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Habitat and Water Quality

Transport projects can have various impacts on wildlife habitat, hydrology (water flow), and water quality, including direct impacts. Transportation corridors can divide animal habitats, making it difficult for animals to move freely without being struck and killed by vehicles. Such disruption can divide an animal population into smaller, less stable groups that may have difficulty surviving. Habitat disruptions and water pollution tend to be particularly severe during project construction. A project can also have indirect impacts if it induces additional land development, for example, if an urban highway expansion project stimulates more automobile-oriented, urban fringe development ("sprawl"). 

Such impacts should be evaluated based on a "with and without" test, that is, the differences in land use conditions that would occur with and without the proposed project.


  • A bridge is built over a river, creating runoff from the roadbed during storms.
  • A road is constructed through the middle of an animal habitat, impeding animal movement through the area.
  • A highway expansion is predicted to stimulate additional low-density urban fringe development that will displace wildlife habitat and reduce water quality in a watershed.

Direct impacts can be measured based on the increase in impervious surface (pavement area), and any degradation of wildlife habitat that the project imposes. This can be particularly significant if a roadway project creates a barrier to wildlife movement, preventing natural migrations or creating habitat islands where wildlife are isolated. Indirect impacts can be quantified using integrated transport-land use models which predict how a transport project will affect future development patterns. 

Various methods can be used to quantify and monetize these impacts, including hydologic impact models (Janke, Gulliver and Wilson 2011) that predict increased stormwater management costs, US Environmental Protection Agency's WaterQuality Scorecard (USEPA 2007) which predicts impacts on water quality (and therefore ecological and water supply costs, for example, if a project degrades the quality of a watershed), and hedonic pricing and contingent valuation to monetize these impacts.

Even if some of these impacts cannot be monetized, they should still be quantified to the degree possible and described in the project analysis. If there are negative benefits that require mitigation, the cost of mitigation should be included with the project's overall cost.

Mitigating Habitat Disruptions

If a transportation project is constructed near an animal habitat, it may be necessary to include features that help animals cross the corridor, such as tunnels under or above a roadway. Also, animals may need to be shielded from noise, runoff, and visual impacts of construction. Special care must be taken if the habitat of a threatened or endangered species is involved.

Water Quality

Transportation projects can have significant effects on water quality. Motor vehicles, for example, deposit particles of rubber, oil, and other pollutants on roads; when it rains, these pollutants are washed into the areas around the road. In some cases, the stormwater may flow through drains directly to a river, lake, or bay, or it may contaminate groundwater or the water in a wetland area. Impacts can be lessened by diverting stormwater away from sensitive habitats or into sewer systems that treat the water before discharge into waterways. The land use impacts of transportation projects can also affect water quality and availability by making the ground less permeable, thus increasing runoff.

Ferry services can have a more direct effect on water quality. Dredging is often required before ferry service begins, which can increase chemical contamination as polluted sediment is disturbed. Ferries can also spill diesel fuel, one of the most toxic types of oil, directly into the water. 


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Robert Burchell, et al (2002), The Costs of Sprawl – 2000, TCRP Report 74, TRB (; at

Caltrans (2007), Storm Water Quality Handbook - Project Planning and Design Guide, California Department of Transportation (; at

Mikhail Chester and Arpad Horvath (2008), “Herbicides and Salting,” Section 5.2.5, Environmental Life-cycle Assessment of Passenger Transportation, UC Berkeley Center for Future Urban Transport, (, Paper vwp-2008-2; at

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LGEAP, Long-Term Hydrologic Impact Assessment (L-THIA) Model (, Local Government Environmental Assistance Program at Purdue University. Internet tool evaluates how land use changes are likely to affect groundwater recharge, stormwater drainage, and water pollution. Includes comprehensive bibliography.

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