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Office of Planning, Environment, & Realty (HEP)
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:
No case studies are currently available for this type of impact.
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:
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.
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.
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.")
Select alternative(s) that minimize impacts. This is part of a process of trading off all the various benefits and impacts of each alternative.
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:
Methods to quantify wetlands loss.
Ecosystem and biodiversity analysis methods to identify and define sensitive habitats.
White applied a probabilistic analysis to estimate the wetland impacts of an expressway. (Louis Berger, 1998)
The Hydrogeomorphic Wetlands System, developed by the Army Corps of Engineers, is a descriptive model to evaluate the functional aspects of wetlands. It can generate data to assist in determining highway alignments and conducting land use planning.
The Watershed Planning System (WPS), developed by the Maryland Office of Planning, is a geographically-based decision support system that examines the effects of land use policies and development on nonpoint source pollution. As an intermediate step, it estimates impacts on wetlands of land transformation from development.
General State Analysis is a method of discussing and tabulating key biodiversity issues (wetlands, endangered species, etc.) in a matrix fashion. General State Analysis requires a relatively low level of effort and is commonly applied in NEPA studies (Bardman).
The Habitat Evaluations Procedure, developed by the U.S. Fish and Wildlife Service in 1976, is a widely used habitat assessment methodology (Whitlock, 1994). It requires the development of habitat suitability indices for individual species.
Gap Analysis combines habitat evaluation data with land management information to assist in land use and conservation planning to maintain critical habitats. Gap Analysis is generally performed at a regional or state level and requires a significant amount of effort to apply. It has recently been tested at the county level in Spokane, Washington (Stevenson, 1998). The focus of Gap Analysis is on the loss and fragmentation of natural landscapes as the principal reason for loss of biodiversity, in contrast with a species-by-species approach to the loss of biodiversity.
The Habitat Evaluation Model is a predictive tool to determine habitat value of lands within a region based on various habitat quality parameters. The San Diego Association of Governments (SANDAG) applied the Habitat Evaluation Model in a county-wide effort to identify areas of critical habitat and develop strategies to preserve these areas. SANDAG subsequently used GIS to overlay habitat maps with proposed transportation facilities. The overlay process was used to identify routes that minimized disruption to the identified areas. The approach is documented in Transportation Case Studies in GIS: Case Study #1 (FHWA, 1999).
Multiscale Biodiversity Assessment is a combination of ecological models at different scales to predict which species and habitats will be most stressed as a result of future development patterns. This method has been applied on a research basis in San Jose, California (Cogan, 1997) in conjunction with land use forecasts from the California Urban Futures Model (Landis, 1995).
Efforts are currently underway to develop an ecological network model for the southeastern U.S. The project involves evaluating and mapping high-quality ecological areas, identifying ecological "hubs," and developing a potential ecological "network" with corridors or linkages between hubs. Data and methods from this project are being considered for use in transportation and land use planning in the Atlanta, Georgia region (EPA, 2000).
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:
The INDEX Model, a GIS-based decision support tool developed by Criterion Planners/Engineers. INDEX is a customized model that takes input files such as site or neighborhood plans and calculates a variety of measures, including impermeable surface area. A version of this model, Smart Growth INDEX, is currently being developed to integrate transportation and land use modeling at a sketch-planning level.
Hydrologic Models. These may be developed at the watershed or small-area scale, and are frequently complicated and data-intensive. An example of a hydrologic model is HSPF. HSPF uses land surface characteristics such as land-use patterns and land management practices to simulate the processes that occur in a watershed.
The Watershed Planning System (WPS), developed by the Maryland Office of Planning, is a geographically-based decision support system that examines the effects of land use policies and development on nonpoint source pollution. The WPS does not model detailed hydrological processes, but it accounts for major factors that are not accounted for in hydrological models, such as subsurface flow and groundwater through riparian areas. The Office of Planning is currently working to integrate this model with the prediction of land use impacts based on transportation accessibility. If successful, this will provide a modeling link from transportation investments and policies to water quality impacts. Funding for this project is coming in part from FHWA's Transportation and Community and System Preservation Pilot Program (TCSP).
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:
The Minnesota DOT has developed a statewide GIS-based model, known as Mn/Model, to predict the probability of archeological impacts from highway projects. The model uses environmental characteristics of known archaeological sites (e.g., proximity to a river or topography) to predict other sites where similar combinations of resources occur. Mn/Model will be used as a planning tool by Mn/DOT and other agencies, allowing planners to avoid areas of high potential for sites. If avoidance is not possible it will allow for more efficient and cost-effective survey efforts by determining locations for intensive survey and areas where no survey may be required at all.
The North Carolina DOT has developed a statewide GIS system for the analysis of environmental impacts in project planning. This system allows planners to overlay the proposed project on designated historic sites and districts, wetlands, and other layers of information. Aerial photographs provide a base. The system provides a centralized repository of data on designated historical sites to assist in planning. NCDOT's system and its uses are documented in more detail in FHWA's Transportation Case Studies in GIS (FHWA, 1999).
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.
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).