Roads

Roads collect high concentrations of phosphorous, suspended solids, bacteria, various metals and generate high volumes of runoff. Additionally, when roads are paved with asphalt they become "heat islands" that elevate the temperature of the first flush of stormwater runoff. Because they are designed to collect, concentrate and convey stormwater through a system of catch basins and pipes to a detention pond, wetland or water resource, researchers have found they are the single most important source of urban nonpoint source pollution.

Road Design

Tips for reducing runoff from roads

Planning/Regulatory Techniques
Reducing Imperviousness
Reducing Impacts

For more information download NEMO's Technical Paper on Roads (PDF - 26k) and NEMO Fact Sheet #9 (PDF - 458k).

Planning/Regulatory Techniques

While collector and arterial roads greatly contribute to an area's impervious cover, local officials spend most of their time reviewing and approving local roads associated with proposed subdivisions. Local road ordinances must be written to encourage and allow designers the flexibility to:

Road travelways can be significantly narrower than the road right of way.

Avoid disturbing natural drainage patterns.
Vary the pavement width according to the proposed use. For example, according to the American Association of State Highway and Transportation Officials (AASHTO) the suggested minimum width for a local two-lane road with a design speed of 30 miles per hour and an average daily traffic up to 400 vehicles is 18 feet. By contrast, the suggested minimum width for a two-lane arterial road with a design speed of 70 miles per hour and a projected hourly traffic volume of over 400 vehicles is 24 feet.
Incorporate alternative stormwater management techniques (such as vegetated swales - see reducing impacts below) where feasible. Many Connecticut municipalities mandate curb and gutter drainage and even go so far as to specify a minimum distance between catch basins.

Review and revise local zoning and subdivision regulations:

To assure minimum lot widths and side yards produce roads that are built for the minimum required pavement width needed to support projected traffic volumes.
To minimize the number and width (radius) of cul-de-sac roads.

Reducing Imperviousness

The transportation system contributes more than 50% of the total amount of imperviousness on a site.
Roads should be built for the minimum required pavement width needed to support projected traffic volumes.
Where cul-de-sacs are built, the radii of turnarounds should be minimized and vegetated islands should be used.
Also, try using permeable alternatives on local and access roads.


Jordan Cove Urban Monitoring Project, road utilizing pavers and a cul-de-sac with a vegetated island, Waterford, CT.	Narrower road width. Watertown, CT.

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Reducing Impacts

Removing the curb from a road design will allow water from the crowned roadway to sheet flow into the adjacent, vegetated shoulder. This is the design utilized by the CT DOT on many state roads. There is a cost savings both in the absence of the curb itself as well as the associated structures.

Swales built to manage storm water in Hebron, CT.

Where cul-de-sacs are built, the radii of turnarounds should be minimized and, where feasible, the center areas sunken below the grade of the road pavement so stormwater runoff can flow into the vegetated area.