Rapid urbanization, which is transitioning native vegetation to man-made engineered surfaces, is taking place on a global scale. In 1950 approximately 30% of the world population lived in urban areas and, by 2030 that number is projected to be at 60%. As an example, by 2004, Phoenix had over 80% of Arizona residents residing in its urban area. Phoenix exploded in growth to become the nation's fifth largest city in population and larger in geographical terms than the City of Los Angeles. The added volume of paved surfaces and engineered materials during this time of growth has contributed to Phoenix experiencing one of the world's strongest Urban Heat Island effects (UHI) at a 0.86 dgrees F/decade-warming rate during the last century. As urban centers continue to expand and/or are being rehabilitated, there is a growing concern over local temperature increases due to solar absorption and associated heat emission. The problem is focused on the extensive network of dark colored horizontal structures (roofs and pavements) in the built environment. These increases are exacerbated by the loss of vegetation and trees in and around our cities due to urban and suburban growth patterns and practices.
Since paved surfaces (streets, highways, and parking lots) comprise 29% to 45% of the urban fabric and the demand/climatic impacts of pavements will continue to increase, a combined research initiative comprising laboratory and field experimentation, and heat transfer modeling, needs to be undertaken. The results of such research will help to understand how pavement designs and materials selection contribute to the Urban Heat Island effect.
The National Center for Excellence on Sustainable Materials and Renewable Technologies Innovations' work plan addresses the need to quantify the reflectance, absorption, porosity, thermal conductivity and emittance of different pavements including Arizona Department of Transportation's Asphalt Rubber – Asphaltic Concrete Friction Course. Another item of interest is the effect of pavement color and texture over time and its impact on mitigating UHI. Additional variables that need to be considered include traffic volumes, elevation of the roadway and spatial morphology.