Environmental Impacts of Roads

Impacts of Roads on Nonliving Systems

Road systems affect hydrospheric processes by altering the flow of water on and below the surface of the land. Roads crossing low-lying areas can interrupt the movement of surface water, affecting the available water on both sides of the road. In rivers, bridge piers and abutments can cause changes to stream geomorphology, erosion [Page 952]and sedimentation patterns, and local aquatic habitat conditions. Drainage swales associated with roads can increase stream density, hastening the removal of water from the landscape, reducing percolation, and increasing peak flows during storm events. Debris flows and landslides occur more frequently in sloping terrain with roads present, especially in tropical environments.

Roads affect atmospheric systems by creating microclimates near roads, changing the airflow and relative humidity, often making the air in the road environment more turbulent, hotter, and drier than in surrounding areas. Other atmospheric effects near roads include higher levels of dust, increased noise, and greater nighttime illumination from both vehicle headlights and roadway lighting.

Roads also affect other natural processes such as fire and biogeochemical cycles. While roadsides are often ignition points for vehicle-related fires, roads also limit the spread of fire and are used to manage prescriptive fires. Biogeochemical processes are affected by the chemical discharge related to road systems and traffic. These include emissions from vehicles, compounds used in maintenance activities, and leachates from infrastructure. The products consist of a wide array of nutrients, greenhouse gases, and contaminants, from heavy metals to hydrocarbons and carcinogenic compounds. Particulate and volatile elements and compounds enter the environment through storm water runoff and atmospheric deposition.

Impacts of roads on Living Systems

Roads and their traffic affect living systems in various ways: They destroy and fragment landscapes, affect mortality rates and behavior in animals, and introduce exotic species. Roadside environments, with higher levels of light, nutrients, and storm water runoff, provide opportunities for some species to thrive, creating linear “edge” ecosystems that extend undisrupted across the landscape. Some of these are exotic invasive species, which compete with native species and disrupt ecological relationships. For some animals, roads provide opportunities for movement and are sources of forage or carrion. However, for many amphibians, reptiles, birds, and mammals, roads act as barriers thwarting their movement. Roads sever contiguous land areas, causing fragmentation of habitats. Roads can isolate organisms from one another, undermining the ability of populations to recover from disturbances and local extinction events. Changing traffic patterns create ephemeral disturbances that enhance the barrier traits of a road. Mortality related to vehicle-animal collisions (i.e., “roadkills”) is significant, especially for large mammals. Traffic noise causes behavior modification in several species. For some, even the presence of small, remote roads is sufficient to affect their behavior and survival.

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