Phoenix attempted to combat extreme urban heat by applying a lighter, more reflective seal to residential streets across eight neighborhoods in a 2020 pilot program. The initiative aimed to reduce the amount of solar energy absorbed by traditional dark asphalt and lower overall neighborhood temperatures. While the treated roads became substantially cooler, the experiment uncovered a complex trade-off regarding human heat exposure.
To evaluate the intervention, Phoenix partnered with researchers from Arizona State University to monitor surface, air, and subsurface temperatures, as well as mean radiant temperature. The assessment utilized vehicles equipped with sensors, helicopter thermal imaging, and a specialized mobile weather station to measure conditions at the pedestrian level across roughly 36 miles of treated residential streets.
Data gathered during the testing revealed that treated road surfaces were 10.5 to 12 degrees Fahrenheit cooler than traditional asphalt during noon and afternoon hours. Subsurface temperatures also averaged about 4.8 degrees cooler. However, because the lighter coating reflected sunlight rather than absorbing it, it redirected more solar radiation back upward toward individuals standing or walking on the roadway.
A subsequent study published in Nature Communications highlighted that while surface temperatures dropped significantly, mean radiant temperature increased over the reflective pavement during peak afternoon hours. Air temperatures measured at two meters remained largely unchanged, and conditions on adjacent sidewalks showed no significant difference. After sunset and before sunrise, mean radiant temperature was slightly lower over the treated sections.
The findings demonstrated that a cooler road surface does not automatically translate to a cooler environment for people. Over a ten-month tracking period, the solar reflectivity of the coating diminished somewhat over time but remained higher than untreated asphalt. Subsequent research reinforced the conclusion that reflective pavements are most effective when deployed in areas with low pedestrian traffic, whereas locations heavily used by pedestrians require complementary cooling strategies such as trees and shading to offset the bounced solar radiation.