Ameliorating the Effects of Roads
Although the development and operation of the transportation system can affect ecosystems and their components at many scales and in many ways (see Chapter 3), many approaches have been developed to avoid and reduce these effects. Protecting natural resources and providing safe and effective transportation are in the public interest, and addressing both of them is good public policy and practice. Ameliorating environmental effects can occur at all phases of road projects—from planning and design through construction and maintenance operations.
The most protective approach is to avoid environmental effects altogether. Another approach is to take measures to minimize effects that cannot be avoided, and a third approach is to compensate for effects. However, it is useful to consider mitigation as a three-tiered hierarchy involving avoidance, minimization, and compensation in a priority order that reflects the principle that it is generally better to leave something alone than to try to fix it after environmental effects have occurred. Discussions of project mitigation often focus on compensatory mitigation, such as mitigation of on-site impacts or compensation through off-site third-party alternatives (mitigation banks or in-lieu fee programs). The focus on compensatory mitigation options, however, can overlook other important ways that environmental factors can be addressed in transportation work. Understanding mitigation as a sequence is an important part of addressing effects comprehensively. This concept can be followed throughout the process of transportation project planning and development.
Managers have a number of opportunities to improve environmental conditions after design and construction phases of road projects. As roads are built and used, new and unforeseen effects often appear. Some of these effects can be addressed during ongoing maintenance operations on the roadbed and roadside.
This chapter discusses opportunities for ameliorating the effects of roads. It begins with a discussion of mitigating effects organized by scales of administrative organization; the largest scale pertains to national or regional perspectives, the medium scale includes state or highway corridor planning, and the smallest or finest scale applies to the decisions and opportunities associated with individual transportation projects. The chapter ends with a section on opportunities for increased environmental stewardship during routine maintenance operations after construction.
SCALE-BASED CONSIDERATIONS OF ENVIRONMENTAL MITIGATION
National or Regional Perspectives (Broad Scale)
At the national and regional scales, opportunities for ameliorating the environmental effects of transportation come in the form of broad public policy for governmental agencies. Environmental issues are often treated as a permitting issue rather than a dimension of project design. Environmental regulations and permits are intended to protect certain types of natural resources, such as wetlands or threatened and endangered species, but this system does not always promote the best, most comprehensive treatment of environmental issues. Actions at the federal level can help to improve the process in the following ways:
Provide policy, guidance, and funding for transportation design and decision making that take ecological processes into account.
Expand the knowledge base for assessing potential effects of transportation activities through nationally funded research projects.
Encourage cross-disciplinary dialogue between engineers, ecologists, and other environmental professionals to raise mutual awareness of each other’s expertise, needs, and challenges.
Share information from practical experience. The larger perspective of national agencies and organizations permits identification and promotion of positive examples of success as well as lessons learned.
State and Highway Corridor Considerations (Medium Scale)
Opportunities to ameliorate the ecological effects of transportation activities at the medium scale equate best to the planning stages of individual projects, larger road corridors, and statewide transportation system plans. These plans identify long-range needs for meeting transportation objectives.
In medium-scale planning efforts, it is possible to set a direction that can avoid and minimize many ecological effects of roads before projects are planned. In most cases, transportation projects consist of improvements to existing systems rather than consisting of new-road construction options. There are fewer options for minimizing effects in improvement projects; however, improving culverts and bridges and the routing are some ways to reduce the ecological impacts of projects on existing systems. Where new alignments are being considered, changes to the overall pattern of roads in a local area can also be considered. For example, roads may be consolidated or realigned to avoid an ecologically sensitive area, such as a wetland. Other examples are provided in the following section.
Regional planning activities typically occur within politically defined areas, such as municipalities, counties, or portions of states. These boundaries make sense when considering the interaction between socioeconomic systems and a regional plan. However, the interaction between ecological systems and a regional plan is most effectively considered within an ecologically defined area, as suggested in Chapters 3, 5, and 6. The appropriate ecologically defined area will depend on the ecological conditions of interest. For example, if water quality is the environmental issue of concern, the watershed would be the most appropriate planning area. If the concern is for persistence of a particular species or population, the range of that species or population should define the planning area. If the concern is for a particular ecosystem process, the appropriate planning area might be an eco-region. Planning boundaries for assessing the interaction between the road system and the ecological systems should coincide with ecologically defined boundaries, such as watersheds, eco-regions, and species ranges, determined by the complexities
of the environmental issue. Multiple concerns should be addressed at the most appropriate planning area and may involve more than one. For example, the appropriate planning area for concerns about the continued survival of a species that is dependent on a particular ecosystem process (improved water quality) may need to consider two ecologically defined boundaries: watershed and species range.
The following discussion on strategies to reduce the effects of roads and traffic considers only the needs of flora and fauna. The committee realizes that transportation authorities must consider and balance multiple interests but offers these strategies to provide guidance on setting priorities for the ecosystem independent of other competing factors.
One way to reduce the ecological effects of roads is to reduce the extent of road corridors in a region, especially through valuable habitat (Figure 4-1). In considering planning options for reducing or limiting the ecological effects of roads, the regional planner is required to first consider whether it is possible to avoid building new roads. In some cases, removal of a road may also be considered. It is usually impractical to remove a road that local residents and businesses depend on, even when substantial ecological benefits may be gained. In addition, road removal is costly. Therefore, road removal is usually not considered to be a practical option.
Many ecological effects of roads are due to traffic rather than to the road itself. For example, pollution from vehicles may alter plant communities to distances of at least 200 m from the road (Trombulak and
Frissell 2000). Increased traffic can also cause an increase in habitat loss because the distance maintained by animals to avoid a road (the road-avoidance zone) increases with increasing traffic volume (Figure 4-2).
There are many possible methods for reducing the total amount of traffic on the roads and concomitant environmental effects. The details of how to achieve these results are outside the charge of this committee and not addressed in this report.
Natural bodies of water, such as rivers, streams, ponds, and wetlands, are particularly sensitive to the effects of nearby roads and traffic. As discussed in previous chapters, roads affect runoff, especially stormwater runoff, and hence have effects on the flows of nearby streams, on rates of groundwater recharge, and on water quality. For new and reconstruction projects that receive federal funding, the Federal Highway Administration (FHWA) requires that “standard management practices” be used to control stormwater. For example, stormwater impacts impacts may be managed by routing the water to infiltration areas, away from the road base, to reduce the amount of pollutants discharged to water bodies. Stormwater management is still a major concern for projects not on the primary road system (for example, local roads under different jurisdictions).
Natural water bodies are a critical component of the life-history requirements of many species, which may be required to move between the water body and other habitat areas on a daily basis or during an annual migration. In mountainous areas of the western United States, roads are intimately associated with major streams and rivers, usually paralleling them in, or next to, riparian areas, which are even more important for
wildlife habitat and movement in arid areas. Roads near water bodies cause high mortality of animals moving to and from the water. For animals that avoid roads or traffic, the presence of a road near a water body can limit the animal’s ability to access the water or the other necessary habitats. Roads also increase human access to natural water bodies, resulting in their indirect degradation. Research by Carr and Fahrig (2001) showed that negative effects of road density on aquatic animal populations occur within 1.5 km of ponds and wetlands for turtles and within 2 km of ponds and wetlands for amphibians.
In most cases, it will not be possible to either remove roads or reduce overall traffic volume in the region. However, planners can strive to reduce the ecological impacts of roads by implementing measures that redirect most traffic away from ecologically sensitive natural areas in the region (Figure 4-3).
The previous suggestions illustrated in Figures 4-1 to 4-3 are aimed at reducing or at least not increasing the road density and traffic volume regionally and locally. However, the regional planner is often faced with a demand for increased traffic capacity, which will adversely affect ecological conditions. There are some general strategies that planners can use to minimizing these effects.
In general, there are fewer ecological effects from one road than from two roads. As discussed above, roads themselves (independent of the traffic on them) have ecological effects, such as hydrological changes. The road-avoidance zone (Figure 4-2) is one of the factors con-
tributing to the road-effect zone, which widens with increasing traffic (Forman et al. 2003, NRC 2003). However, the width of the road-effect zone increases at a declining rate with increasing traffic (Figure 4-4; Foppen and Reijnen 1994, Reijnen and Foppen 1994, Reijnen et al. 1995, Forman and Deblinger 2000). Measures that reduce roads and traffic within 2 km of water bodies are particularly effective in ameliorating ecological effects of roads. When an increase in traffic must be accommodated, the ecological effects will be less when adding to an existing road rather than building a new road, all else being equal.
All the suggestions illustrated in the figures depend on the particular circumstances under consideration. For example, if the existing road is near a water body, more ecological damage might be caused by increasing traffic on it than by building a new road if it is in a less ecologically sensitive location. There also can be social and political reasons for not increasing traffic volume on existing roads. When more traffic must be accommodated but the volume on existing roads cannot be increased, construction of a new road may be the only viable option. The ecologi-
cal effects of a new road can best be minimized by building it as far from natural areas as possible and through developed areas, such as urban areas or areas of intensive agriculture (Figure 4-5). In fact, if a road is placed through a high-intensity agricultural area, roadside verges of seminatural habitat can represent an ecological benefit of the road.
In summary, fewer larger, high-volume roads are preferred to many smaller, low-volume roads, because the impact of roads on the local ecology increases with distance and volume at a declining rate. Another benefit to this strategy is the preservation of larger habitat areas and less habitat fragmentation. Research on rural roads in The Netherlands indicates that habitat fragmentation can be reduced by using techniques for easing the impact of traffic in residential areas, referred to as “traffic calming” (Jaarsma 1997, Jaarsma and Willems 2002). Specifically, traffic calming is used to concentrate traffic flows so that the majority of the traffic is on major roads. The result of these tactics is less construction of new roads and therefore reduced habitat fragmentation, suggesting that a less dense road network with higher volumes results in less habitat fragmentation.
Sometimes, political and economic conditions might not allow implementation of measures to reduce traffic on roads near natural areas. A new road might even be built near or through such areas. Measures can
be implemented that permit water and animals to pass safely under the road, such as lengthening bridges or constructing viaducts, or that permit animals to move over the road, such as wildlife overpasses (Figures 3-4, 4-6). For example, where roads cross small streams, culverts can be replaced by bridges that are long enough and high enough to permit movement of stream animals, as well as terrestrial animals, under the road. Wildlife overpasses also can be used to connect otherwise disconnected populations. Fencing along both sides of the roads discourages animals from attempting to cross the roads and funnels them to the underpasses or overpasses where they can safely cross.
Using Environmental Data in Medium-Scale Planning Efforts
Factoring ecological information into transportation planning relies on quality information about the environmental resources in the planning area. Due to the broad landscape context of road systems, landscape patterns and processes must be incorporated into the planning and construction process (Forman 1987). When used in a geographic information system (GIS) environment, regional or landscape habitat connectivity models can facilitate decision making in identifying, setting priorities
for, and designing amelioration measures (for example, corridors for animal movement) (van Bohemen et al. 1994, Bekker et al. 1995). Use of such models provides for the development of a more integrated land-use strategy by taking into account different land-management practices and priorities of habitat conservation concerns.
At the medium-scale planning stage, evaluating potential environmental effects can take the form of relatively simple screening of project areas (sensitive wetlands, streams, or habitat for species with special management considerations) and pollution sources (contaminated sites or air quality). Evaluation can also consider adjacent land uses, especially where they involve natural-resource management. Other resources could include information on wildlife and vehicle collisions in the area. Rapid assessment methods using GIS hold promise for streamlining the planning process when the effects of transportation on the environment are considered from the onset. These methods are discussed in detail in Chapter 6.
Governmental agencies and nongovernmental organizations that manage natural resources are involved in many types of conservation planning. Some examples include state biodiversity plans, endangered-species recovery plans, and watershed-restoration plans. Many of these plans are not conducted in coordination with transportation planning information and as a consequence do not take the existing transportation network or potential expansion into account. Land-use and environmental-restoration plans present opportunities to link transportation planning with environmental protection. Linking the plans can help to (1) identify natural resources that should receive special consideration of avoidance and minimization strategies; (2) identify ways for transportation projects to contribute to environmental enhancement as part of future project planning and implementation; and (3) provide direction for the location and design of compensatory mitigation sites so that these sites can help to maximize benefits by supporting larger conservation objectives.
A number of examples of this collaborative planning exist in the United States. These joint efforts provide a good opportunity for addressing a broader set of environmental concerns early enough in the transportation project to shape and modify the project. The California Department of Transportation and the Nature Conservancy have undertaken such collaborative projects (Box 4-1). Other examples of such planning efforts can be found in specific transportation plans at the
BOX 4-1 Caltrans and TNC
The California Department of Transportation (Caltrans) and the Nature Conservancy (TNC) have collaborated on a partnership to minimize the environmental impacts from road projects. Developed in 2002, Caltrans and TNC combined California Transportation Investment System data highlighting existing paved roads, current projects, and planned development for the next 20 years and TNC portfolio sites displaying conservation and biodiversity data at multiple scales. The GIS overlays provide both parties a visual representation of key land areas of concern and possible impacts on individual species that may be occurring now or in the future because of road projects. Caltrans and TNC are discussing how the GIS tool can be implemented in transportation planning within the state.
transportation-department websites of Florida (see also Chapter 6) and Washington states.
Environmental Mitigation at the Individual Project Scale (Small or Fine Scale)
The transportation activities in individual projects on road segments correspond to the finest scale of analysis. At this scale, transportation actions include designing and evaluating alternative designs, obtaining environmental permits, developing project-specific mitigation measures, acquiring right-of-way lands, and constructing the road. Ongoing maintenance and road operations also occur at this scale.
Mitigation Considerations During Project Design
Environmental issues, needs, or desired conditions need to be known at a fine scale of resolution for factoring into project development. In the early stages of the project design, context-sensitive design and alternative development can be selected to avoid and minimize adverse environmental effects. A simple example would be to increase the slope along the sides of a roadway to minimize wetland fill. Other examples include selection of roadway alignment (such as following topo-
graphic contours rather than cutting across them), use of alternative construction methods, or timing of construction.
Many new and alternative construction techniques have been introduced to minimize environmental effects—for example, noise from pile-driving, which can be disruptive and harmful to wildlife, can be reduced by the use of vibratory hammers instead of impact hammers. Underwater bubble curtains can be used to reduce sound-pressure transmission underwater, which is known to harm aquatic animals (Turnpenny et al. 2003). Fish-startle systems that produce high-frequency signals have been used to reduce the impacts of aquatic blasting by temporarily relocating fish from a project site (FHWA 2003a). The timing of construction projects can be adjusted to avoid periods that are especially sensitive for fish and wildlife.
Traffic, however, is the primary source of noise that affects animal behavior (Forman et al. 2003). Strategies to reduce traffic noise, typically intended for human benefit, include improvements in road surfaces and vehicles and reducing the volume of trucks in ecologically sensitive areas. Results of studies in The Netherlands showing that effects of traffic noise can result in decreased breeding-bird densities within 1,000 m of high-volume roads (Van der Zande et al. 1980, Reijnen et al. 1996) and other traffic noise-related studies have led the Dutch government to begin repaving high-volume roads with ZOAB, a noise-absorbing porous asphalt (Piepers 2001). Similar amelioration measures to reduce traffic noise are under way in the United Kindgdom where the Highways Agency’s goal is to reduce noise on 60% of their trunk roads (major highways) during the next 10 years (Price 2003). In Germany, a recently developed tire can produce half the pavement noise of a conventional tire at the same price (Carstens 2003).
Other noise-reducing strategies include the use of soil berms (low, smooth earthen ridges) and vegetation that, unlike noise walls, are not barriers to wildlife movement. Noise reduction can be further improved by the combination of narrow soil berms along below-grade roads (Forman et al. 2003).
Wildlife crossings are perhaps the most conspicuous amelioration measures on roadways. Wildlife passages are found in 23 states, 17 of which are beginning to systematically incorporate wildlife crossings into roadway designs (Evink 2002) and across Europe (Damarad 2003). Some of the most successful examples come from Florida, where 32 underpasses were built along Interstate 75, and from Banff National Park, Alberta, where 24 crossings, including two overpasses, were installed on
the Trans-Canada Highway. In both locations, the measures were effective in dramatically reducing wildlife and vehicle collisions and barrier effects to animal movement (Clevenger et al. 2002b, Evink 2002).
Central median (or Jersey) barriers are frequently used to separate lanes of traffic and often extend over many miles. These 1-1.5-m high concrete structures are designed to prevent head-on collisions, but they also form a wall that can disrupt wildlife movements (Servheen et al. 1998, Forman et al. 2003). Despite the potential impact on wildlife movements, mortality, and driver safety, the “Roadside Design Guide” does not address this issue (AASHTO 2002b).
As a means of increasing road permeability for wildlife, many state departments of transportation are installing Jersey barriers with a modified design that allows passage of small fauna. However, there is no information regarding how effective these modified barriers are, and there are no standard guidelines regarding their placement. Installation of large wildlife crossings is generally an issue of maintaining habitat connectivity for key and wide-ranging, fragmentation-sensitive wildlife species. For many small and medium-sized mammals, drainage culverts have been found to provide critical habitat linkages (Yanes et al. 1995, Clevenger et al. 2001, Foresman 2003).
Spacing intervals for wildlife crossings vary among environments, species for which crossings are designed, and conservation objectives (which might range from simple genetic interchange to more complex restoration and maintenance of ecosystem processes). Therefore, guidelines for spacing have not been developed.
Some environmental concerns can be addressed through setting design standards for projects. For example, wider bridge spans and reducing or eliminating in-water piers help to limit effects on aquatic systems. Design standards for stream crossings that routinely incorporate in-water piers can help to improve environmental conditions. The periodic reconstruction of highway bridges that span riparian areas is an excellent opportunity to improve wildlife passage along riparian corridors by widening bridge spans or by habitat enhancement.
A series of best-management practices (BMPs) for road construction has been developed for reducing or ameliorating the adverse effects of roads on aquatic ecosystems described in Chapter 3. The BMPs often focus on mitigating effects on fish passage or stream habitat (e.g., TranSafety 1997, Moore et al. 1999, Robison et al. 1999, Bates et al. 2003). The techniques, summarized by the National Research Council (NRC 2004), include the following approaches:
Careful planning of the road’s route to keep the road on terrain that is resistant to landslides and erosion and to minimize the number of stream crossings.
Designing bridges and culverts with hydraulic characteristics that allow aquatic organisms to pass through them in both directions, as appropriate to different life stages.
Engineering techniques, including the appropriate use of vegetation, to stabilize embankments.
Managing stormwater runoff to reduce hydraulic connections between roads and streams. To the degree that a road is impervious, as paved roads generally are, stormwater runoff is enhanced and concentrated unless provision is made for adequate drainage. Even unpaved roads enhance and concentrate runoff.
Controlling soil erosion on newly constructed roads, stream crossings, and related structures.
Maintaining the crossings regularly to prevent debris and beaver dams from allowing culverts to become clogged. A clogged culvert can turn a road crossing (embankment) into an earthen dam, whose inevitable failure will lead to a torrent of water, debris, and sediment.
Unlike other areas of transportation research, there are virtually no design guidelines for building wildlife-crossing systems (Evink 2002). This area of research is an emerging science. Consequently, there are few published studies to refer to for design criteria for habitat connectivity structures for wildlife (for example, wildlife passages) or guidelines for multiple species or fragmentation-sensitive species (for example, wide-ranging large carnivores). The National Cooperative Highway Research Program is currently funding a project that will be the first attempt to develop a state-of-the-art guidance and a decision-support tool to enable well-founded decision making on habitat connectivity structures for wildlife (TRB 2004).
Some amelioration measures are expensive, making it difficult to decide what actions are appropriate and what level of investment is justified. There are definitive standards for some mitigation, such as placement of noise walls. For many impacts, however, there is discretion and flexibility in deciding how to mitigate. Competing interests, costs, and operational issues must be balanced with environmental needs, so it is important to know what scale of enhancement will provide a meaningful ecological benefit. These decisions become more difficult when basic
information is lacking, such as preconstruction criteria needed to evaluate the performance of wildlife-crossing structures.
Mitigation projects should have a priori criteria or performance standards that are agreed upon by all responsible for supervising the implementation and functioning of the mitigation measures (NRC 2001). The standards can be designed with some flexibility (or ranking) in terms of goal attainment and target dates and later be refined and updated if required. A small but growing body of information on methods to measure mitigation effectiveness is becoming available as more mitigation projects are undertaken.
Determining how well amelioration measures perform requires long-term study (NRC 2001). Research needs to be an integral part of a highway mitigation project, even long after the measures have been in place. Mitigation is costly, requiring an important investment of public funds. Transportation agencies and biologists responsible for designing approaches to ameliorate highway effects are hampered by the lack of information on the performance of the measures because few post-construction performance studies have been carried out (Romin and Bissonette 1996). Such studies would provide useful information for future decisions.
Long-term research in Banff National Park, Alberta, has been successful in providing valuable information about performance of a variety of measures (including fencing, warning signs, and reflectors) designed to reduce the effects of the Trans-Canada Highway on wildlife populations (Clevenger et al. 2002b). More than 6 years of research and monitoring of 24 wildlife passages, landscape changes around them, and wildlife populations that the passages are designed to sustain have shown the importance of long-term study. A Transportation Research Board report cited the highway mitigation research in Banff National Park as having worldwide importance and as being one of the most successful research projects for wildlife connectivity (Evink 2002).
Data from Banff suggest that the annual patterns and trends of wildlife use of the passages provide strong evidence that there is a learning curve or adaptation period for all wildlife species regardless of passage type (overpass or underpass). Small sampling time periods, typical of 1-or 2-year monitoring programs, are too brief, can provide spurious results, and do not adequately sample the range of variability in wildlife-crossing-use patterns in landscapes with complex wildlife and human land-use interactions. For wildlife passages to be effective over the long term, they will have to be able to accommodate the fluctuations in species, their demographics, and variances in animal behavior while main-
taining viable populations around them. Continuous long-term monitoring of wildlife-crossing structures and wildlife populations is critical for assessing the conservation value of mitigation passages for wildlife.
Wildlife crossings are expensive measures, but the gap in devising cost effective designs and decision-support tools based on ecological and engineering criteria leaves no alternative. Roads do not affect wildlife populations equally (Forman et al. 2003). Road mortality has a more immediate impact on population viability compared with barrier effects on genetic isolation. Thus, different levels of effort are required to assess the ecological benefits of wildlife crossings in regard to restored movement patterns, genetic interchange, or road-kill reduction.
Wetlands are ecosystems that must be specifically addressed under the Clean Water Act (see Chapter 5). A review by the NRC (2001) reported on how effective the “no net loss” rule has been for compensatory mitigation of wetlands permitted under Section 404 of the Clean Water Act and how it may be useful for mitigation of transportation project effects on wetlands. The report stated that wetlands processes should be evaluated in the context of the watershed or region within which a wetland exists to maintain wetland diversity, connectivity, and appropriate proportions of upland and wetland systems needed to enhance the long-term stability of the wetland and riparian systems. The report suggested avoiding such wetland types as bogs or fens that are difficult to restore and paying special attention to riparian wetlands. The report recommended that self-sustaining wetlands be the goal of mitigation or restoration. The report’s operational guidelines for self-sustaining wetlands are listed in Box 4-2.
OPPORTUNITIES FOR ENVIRONMENTAL STEWARDSHIP
Although mitigation of environmental concerns is being addressed at a variety of scales during the planning and construction phases of road development, a tremendous opportunity exists for transportation agencies to become better environmental stewards through ongoing maintenance and operations activities. These opportunities focus on managing vegetation in road corridors and rights-of-way, on watershed management, and on improving other targeted ecosystem processes.
BOX 4-2 Operational Guidelines for Creating or Restoring Self-Sustaining Wetlands
Roadside Maintenance and Management
Twelve million acres of land are in public rights-of-way, an area larger than many states (White and Ernst 2003). Both directly and indirectly, these roadside areas are habitats for nonnative invasive species and provide corridors for the expansion of nonnative species. The social values associated with roadside vegetation have changed over time. Initial focus for roadsides was on “beautification” projects or bank stabilization and led to plantings of exotic grasses and plants, such as kudzu (see Chapter 3). Beginning in the 1970s, these aesthetic goals were supplanted by more ecological goals, although the two are not mutually exclusive. Roadside vegetation management seems to be moving in the direction of native species’ restoration and maintenance (Figure 4-7). The state of Iowa’s roadside management program is an example where transportation agencies are partnering with other groups to achieve goals of restoring large areas of prairie ecosystem (White and Ernst 2003). The committee heard reports from the New York Department of Transportation of similar opportunities for roadside ecosystem restoration carried out as part of normal maintenance operations. The Washington Department of Transportation has a soil bioengineering program that uses native plants and seeds, which are well adapted to local climate and soil
conditions, to provide erosion control, slope and stream bank stabilization, wildlife habitat, and other benefits. These projects usually require less heavy machinery and can be installed when the site problem is small and during slow construction periods, thereby costing less and causing fewer impacts. The development of training manuals and the sharing of experiences, data, and information would help to build a body of knowledge on ways to achieve these ecological goals. Maintenance operations
Improving Specific Ecosystem Processes
The committee also uncovered numerous examples of small-scale projects that produced environmental benefits. These ranged from sensitive-species mapping and planting, stream habitat improvement, construction of stormwater treatment facilities, and improved aquatic habitat. Other small-scale projects focused on endangered species recovery and restoration, such as riparian and stream restoration for salmon in the Pacific Northwest and fencing for protection of the desert gopher tortoise. Other projects included attempts at decreasing habitat fragmentation by construction of multifunctional crossings, widening of bridges, and improving wildlife connectivity. These projects integrate a variety of objectives, such as improving fish passages and stormwater and noise management, in environmental retrofit programs and can lead to broader-scale, such as watershed scale, improvement of ecosystem functioning (Figure 4-7c,d).
This chapter has presented a set of existing and potential opportunities for mitigating the ecological effects associated with three major phases of road projects: planning, construction, and maintenance. At the national scale, the types of conclusions made in Chapters 3, 5, and 6 are all germane. Environmental issues should be addressed earlier in the planning and design phases. Interdisciplinary teams could be developed to share data, understanding, and expertise. At the medium and fine scales, many designs, activities, and projects have been done to mitigate effects. However, information, studies, and databases on the long-term functionality of these mitigation efforts are sparse. Long-term studies and a greater set of analytical tools are needed to help inform decision makers about the implementation of mitigation efforts, such as wildlife habitat connectivity structures. These tools could help to assess appropriate locations and design measures for these efforts. Many other opportunities exist for improving environmental conditions in road project operation and maintenance. Projects have addressed improving water quality, aquatic habitat, habitat for species of concern, control of nonnative plants and animals, and reestablishment of habitat connectivity.