Porous Asphalt Study: “Effect of Road Shoulder Treatments on Highway Runoff Quality and Quantity”

The Washington State Transportation Center (TRAC) studied the effectiveness of porous asphalt road shoulder treatments at reducing the quantity and improving the quality of highway stormwater runoff, compared to traditional asphalt and gravel. Out of the three treatments tested, the porous asphalt shoulders produced both the lowest volume of runoff and runoff with the lowest concentration of pollutants. The report suggests that although porous asphalt may have higher installation costs than traditional asphalt, the use of porous asphalt road shoulders may have long-term economic benefits along with safety and environmental advantages. Improved stormwater infiltration and reduced runoff volumes from porous asphalt road shoulders may also help reduce impacts of precipitation increases due to climate changes.

TRAC conducted a series of experiments simulating both rainfall and water flow to determine the ratio of runoff to rainfall volume of each surface. Although this ratio varied depending on storm size, it was consistently lower when porous asphalt was used rather than traditional asphalt or gravel. TRAC also tested the infiltration rates on gravel and porous asphalt, with porous asphalt showing significantly faster infiltration rates. Experimenters produced a sheet of water over the porous asphalt which infiltrated in under a minute.

In addition to testing infiltration rates, TRAC studied each surface’s ability to produce high water quality runoff and therefore keep pollutants from reaching local waterways. TRAC tested runoff samples from each type of shoulder treatment for pollutant concentrations. Porous asphalt outperformed both gravel and traditional asphalt, notably producing runoff with only three-percent of total suspended solid loads from traditional asphalt.

The report acknowledged that, despite the significant operational and environmental benefits of porous asphalt shoulders, further research is necessary to determine if such shoulders are likely to clog, thus reducing their water-quality benefits. If future research finds that porous asphalt shoulders will not become completely clogged in five years, TRAC recommends the installation of porous asphalt shoulders. Porous asphalt would also likely help address safety and environmental concerns raised by increased rainfall or extreme weather because it performed better than other shoulder treatments at reducing runoff volume and improving water quality in high-volume storms. 

The Washington State Department of Transportation (WSDOT) has recognized the effectiveness of permeable pavements for stormwater management, which is a high priority for the agency. WSDOT’s Pavement Policy (e.g., this version is from September 2018) encourages the use of permeable pavements in certain new pavement design applications. Specifically, it notes that permeable pavement is suitable for new installations on very low volume, slow speed locations with little truck traffic, and should be considered and used in parking lots, driveways, and other light access areas like maintenance/enforcement areas on divided highways.


This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on October 26, 2015.


Publication Date: July 1997

Related Organizations:

  • Washington State Department of Transportation


Resource Category:

Resource Types:

  • Academic research paper
  • Best practice

States Affected:


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