effectively retain heat with low permeability to water). Compared to rural areas, temperatures in cities can typically be warmer by several degrees. This heat is transferred to the surrounding air and increases air pollution problems (tropospheric ozone and smog) and the demand for air conditioning and even heat-related illness and dead in vulnerable people.
However, this effect can be minimised through the use of green roofs and lighter-colored surfaces, which reflect more sunlight and absorb less heat.
A recent study by Synnefa, A., Karlessi, T., Gaitani, N. (2010) tested the use of cool colored thin layer asphalt and estimated its potential to improve microclimate. This solution could significantly lower surface temperatures in hot urban areas, making living conditions more comfortable and reducing the amount of energy needed to cool buildings. Paved roads are typically made of a mixture of asphalt (bitumen) and aggregate material. But different ways are being developed: in this study they used a colourless, flexible asphalt binder and various pigments and sizes of coloured aggregates (granular materials), resulting in beige, off-white, green, red and yellow paved material that were tested for their ability to reflect solar energy and compared to a thin-layer sample of conventional black asphalt. All samples reflected back higher levels of solar radiation than conventional black asphalt, ranging from 55% (off-white) to 27% (red and green), compared with 4% radiation reflection of black asphalt.
However, lighter coloured surfaces can cause glare, which can be a problem for drivers. But some 'invisible' reflectance is possible if the reflectance is in the 'near-infra-red' part of the spectrum.
In conclusion, the thin-layers of coloured asphalt could be applied onto existing or new asphalt road surfaces. Computer modelling suggests an off-white coloured surface on a road could lower the average air temperature by 5°C in low wind speeds.

Reference: Synnefa, A., Karlessi, T., Gaitani, N. (2010) Experimental testing of cool colored thin layer asphalt and estimation of its potential to improve the urban microclimate. Building and Environment. 46: 38-44.

Contact: asynnefa@phys.uoa.gr