Description: As the effects of climate change become apparent around the world, extreme temperatures are becoming an increasingly urgent concern. This increase in extreme heat is particularly severe in densely populated urban areas where concentrated urban development and structures of metal, concrete, and other heat-trapping materials absorb the sun’s heat. These pockets of heat, known as “heat islands,” experience higher ambient air temperatures than the surrounding areas. Daytime temperatures in dense urban spaces can range from 1–7°F higher than less developed spaces in the same area. During the summer, triple-digit temperatures turn arid cities like Phoenix, AZ and Austin, TX into dangerous and exhausting environments with heat visibly radiating off the roadways. In heat island areas, the sun’s energy is absorbed by buildings and pavement during the day and re-emitted after the sun sets, keeping temperatures abnormally high during the day and into the night. To combat this phenomenon, cities are seeking to implement various measures such as cool pavement technology, cool roofs, shade structures, and maintaining shade trees within the larger urban environment.

Los Angeles, CA
The Los Angeles Bureau of Street Services (also known as StreetsLA) launched a pilot program in 2017 to investigate the cooling effect of light-colored pavement coatings. The Cool Street Project involved applying a light gray, water-based coating onto the pavement enabling it to absorb less heat than darker asphalt due to the albedo effect. In high temperature and brightness conditions, these measures can produce a cooling effect as significant as 11-13°F on the treated surfaces. The goals of this program included reducing the risk of heat-related illness and death as well as reducing energy used for air conditioning.

The success of the Cool Street Project led to the 2019 Cool Neighborhood pilot projects, where 60 miles of ‘cool pavement’ have been added to a number of neighborhoods and nearly 2,000 trees were planted. This followed a 2014 ordinance that required new and renovated buildings to use roofing materials with a high solar reflectance rating. Cool pavement is an asphalt treatment applied on top of existing pavement in order to lighten its color and increase its reflectivity, thereby reducing the amount of heat absorbed. In the Winnetka neighborhood, locations within 100 meters of cool pavement were on average 2°F cooler than surrounding blocks. Following these positive outcomes, additional Cool Neighborhood projects were implemented in the neighborhoods of Pacoima, Sun Valley, and South Los Angeles using pavement coatings from three different suppliers. More than 181 lane-miles of cool pavement have been installed in the city since 2017.

In the next phase, the Bureau of Street Services is planning a project in the Sherman Way Metro Station of the Canoga Park neighborhood. The dual objectives of this are to simultaneously mitigate instances of extreme heat in a local hotspot while also improving transit access for low-income renters dependent on public transit resources. Cool pavement coating will be applied along with other urban cooling features, such as shade trees and hydration stations. Construction is intended to start in 2025 and be completed by 2028.

Phoenix, AZ
In 2020, researchers from Arizona State University (ASU) began studying the effectiveness of a reflective asphalt sealcoat in Phoenix’s Cool Pavement Pilot Program. They found the temperature of the treated surfaces was reduced by as much as 10.5 to 12°F during the hottest part of the day. These significant results led to cool pavement becoming part of the city’s routine street maintenance program in 2021. Other findings show temperatures below the street surfaces were cooler by an average of 4.8°F. However, while the difference in surface temperature of cool pavement is significant, the temperature variance quickly diminishes above the road surface. At 6 feet above the pavement surface, the air temperature was reduced by only 0.3°F during the day and 0.5°F at night.

The city of Phoenix launched a plan in 2022 to create more shaded pedestrian walkways such that all residents are consistently within a five-minute walk to a tree canopy. Since shade is the simplest means to reduce the effects of outdoor heat, trees can decrease the apparent temperature by 30-40°F compared to full sun exposure. The temperature is further lowered through the evaporation of water via the tree system itself. The goal is to shade 25 percent of the city by 2030 by establishing one hundred ‘cool corridors’ in the most heat-vulnerable neighborhoods.

Austin, TX
The city of Austin partnered with the University of Texas in the fall of 2022 to test cooling pavement treatments on a street in an urban area. The nearly mile-long street was treated with a light-colored, environmentally friendly pavement seal sprayed onto pavement. Ambient air temperatures registered 2-4°F lower at the testing site compared to surrounding areas. In some cases, the light-colored pavement seal reduced asphalt temperatures by as much as 10°F. Since the study was only a year long, it is difficult to determine if the benefits diminish over time and how often it will need to be reapplied, necessitating further research in the future. The program cost the city approximately $17,000 and was completed in Nov, 2022. Funding is currently being sourced to expand the study to a larger expanse of streets and obtain more data.

San Antonio, TX
The city of San Antonio tested cool pavement technology on a small section of asphalt at one end of the Hays Street Bridge in 2021. The city, in partnership with the University of Texas at San Antonio, is expanding the Cool Pavement Pilot Program by applying different cool pavement treatments to portions of roads in all ten Council Districts to compare and contrast their relative efficacy over the course of six months. The program will cost about $1 million and be funded by the city’s Resilience, Energy Efficiency, and Sustainability Fund. This figure includes the costs of installing the cool pavement as well as assessing the materials for temperature and product performance.

The ASU study results and other reports have revealed possible limits as well as best practices for cool pavement applications. Treated pavement reflects a portion of the sun’s rays instead of absorbing it; as a result, a person could feel 5-7°F warmer when standing or walking on a treated pavement during the hottest portion of the day. For this reason–and despite the reduced surface and subsurface temperatures associated with cool pavements–pavement treatments may not be suitable for locations with high foot traffic such as plazas and playgrounds. In certain situations the reflected radiation, called incident radiation, can also impinge on nearby buildings and increase the demand for air conditioning. Nevertheless, carefully considered cool pavement applications on large parking lots, streets, and other large expanses of pavement still offer a way to meaningfully reduce the heat island effect, especially when combined with increased shade from trees and other structures as well as reflective roofing on buildings.

Goal: Reduce heat island effect and thereby heat-related illness and energy use for air conditioning