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Toolbox for Regional Policy Analysis Report (2000)

Impact Methodologies - Environmental - Physical


Physical environmental impacts result from the presence and characteristics of transportation facilities and land development. Some types of physical environmental impacts include:

Key factors that influence physical environmental impacts include:

Case Studies

No case studies are currently available for this type of impact.

Forecasting Methods

Sensitive Habitat and Wetlands

Impacts of transportation facilities on sensitive habitats and natural areas are typically addressed at the project level, for example, through the National Environmental Policy Act (NEPA) process which requires the development of an Environmental Impact Statement for major projects. Responses may include choosing an alternative that minimizes impacts on sensitive areas, or developing mitigation measures such as wildlife crossings or wetlands replacement. At the local level, sensitive areas may be set aside from development, for example, through restrictions on the filling of wetlands or designation as natural reserves.

At the same time, awareness is increasing of the need to consider habitat and open space preservation from a broader, regional perspective. Ecological systems do not adhere to city or county boundaries. Similarly, transportation and land use decisions have implications for development that cross jurisdictional lines.

A typical approach to incorporating sensitive habitat and open space considerations into regional transportation planning might include the following steps:

  1. Identify areas that are important to preserve. Wetlands and riparian areas can be readily identified using existing USGS databases. Delineation of other areas of sensitive habitat is more complex process; some approaches are described below.

  2. Analyze transportation alternatives to determine their respective impacts on these areas. Amount of wetlands destroyed is a basic measure. Measures can also be developed of fragmentation of contiguous natural areas.

  3. The impacts measured in step (2) also should include secondary impacts - i.e., those resulting from shifts in land development patterns following the transportation investment. This step requires some form of land use modeling (see "land development impacts.")

  4. Select alternative(s) that minimize impacts. This is part of a process of trading off all the various benefits and impacts of each alternative.

  5. Develop mitigation measures to minimize the impacts of the selected alternatives. The mitigation measures may include local land use controls as well as modifications to the design of the transportation facilities.

The appropriate and available methods vary according to the type of impact being measured. These are discussed under the following categories:



Water Quality

While point source pollution (e.g., from industrial discharge pipes) has been significantly reduced over the past three decades, nonpoint source pollution has been much harder to control. Urban runoff - from roads, parking lots, buildings, and other impermeable surfaces - is a significant contributor to nonpoint pollution. Runoff may contain road salt, oil, fertilizers, pesticides, and other pollutants. It can also create high-sediment loadings in streams and rivers. High-sediment loadings can also result from erosion during construction activities.

The primary determinant of urban runoff is the proportion of ground cover that is impervious and does not allow rainwater to soak into the soil. As the proportion of impervious surface increases, the velocity and volume of runoff increases; flooding, erosion, and pollutant loads in receiving waters increase; groundwater recharge and water tables decline; stream beds and flows are altered; and aquatic habitat is impaired. The relationship between impervious cover and watershed degradation is not necessarily linear. Stream degradation occurs as impervious cover exceeds 10 percent, which can happen at residential densities of one unit per acre or less. At 30 percent cover, a watershed may be considered generally degraded. Industrial, commercial, and shopping center development can bring 75 to 95 percent imperviousness. (Benfield, Raimi, and Chen, 1999)

The percentage of impervious cover can be used as a proxy for water quality impacts associated with various types of development. Generic estimates of the percentage of impervious cover according to type and density of development have been developed for use in various models. A site-specific assessment can produce more refined estimates based on site plans showing street patterns, driveways and parking lots, building footprints, etc. The modeling of actual water quality impacts requires a more extensive hydrological model to translate runoff into sediment, nutrient, and pollutant loadings and other impacts.

Models that can be used to assess the water quality impacts of transportation and land development include:

Historical and Archeological

Federal law mandates that federal agencies must consider the impacts of their projects to historic properties, including archaeological sites. Like wetland and sensitive habitat impacts, historical and archeological impacts traditionally have been considered in project-level planning rather than regional planning. At the state or regional level, however, GIS tools are increasingly being applied to inventory historical and archeological sites and to use this information in the siting and design of transportation investments.

If sites of archeological importance can be identified at a regional level, projects can be routed to avoid these sites, or excavations can be performed in advance of the project. Frequently, however, sites of archeological importance are not discovered until digging for the project has commenced. At this point in the project, delays, or modifications to the project can result in significant additional expenses.

The identification of other historical sites can also be difficult. Historic resources can include entire districts or areas as well as individual sites. The extent to which specific sites or districts are worthy of historical protection is often open to question. Also, some areas with potential historical significance may not have been explicitly identified, or a decision has not been made on the extent to which they should be protected.

The following examples illustrate the use of GIS in identifying sites of archeological and/or historical significance:

Community Impacts

Other community impacts, aside from those addressed above such as noise and air pollution, may include the creation of barriers that restrict physical movement within a community, displacement of residents or businesses, or undesirable aesthetic impacts resulting from transportation facilities. These impacts are typically addressed in transportation planning through the public involvement process. A variety of public involvement techniques are available to elicit feedback on potential impacts, and to help community members identify alternatives with the least negative or the greatest positive impact. Newly-emerging tools such as computer-aided visualization techniques can assist in this process.

General References

Bardman discusses options and current practices for biodiversity assessment for transportation projects by state DOTs.

Garrett and Bank (1995) discuss the ecosystem approach and its relationship to transportation development in an address to the American Association of State Highway and Transportation Officials.

NCHRP Report 403, Guidance for Estimating the Indirect Effects of Proposed Transportation Projects, (Louis Berger Associates, 1998) reviews and discusses methods for analyzing the land use and environmental impacts of transportation projects within the EIS process. The report includes case studies of best practices.

FHWA's "Community Impact Assessment: A Quick Reference for Transportation" provides an introduction to community impact issues and assessment techniques for transportation planners (FHWA, 1996).

Updated: 4/24/2012
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