Roads are considered one of the “higher pollutant” types of impervious surfaces, as they often contain contaminants from oil, brake dust, and other vehicle-related pollutants. In addition, road construction and maintenance represents a cost to developers that is passed on to homebuyers; municipalities bear the cost of road maintenance.
Acres of new right-of-way per approved unit in residential subdivisions, 1970 to 2014
The amount of impervious area devoted to roads (per home) has decreased over time in residential subdivisions.
Over the last three and a half decades, the amount of area used for right-of-way in residential subdivisions has decreased by over 40 percent on a per-home basis. In part, this is due to a greater proportion of new subdivisions being located in urban areas where density – or the number of homes per acre – is higher than in the rural areas. In rural areas, the amount of impervious area per home has been fairly variable.
This indicator is influenced by two primary factors:
Residential density (or the number of lots per total subdivision area)
Street width – which translates to the amount of land placed into the subdivision’s right-of-way.
Right-of-way (square feet) per Home in Residential Subdivisions, 1970-2014
Subdivision Approval Year
Right-of-way (square feet) per Home
Number of Units Approved
Source: TRPC Data Program. Subdivision plats from Thurston County Assessor's Office. Note: This table does not include residential lots created in mobile home parks or subdivisions without right-of-way separated from residential tax parcels; represents scenario if subdivision were completely built out.
Examples of a Subdivision
Subdivisions are typically divided into right-of-way, residential lots, and open space. As shown in the example, the right-of-way includes the street surface, sidewalks, street planter strip, and an additional area behind the sidewalk. Not all of the area within the right-of-way is impervious.