Tennessee Valley Authority Economic Development
Sustainable Development Guide
Design Principles for Roads
Roads
Reduce the total length of roads
Treat storm water at the edges of roadways
Minimize roadway width
Roadways should provide the minimum pavement width to support travel lanes and emergency, maintenance, and service vehicle access. This width should be based on traffic volume and minimum road speeds. For example, 20 feet is the suggested minimum width for a two-lane road with a design speed of 30 miles per hour and an average daily traffic of 50 to 250 vehicles. (3)
Refer to the following document for more information about this best practice:
Environmental Protection Agency Post-Construction Storm Water Management
Reduce the total length of roads
Roads can be shortened by examining alternative layouts that increase the number of parcels per unit length. Because streets are usually designed to accommodate a rapid, smooth traffic flow, total street length is rarely the most important design consideration. (4) Shorter roads reduce the amount of pavement, curb, gutter, and storm-sewer construction, saving money on construction, design, and materials. Additional long-term savings come from reduced maintenance costs.
Minimize right-of-way widths
A right-of-way that is wider than necessary results in needless loss of trees. It consumes land better used for lots. Right-of-way reduction may not be desirable, however, if storm water is conveyed by swales along the road. They provide the best storm-water treatment, but to prevent standing water they may require 10 to 12 feet along one or both sides of the road. (5)
The table summarizes the pluses and minuses of choices for reducing a road’s right-of-way width:
Major components |
Design techniques |
Pluses/Minuses |
1) Pavement width |
reduce on some streets |
+Traffic volume on commercial streets remains relatively constant - Narrow widths do not allow for future road widening |
2) Sidewalks |
Reduce width or only one side or eliminate them where they don’t make sense. |
+ Grade so they drain towards the yard. - ADA concerns. - It will limit access of sidewalk for employees. |
3) Border widths |
Separates sidewalk from street—relaxed |
- Pedestrian safety considerations. |
4) Utilities |
Installed beneath the street pavement at time of construction |
+ New technologies can reduce the amount of pavement torn up during maintenance. - Will need to be accessed and maintained for repair or replacement. - Necessary to repave after utility work |
Table 2: Pluses and minuses of right-of-way width choices.
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Potential design options for narrower right of way. From Georgia Stormwater Manual, Vol. 2. |
Treat storm water at the edges of roadways
Where topography, soil, and slope permit, use vegetated open channels in the road right-of-way to convey and treat storm-water runoff. Curbs and gutters provide no storm-water treatment benefit. They help turn roads into a pathway for pollutants from tires, brake-pad wear, lawns, and bare ground. The Center for Watershed Protection’s Better Site Design Manual lists three practices that convey storm water while providing water quality treatment:
- Open channels
- Grass channels
- Dry swales.
Open channels cost as little as one-third of curb-and-gutter or drainpipe systems. (6) Roadside swales are not roadside ditches. Properly designed and installed, they experience few of the nuisance problems associated with roadside ditches. (7)
See more information in the section on storm-water treatment for parking lots.