Figure 1. Comparison of street design in (a) a conventional residential subdivision and (b) a mixed use, traditional neighborhood design. Having well connected streets is one objective of Smart Growth street design. (Figure courtesy of Tom Low of Duany Plater-Zyberk)

Two aspects of street design relate to stormwater and its impacts. The first is the street itself. EPA, watershed researchers, and local governments have developed guidance on designing "green streets," that focuses on narrower widths, infiltration opportunities, and eliminating curbs and gutters. However, the underlying street patterns are just as influential, particularly as they relate to development patterns in a neighborhood and region.

Smart Growth Street Designs are based on a network of well-connected streets that support multiple transportation modes. Some smart growth approaches to street design include decreasing street widths, adjusting the vehicular level of service (LOS), creating LOS for other modes of transportation, and designing connected street networks to support multiple uses.

Regional Applicability

A variety of agencies control street and road designs on the regional level. State Departments of Transportation typically control the design and operations of highways and larger arterial streets. When developing streets, state DOT's most likely refer to manuals such as the American Association of State and Highway Transportation Official's "reen Book," or manuals developed by the Institute for Transportation Engineers (ITE).

Conventional street layouts today tend to follow a hierarchical system, with a multitude of smaller roads serving residential areas feeding into larger roads and arterials. These arterials funnel traffic onto larger regional roads and highways. This system is often highlighted for its role in congestion since the funneling of traffic creates congested chokepoints which prevent few, if any, alternative routes from place to place. This system also arose as part of a highly separated and dispersed land use system.

As local governments and states demand connected, multi-modal street networks, these two organizations have recognized the need for alternative standards. In response, ITE has published "Traditional Neighborhood Development Street Design Guidelines" (1999) and "Neighborhood Street Design Guidelines" (2003).

On a local level, several cities and counties (Cary, N.C., Portland OR) have enacted "connected streets" policies so that new residential or mixed-use development projects have more than one connection to neighboring retail, commercial or transportation centers.

In urban areas, the street system is likely to be well established. However, some cities are finding the need to modify existing street patterns to meet new needs. Charlotte, North Carolina, recently added streets to the area surrounding a planned transit station to enhance circulation and multi-modal connections.

Standard road design practice has been to make decisions about stormwater BMPs after the roadway has already been designed. This not only limits options, but often focuses attention on end-of-pipe treatment BMPs rather than in-line measures or preventive measures, which are generally less expensive to build and maintain, and more effective at protecting water quality. For new development or redevelopment of any part of a transportation system, stormwater management features should be an integral part of the design, not 'add-on' features.

Portland's SW 12th Avenue Green Street Retrofit project (source: City of Portland)

Where possible, a street retrofit should take advantage of opportunities to improve the drainage system or add structural and non-structural BMPs to lessen the flow of stormwater volumes or filter pollutants. This will require a new approach to street repair and retrofits. Departments of Public Works and stormwater engineers will need to consult with land use planners and site designers on reducing volume and treating stormwater before runoff enters the public conveyance system. In some areas, the stormwater inlets can be retrofitted with trash separation and filtration controls.

Siting and Design Considerations

Smart Growth street designs can be divided into two categories . street design in new projects and modification of existing street patterns.

Smart growth street designs incorporated into new projects are typically part of an overall site design that seeks to meet transportation, economic and multi-modal objectives. Though there is not one set standard, street designs should meet the following objectives:

• support a mix of uses
• develop parking plans to optimize the number of spaces and layout for multi-modal connections
• incorporate features such as boulevard islands, rotary islands, parking lot islands, swales, sidewalk tree and groundcover planters designed to capture, filter, and infiltrate runoff. These features are often already incorporated as aesthetic amenities and for traffic calming purposes; use them to manage stormwater as well
• integrate sidewalks, crosswalks, and traffic calming approaches to support bicycling, walking, and automobile traffic
• design for shorter block lengths
• engineer narrower street widths to facilitate pedestrian crossings and moderate automobile speed while meeting the needs of emergency responders
• provide access lanes, on-street parking and turning lanes to complement the land development design, sidewalks and building setbacks

Once the underlying layout has been established, transportation and stormwater engineers can look for additional strategies (e.g., pavers for low traffic areas) to further reduce stormwater volume and pollutants.

Separate stormwater sewers typically discharge runoff with little or no treatment into receiving bodies. Thus, avoiding or minimizing the use of large collection and conveyance systems (i.e., standard curb and gutter) should be a goal of any project. Bio-retention and infiltration measures should be standard components of any design manual.

Poorly draining soils do not have to preclude the use of these measures, as good designs and soil amendments can facilitate some level of infiltrative capacity almost anywhere. In areas with existing curb and gutter, and limited short-term options for major retrofits, the inlets and catch basins of storm sewers in a smart growth development or redevelopment project might require additional BMPs or design modifications.

Local governments can use several methods to incorporate smart growth features and stormwater benefits to existing streets. Some of these strategies will include:

• connecting disconnected streets, lanes and cul-de-sacs
• where a new street is impossible, adding paths to link housing and other uses,
• utilize unused streetscape to add public parking, increase the number of spaces, and introduce bike lanes.

These strategies are often used in connection with site design features like tree planters and vegetated bulb-outs. These features can be designed to handle and treat stormwater.

A connected system need not be a formal grid of streets. Often the connections are determined not only by the street layout, but by linkages among activity centers (e.g., schools, jobs). In addition, site planners might need to avoid introducing streets and hardscape in or around environmentally sensitive land or water resources.

The main benefit of smart growth street designs rests on the ability to support a higher level of development intensity on a smaller footprint. This benefit manifests itself well at the regional and neighborhood levels. Although most literature on stormwater management discourages "connected impervious surfaces," local governments need to recognize that as part of an overall smart growth design, "connected streets" confer stormwater benefits. The placement of intense and connected development is not always appropriate in every part of a watershed. However, concentrating growth and development in certain parts of the watershed in order to protect more sensitive areas, such as headwaters, can be a viable strategy.

In addition, developers who are accustomed to a conventional, separate pattern of development may resist new rules requiring connecting internal streets to neighboring projects, adding sidewalks or introducing a mix of uses. Likewise, residents on unconnected streets may oppose efforts to improve connections within existing neighborhoods.

Limitations to implementing innovative street designs might also occur within the stormwater regulations themselves. Blanket regulations that require land set-asides, mandatory infiltration, or swales can pose barriers to better site design. For example, mandated sizing requirements for swales might consume land needed for connections to a higher intensity transit district. While preserving these standards for certain parts of the watershed, incentives can be created for alternative street designs by modifying stormwater management requirements in targeted areas. In addition, there are reasonable, low maintenance, stormwater management measures that can be used even in densely developed, highly impervious areas that result in very low runoff.

Finally, the street system alone will not bring about stormwater benefits. The relationship among the street layout, the development plan, and existing activity centers is crucial for obtaining stormwater benefits.

Even in circumstances where the overall surface area of a smart growth street layout results in less impervious coverage, there are maintenance considerations.

Separate stormwater sewers typically discharge runoff with little or no treatment into receiving bodies. Thus, typical maintenance considerations for curb and gutter designs include street sweeping, catch basin cleaning, clearing blocked sewer lines, repairing and replacing failed pipes, and other aspects of maintaining buried, hard infrastructure. Maintenance of above ground bio-retention/-infiltration features such as swales and infiltration trenches largely include vegetation maintenance. Depending on locations and designs, removal of accumulated sediment and debris is also usually necessary. Porous or pervious surface materials generally do not have additional maintenance requirements. In-line and end-of-pipe commercial swirl or filter devices require regular clean-out.

In northern climates, storing plowed snow from streets is a major consideration. Narrower streets translate into less on-street snow, though multi-use streets (e.g. with parking on each side and frequent intersections) requires advanced planning for snow storage. As with stormwater runoff, snowmelt can carry pollutants and water volume, so techniques to filter pollutants and reduce the velocity of melting snow are also important. Some communities are planning for storage by designating park areas or infiltration strips for the handling and eventual release of collected snow from streets.

All types of systems should have regular inspections to ensure they are functioning properly.

The effectiveness of a smart growth street design can be at the street, neighborhood, and watershed levels.

At the watershed level, the benefits of smart growth street designs for both redevelopment and new development emerge from absorbing development demand on a smaller footprint. During initial construction, less land disturbance results in less exposure and risk of sedimentation. Quantitatively, the best management practices will be preventative in nature since development takes place on a smaller area.

The Atlantic Station redevelopment project is a good example of how streets, the development plan, and stormwater control were considered in an integrated fashion. The 138 former steel mill site was planned to hold 15 million square feet of residential, office and retail space. To assess the regional environmental benefits of the site, an alternative scenario of where growth could occur in a greenfield location was developed. The Greenfield development scenario reflected prevailing development and street patterns of the outlying area. Figure 2 illustrates site runoff comparisons. The distinguishing feature of Atlantic Station is its smaller footprint.

Figure 2 - Runoff characteristics between the site design for Atlantic Station and a hypothetical greenfield site with the same development use profile. All parameters were lower at Atlantic Station than the greenfield site. (source: USEPA)

The site designers also separated the combined sewer system serving the site and installed pre-treatment BMPs for runoff within the site. The first office tower, 171 17th Street Building, achieved a silver LEED rating, which includes project-specific stormwater benefits as well.

By providing transportation options, there can also be fewer impacts related to eliminating or shortening automobile trips. The benefits of shrinking the footprint of parking and better managing existing streetspace are straightforward, but watersheds also benefit from less tailpipe-related deposition and from devoting what was single use land development (parking) to multi-use (shared parking and retail, for example). This efficiency also represents environmental benefits.

The most basic cost calculation is a streetscape's surface area. Smart growth street designs can involve more coverage per acre in a district, but far less on a subwatershed scale.

For redevelopment, retrofitting streets might include the costs of land acquisition, burying utilities, and/or the complexities of negotiating with multiple landowners. In addition, the cost may not be limited to a single street, but rather require upgrades to several streets (and their storm sewers), that can add costs.

For new development projects, costs might be higher in the planning and site preparation phases, since the streets will be coordinated with the land use plan, with public space, and with surrounding streets and arterials.

For both redevelopment and new development projects, installing sidewalks, curbs and gutters is typically more expensive than the installation of the roadway itself.

For more information on the integration of smart growth and water quality, see [http://www.epa.gov/smartgrowth].

CNU has published a literature review of street designs for traditional neighborhood design and smart growth projects. This literature review will be used to support further work with the Institute of Transportation Engineers on the subject, which should be published in 2005. For the literature review, see [http://ite.org/bookstore/RP036.pdf [PDF - 19 KB - 5 pp] ].

The Institute for Transportation Engineers has developed two recommended practice guidelines: "Traditional Neighborhood Development Street Design Guidelines" (1999) and "Neighborhood Street Design Guidelines" (2003). These are available through ITE's bookstore at [http://www.ite.org ].

The American Planning Association has issued a report, "Planning for Connectivity: Getting from Here to There," Report PAS # 515. The report, written by Susan Handy, Robert Paterson and Kent Butler, is available through APA's bookstore at [http://www.planning.org ].

Portland, Oregon is retrofitting its existing street system through a "Green Streets" Program. See the Portland Bureau of Environmental Services home page at [http://www.portlandonline.com/bes/ ].