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Water: Best Management Practices

Alternative Turnarounds

Minimum Measure: Post-Construction Stormwater Management in New Development and Redevelopment

Subcategory: Innovative BMPs for Site Plans
  
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Description

Alternative turnarounds are end-of-street vehicle turnarounds that reduce impervious cover in neighborhoods by replacing cul-de-sacs.  Cul-de-sacs are local access streets with closed circular ends that allow for vehicle turnarounds.  Many cul-de-sacs have radiuses of more than 40-feet.  From a stormwater perspective, cul-de-sacs create a huge bulb of impervious cover that increases the amount of stormwater runoff.  Reducing the size of cul-de-sacs, either though the use of alternative turnarounds or by eliminating them altogether, can reduce the amount of impervious cover created at the site. 

There are numerous alternatives to the traditional 40-foot cul-de-sac, all of which reduce impervious cover.  One alternative reduces cul-de-sacs to a 30-foot radius.  Others create hammerheads, loop roads, and pervious islands in the cul-de-sac's center. 

Applicability

Alternative turnarounds can be applied in the design of residential, commercial, and mixed-use developments. Combined with alternative pavers, green parking, curb elimination, bioretention, and other techniques, the total reduction to site impervious cover can be dramatic, reducing the amount of stormwater runoff from the site. With proper designs, much of the remaining stormwater can be treated on site.  For instance, a rain garden can be placed in a pervious island.  Doing so will reduce impervious cover and treat stormwater from neighboring impervious areas.

Implementation

Sufficient turnaround area is a significant factor to consider in the design of cul-de-sacs. In particular, the types of vehicles entering into the cul-de-sac should be considered. Fire trucks, service vehicles, and school buses are often cited as examples for increased turning radii. However, research shows that some fire trucks are designed for smaller turning radii. In addition, many new larger service vehicles are designed using a tri-axle, and school buses usually do not enter individual cul-de-sacs.

Implementation of alternative turnarounds will also have to address local regulations and marketing issues. Communities may have specific design criteria for cul-de-sacs and other alternative turnarounds. Also, although cul-de-sacs are often featured as highly marketable, actual research on market preference is not widely available.

Limitations

Local regulations often dictate requirements for turnaround dimensions, and local codes may not allow some of the alternatives. In addition, marketing perceptions may also dictate designs, particularly in residential areas. While changing local codes is no small effort, by initiating a local site-planning roundtable, communities can change some of these regulations through a cluster ordinance or through a collective effort to review local codes to promote better site design.

Maintenance Considerations

If islands are constructed as part of a turnaround, they will need to be maintained. Kept as a natural area, the costs could be minimal. Bioretention areas will also require maintenance. The other options create less asphalt to repave, and maintenance will remain the same and cost less.

Effectiveness

Comparisons of several types of turnarounds found that hammerheads create the least amount of impervious cover, as shown in Table 1.

Table 1. Impervious cover created by each turnaround option (Schueler, 1995)

Turnaround Option

Impervious Area (square feet)

40-foot radius

5,024

40-foot radius with island

4,397

30-foot radius

2,826

30-foot radius with island

2,512

Hammerhead

1,250

Costs

Alternative turnarounds reduce impervious cover.  Consequently, they also reduce construction costs (asphalt alone costs $0.50-$1.00 per square foot).  At an estimated $6.40 per cubic foot, bioretention costs more than providing naturally vegetated areas, but it can help reduce overall stormwater management costs. 

Information Resources

American Society of Civil Engineers, National Association of Home Builders, and Urban Land Institute. 1990. Residential Streets (2nd edition). Urban Land Institute, Washington, DC.

Brown, W.E., D.S. Caraco, R.A. Claytor, P.M. Hinkle, H.Y. Kwon, and T.R. Schueler. 1998. Better Site Design: A Handbook for Changing Development Rules in Your Community. Center for Watershed Protection, Inc., Ellicott City, MD.

Bucks County Planning Commission. 1980. Performance Streets: A Concept and Model Standards for Residential Streets. Bucks County Planning Commission, Doylestown, PA.

Institute of Transportation Engineers. 1993. Guidelines for Residential Subdivision Street Design. Institute of Transportation Engineers, Washington, DC.

Schueler, T. 1995. Site Planning for Urban Stream Protection. Metropolitan Washington Council of Governments, Washington, DC.


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