PT-4: Cool Pavement

Greenroads
Jun 15th, 2011

To KarenF (NAPA): agreed that this is a rapidly evolving field. We will seriously consider these research results when revising the manual for the next version.

KarenF
May 9th, 2011

On behalf of National Asphalt Pavement Association, the following comment is submitted with regards to PT-4: Cool Pavement.

The reliance on an absolute albedo value of 0.3 (or Solar Reflectance Index (SRI) of 30) to mitigate UHI is scientifically unfounded and appears arbitrary. There are three reasons to strike this requirement.

First, there is little, if any, credible or published evidence that the albedo of pavement is determinative of Urban Heat Island (UHI) effect. In fact, bright pavement surfaces may pose additional health risks.

Currently, a number of researchers are investigating the ability of the built environment, including pavement materials, to impact building cooling loads and UHI. Some results will be presented at the upcoming 2012 Transportation Research Board meeting. Arizona State University (ASU), leaders in understanding the impact of the built environment on UHI, are also finishing up a research study that examines the impact of pavement characteristics on air temperature and resultant UHI effects. This research will be completed in February 2012. The uncertainty associated with the impact of pavement materials on UHI, illustrated by the numerous ongoing research studies that are investigating this effect, illustrates concern for setting bright-line criteria on pavement albedo or SRI indices.

NAPA believes that any pavement albedo requirement should be based on sound science. This study and additional research must be completed before codes and criteria are adopted that are not based on sound science.


Second, there are ancillary negative effects of imposing an absolute albedo value of 0.3 that has not been considered. For example, the National Institutes of Health, Centers for Disease Control and Prevention (CDC) suggest refraining from using highly-reflective hardscapes especially in school yards, in an effort to protect children from reflected UV radiation. CDC specifically recommends that: “Materials with a lower reflectance are more desirable.” (CDC, p.6)

And this has been echoed by a number of international occupational health agencies -- for example, the Canadian Centre for Occupational Health and Safety (CCOHS) indicates that "water, white sand or concrete, snow, and ice can reflect from around 10 percent to 85 percent of the sun's ultraviolet radiation. Skin may require extra protection against these indirect, reflected rays." (CCOHS; Reducing Exposure section)

Similarly, National Oceanic and Atmospheric Administration (NOAA) indicates: "Bright surfaces can reflect the sun's rays and increase the UV exposure. Snow, sand, water, and concrete will reflect most UV radiation. Thus, your skin will receive a much higher dose of UV radiation when near one of these materials. Contrary to expectations, a snow skier can be sunburned during the middle of winter while skiing on a mountain slope due to this factor." (NOAA, intensity of UV radiation section)

These studies indicate that bright surfaces reflect UV radiation and may pose additional health risks. In addition, any reflected solar radiation will be absorbed by surrounding structures and transfer that energy into thermal heating of the structure(s).


Third, the proposed pavement albedo requirement oversimplifies a very complex problem.

Specifically, regarding the reliance on an absolute albedo value of 0.3 for pavements to mitigate UHI, this appears unnecessary and arbitrary. Leading researchers in this area recently indicated: “To simplify the UHI approach by . . . increasing [pavement] reflectivity . . . is to grossly overstate and simplify the value of . . . mitigation strategies (Golden and Kaloush, p.1). Pavement color or reflectivity is only one aspect that affects pavement surface temperature. Density, heat capacity, thickness, porosity, and a myriad of other factors affect pavement surface temperature. Although not mentioned by the authors, the hottest surface temperature signature in Phoenix Arizona is in the south-central part of the city, at the airport with its very thick reflective pavement design -- and is easily viewed in the article's thermal satellite image (see Golden and Kaloush, p.3). Further, pavement surface temperature is a poor indicator of both the effective heat sink capacity of a material and the impact that material might have on adjacent building cooling loads. Again, solar radiation that is reflected off bright pavements will be absorbed into adjacent buildings, similarly causing increased cooling loads.


As other sections of this credit address, there are other proven technologies that can reduce UHI, including permeable pavements. Shade trees and structure cover are also recognized and proven strategies. In addition, strategies used to reduce summertime urban temperatures should also be balanced with strategies that help reduce wintertime cold, including the ability of pavement to assist in roadway snow melt and the ability of pavement to temper urban winter temperatures and building heat requirements.

Because there are a myriad of pavement characteristics that effect pavement surface temperature, and because there has been no relationship established between pavement albedo or pavement surface temperature and UHI, the strategy identifying minimum pavement albedo should be removed from the PT-4 credit.


Citations:

Centers for Disease Control and Prevention (CDC). Shade Planning for America’s Schools. Available at: http://www.cdc.gov/cancer/skin/pdf/shade_planning.pdf

Canadian Centre for Occupational Health and Safety (CCOHS). Skin Cancer and Sunlight. Available at:
http://www.ccohs.ca/oshanswers/diseases/skin_cancer.html

National Oceanic and Atmospheric Administration (NOAA). What is UV radiation and how much does it increase with altitude? Available at: http://www.wrh.noaa.gov/fgz/science/uv.php?wfo=fgz

Golden, J and Kaloush, K. 2005 A hot night in the big city. Public Works Magazine. Available at: http://www.pwmag.com/industry-news.asp?sectionID=760&articleID=268116&artnum=1

Greenroads
Apr 16th, 2011

Good catches, we'll address these.

HelderB
Mar 18th, 2011

In the glossary, please add the words "UHI - Urban Heat Island" and "CP - Cool Pavement."

HelderB
Mar 18th, 2011

On page 451, item 1: Please check the text "REDUCE solar reflectance". There should be "Improve solar reflectance"?