Wildlife & Traffic

A European Handbook for Identifying Conflicts and Designing Solutions

7 Fauna Passages and other Technical Solutions
Original version (2003) - New version will be available by the beginning of 2022 -
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7.1 General approach

7.1.1 How to use this chapter

Description of measures

This chapter describes individual technical measures designed to mitigate the negative effects of transport infrastructure (see Chapter 3). For each measure a general description is given followed by important information on design and points for special attention. Technical specifications for materials and technical design details are presented if they are of particular importance to ensure the functionality of the measure. However, giving exact design instructions to the engineer would be beyond the scope of this handbook, which is intended for use throughout Europe. In several countries handbooks have already been produced, which deal with some of the issues presented here and which often provide more detailed information. In addition, technical design handbooks may be helpful to find appropriate solutions for construction. A list of national and regional handbooks can be found in Annex 5.

State of knowledge

Some measures have been well tested and considerable knowledge has been accumulated. Others are new and are still being developed and tested. The amount of information presented for each measure reflects this disparity, but best practice according to current knowledge and experience is presented. This means that some recommendations may be different from those in existing handbooks, especially the earlier ones. In some cases, recommendations in a particular country may differ from those presented here because they take into account regional issues such as a specific climate or habitat.

Measures that are not recommended

Some measures that are still widely used have been shown not be effective. Such measures are mentioned in the text, but no design details are given, since their use is not recommended in future schemes.

Relation to other chapters

This chapter should not be read in isolation. Minimising fragmentation starts with general infrastructure planning and avoidance is the first priority. Specific mitigation measures have to be viewed as small parts of an integrated solution. Before looking at the detailed description of a measure, the reader should look at Chapters 4, 5 and 6. Sections 7.1.2 to 7.1.8 provide more information on the relationship between different measures and how to choose the most appropriate ones. In these sections, the emphasis lies on fauna passages as they are the measures most specifically designed to mitigate habitat fragmentation. 

Structure of the chapter

For clarity, different measures are described separately, but often a combination of measures is required. The chapter starts with fauna passages designed specifically for wildlife. Adaptations of infrastructure elements to enhance their use by animals are also described here. Fences and other measures that mainly aim at reducing the numbers of animals killed are then described. In this section, some related measures to protect animals alongside roads or railway lines are included as well as measures specific to artificial waterways.

7.1.2 Types of measures and their primary function

Providing links versus reducing mortality

Measures to protect wildlife along transport infrastructure and to reduce habitat fragmentation can be divided into two groups

  • Measures that directly reduce fragmentation by providing links between habitats severed by the infrastructure, e.g. wildlife crossing structures or fauna passages (overpasses, underpasses etc.).
  • Measures that aim to improve road safety and reduce the impact of traffic on animal populations by reducing traffic-related mortality.

In practice this distinction is often blurred. Measures may fulfil both functions but can also have an associated negative impact. For example, fences are a good means of reducing the number of collisions between large mammals and cars, but at the same time they increase habitat fragmentation. Thus, fences can be regarded as a mitigation measure for fragmentation only in combination with fauna passages that compensate for their negative barrier effect. As a further example, well designed underpasses for otters both link the habitats on either side and reduce the numbers of animals killed on the road or railway line.

Measures designed to reduce animal mortality also include the adaptation of engineering structures that may be fatal traps, particularly for small animals, e.g. drains, channels and gullies alongside roads.

Disturbance from roads or railway lines can significantly contribute to the effects of habitat fragmentation on wildlife (see Chapter 3). Measures to reduce disturbance are not dealt with here in detail, but they should be considered together with other measures to reduce traffic emissions (noise, light and chemical pollution).

Specific measures versus modified structures

A further distinction can be made regarding the objective of particular engineering measures. Fauna passages may be designed specifically for animals with human access prohibited. On the other hand, bridges, culverts or other structures built for people can be modified to increase the permeability of the infrastructure for animals. Again, there is no clear distinction between the two groups. A dedicated wildlife overpass, for example, can be combined with a forestry track where foresters occasionally need to cross the infrastructure. Modifying engineering works is often the most appropriate way to reduce the barrier effect of existing roads and railway lines. Many such adaptations are not costly but can significantly increase the permeability of the infrastructure.

Figure 7.1.2 - Types of measures to mitigate habitat fragmentation

7.1.3 Fauna passages as part of a general landscape permeability concept

Fauna passages and other structures adapted to increase the crossing of transport infrastructure by animals should never be considered in isolation. They should be part of a general 'permeability concept' to maintain connectivity within and between populations of animals. This concept emphasises connectivity between habitats at least at a regional scale and considers not only the transport infrastructure but also the distribution of habitats and other potential barriers such as built-up areas. Fauna passages can then be regarded as small but important elements used to connect habitats by enhancing the movements of animals across transport infrastructure.

At a more specific level, a permeability concept can be produced for a particular road or railway project. All connecting elements, such as tunnels, viaducts or elevated roads, stream and river crossings, culverts, and passages designed specially for animals should be integrated into the permeability concept. Again, the primary objective must be to maintain the permeability of the transport infrastructure for wildlife to ensure the connectivity of the habitats at a larger scale.

Mitigation measures, in particular fauna passages, are necessary if transport infrastructure bisects important patches of habitat, creates a barrier to migration routes and if avoidance by altering the route is impossible (see Chapters 3 and 4). Fauna passages are necessary for animals where:

  • A road or railway line results in significant damage or loss of special habitats, communities or species.
  • A road or railway line affects species particularly sensitive to barriers and traffic mortality.
  • The general permeability of the landscape, i.e. the connectivity between habitats in the wider countryside, is significantly impaired by the infrastructure development. • Fauna passages are considered to be a suitable solution for mitigating the barrier effect in the specific context. • Other less costly measures are unlikely to be effective.
  • The road or railway line is fenced along its length.

7.1.4 Choice of appropriate measures

Fauna passages and modifications to infrastructure that enhance the possibility of safe animal movements are the most important measures for mitigating habitat fragmentation at the level of a particular infrastructure. Many principles regarding, for example, the location or number of passages are the same for different types of passage. The following sections deal with these more general aspects.

Types of passages

The selection of the most appropriate type of fauna passage requires consideration of the landscape, habitats affected and target species. The importance of the habitats and species should be evaluated at a local, regional, national and international scale as part of an EIA (see Chapter 5). In general, the more important habitat connectivity is to the target species, the more elaborate the mitigation measures have to be (see Figure 7.2). Thus, where an internationally important corridor for the movements of large mammals is cut by an infrastructure development and this cannot be avoided, a large landscape bridge may be the only measure which can help maintain functional connectivity. In contrast, a small culvert may be sufficient to maintain a migration corridor for a locally important population of amphibians. In practice, however, there is rarely just one measure required to effectively mitigate habitat fragmentation. Instead, a package of integrated measures is required that address problems at specific sites and for the infrastructure as a whole. A combination of diverse measures suitable for different groups of animals will often be the best solution.

Overpasses versus underpasses

There are few general rules regarding whether one is more suitable than the other. The choice is partly determined by the topography. In hilly terrain it is often easy to construct either over- or underpasses, whereas in flat landscapes underpasses may be easier to construct, if the ground water level is not too high. Examples of over- and underpasses in different topographical situations are given in the respective sections. Overpasses have the advantage that it is easier to provide different microhabitats, because vegetation grows more easily than in underpasses. A wider range of species may therefore use them. On the other hand, conditions on overpasses are usually dry, and underpasses therefore seem to be better suited to animals requiring wet or humid conditions. The choice thus also depends on the adjacent habitats that are being connected. Monitoring has shown that, where overpasses and underpasses are close to each other, moose and deer prefer to use overpasses. For burrowing animals, the opposite may be true.

Figure 7.1.4 - The choice of different types of fauna passages depends also on the importance of an area or corridor.

Target species

Any species native to the region can be a target species for fauna passages. Non-native species should not be target species for fauna passages, as they are not part of the natural ecosystem and their spread should not be encouraged. In practice, the expense of building fauna passages will mean that priority will be given to locally or regionally important species threatened by the infrastructure development. Identifying target species is an important step in the planning process where the location and design of fauna passages is, to a large extent, determined by the location and movement patterns of target species (see Section 5.3). Identifying target species is also important as a basis for the planning of monitoring procedures to evaluate the success of a measure (see Section 7.1.8 and Chapter 9).

Even if target species are important in deciding if and where fauna passages are necessary, the design of passages should not just consider one single target species. For example, an overpass across a motorway that is built to preserve a migration route for red deer should also form a habitat connection for populations of invertebrates (e.g. insects) or small vertebrates (e.g. lizards or mice). Nevertheless, experience has shown that some designs are better suited for particular species than others. Table 7.1 gives some indications that outline the appropriate type of passage for particular species or groups of species.

Figure 7.3 - The Iberian lynx is threatened by extinction. Providing safe connections across motorways is an essential part of conservation action for this species, for which very large home ranges are typical. (Photo by Ministerio de Medio Ambiente, Spain)

Figure 7.5 - Small vertebrates such as this tortoise, other reptiles and small mammals can be target species for fauna passages as well as non-flying insects and other invertebrates, which also need connections between their habitats. (Photo by H. Bekker)

Figure 7.4 - Collisions between moose and vehicles can have fatal consequences. Moose and other large mammals are therefore often target species for fauna passages, even when their populations are not threatened. (Photo by V. Hlaváˇc)

Figure 7.6 - Common toads suffer from high mortality when they have to cross roads to reach their breeding sites. They are therefore often target species for specific amphibian passages. (Photo by N. Zbinden)

7.1.5 Density and location of fauna passages

Figure 7.7 - The three-lane highway B31n in Southern Germany cuts off an area along Lake Constance. A high density of passages prevents the habitats from becoming isolated. Passages include wide wildlife overpasses as well as narrower ones combined with agricultural and forestry tracks. (Photo by V. Keller)

Density of passages

The density of fauna passages required to effectively maintain habitat connectivity is a major decision in planning mitigation measures. Deciding on the number and the type of measures required will depend on the target species and the distribution of the habitat types in the area. In some cases one or more wide passages will be appropriate whereas other problems will be better tackled by a larger number of smaller-scale measures. An additional argument for constructing several passages is to spread the risk in case a passage is not used as predicted.

In order to determine the number of passages required, the behaviour of target species can be used as a guiding factor. The catchment area of fauna passages, (i.e. the area where animals come from) is limited even for mobile species. For most invertebrates, if there are habitat corridors leading to the bridge, the catchment area is at most 200-300 m. For larger animals, individual home ranges and social interaction between individuals limit the range from which animals will be able to use a passage.

When determining the frequency of passages, all opportunities for animals to cross an infrastructure have to be considered, including the ones that may already be available, e.g. due to a road being led through a tunnel.

In general, the density of passages should be higher in natural areas, e.g. forests, wetlands and in areas with traditional agriculture, than in densely built-up or intensively-used agricultural areas. However, in areas where there are many artificial barriers due to transport infrastructure or built-up areas, fauna passages can be essential for maintaining the general permeability of the landscape. In such cases, solutions can be integrated with all remaining open corridors.

The density of passages required in relation to environmental goals has been poorly studied and more research is needed.

Example: Recommendations for the density of fauna passages for mammals in the Czech Republic.

The landscape in many parts of the Czech Republic is relatively unfragmented by infrastructure, and a patchwork of forests and agricultural areas offers good habitat for many mammal species. The motorway network will be enlarged in the coming years. In order to preserve landscape connectivity for mammals, the following steps have been taken:

  1. Actual and potential distribution and movement corridors of large and medium-sized mammals were mapped.
  2. Based on these data the overall importance of the different regions for mammals was classified.
  3. The use of different types of passages and the behaviour of mammals in the neighbourhood of the existing motorways was investigated.
  4. Based on these results, recommendations were formulated on the density of passages.


Figure 7.8 - Categorisation of the territory of the Czech Republic according to importance for mammals.

Location of passages

The location of passages has to be decided on the basis of sound knowledge regarding animal movements and the distribution of important habitats. Where clearly defined animal trails exist, passages should be placed as close to them as possible. Often topography and landscape structure can help to identify likely migration routes such as valley bottoms, streams, hedgerows, and continuous woodland. Where the principal aim of a passage is to link particular types of habitat, the passage has to ensure connectivity to suitable habitat on either side of the planned infrastructure. Other barriers existing in the surrounding landscape have to be considered when locating passages. Access to the passage must be guaranteed in the future.

Ensuring that passages are built at all known 'conflict points' (see Chapter 5) must be the first step in defining the location of passages. If this results in a density of passages considered too low to create the necessary level of permeability of the infrastructure in the particular region, additional locations should be identified.

Integration into the surroundings

Fauna passages should be well connected to their surroundings, either by way of habitat corridors leading towards passages for small animals or with guiding lines for larger animals. The probability of an animal encountering a fauna passage can be improved considerably with guiding structures. Barriers that prevent or hinder animals from reaching passages need to be removed or mitigated. Where other infrastructure elements occur in the vicinity, an integrated approach to defragmentation is required.

Figure 7.9 - The acceptance of fauna passages by animals depends on good guidance to the entrance. Linear (man-made) structures providing shelter improve the guidance. Some examples of guiding structures (from top left to bottom right): hedgerow, row of trees, cattle fence, ditch, heaps of stones, stone wall, small stream (after Oord 1995).

7.1.6 Adapting engineering works for use by animals

Engineering works are designed and constructed for crossings of two different flows. These can be two flows of traffic (e.g. one road crossing the other with an overpass), water and traffic (e.g. a culvert leading water under a road or an aqueduct leading water over it) and, more recently, traffic and fauna. Road bridges or culverts are mostly not used by animals to cross a road or railway line, because they don't fulfil the requirements for more demanding species. However, if the demands of animals are taken into account, these existing structures can often be adapted to serve as fauna passages. These passages, which combine the flows of fauna and traffic or fauna and water, are called joint-use passages.

For viaducts and other large structures, often little adaptation is needed for the structures to be a genuine alternative to specific fauna passages. At important sites, however, modified over- or underpasses are usually no alternative to specific fauna passages. Nevertheless, modified structures can help to increase the permeability of infrastructure at little additional cost.

Existing guidelines for the design of roads, over- and underpasses and culverts, etc. mainly focus on drainage, traffic safety and related issues. In many cases, provisions for wildlife at such structures can be implemented without compromising safety aspects. The planning of these structures has to be undertaken jointly by wildlife experts and engineers.

Integrating wildlife requirements in the planning phase of infrastructure development is the best and easiest way to develop costeffective solutions. Nevertheless, many modifications can also be carried out at existing sites (see Section 7.1.7).

Many design principles relating to specific fauna passages are also applicable for modified and joint-use passages. Some general considerations may however improve joint use by animals and people:

  • Both the ecological and engineering requirements have to be known and possible conflicts identified.
  • Larger dimensions facilitate joint use.
  • As far as possible the flow of human activities (traffic, pedestrians) and the flow of animals have to be separated.
  • Providing shelter for animals may reduce disturbance and increase levels of use by animals.
  • Lowering the amount of traffic permanently or at certain times (e.g. at night) may increase the use by animals.

7.1.7 Solving problems on existing roads and railway lines

In Europe, thousands of kilometres of motorway, other road and railway line have been built before people became aware of the potential problems they caused for wildlife. An obvious need for adapting existing structures arises when there is a high number of collisions between animals and vehicles. High levels of animal mortality, and the need to re-establish movement corridors may require measures to be taken while a road or railway line is in use.

When planning adaptive measures for existing infrastructure the general principles discussed in this handbook should be considered, not just the particular local situation. This is particularly the case when fences are installed to reduce the number of collisions between vehicles and animals. Fences will increase the barrier effect and should never be installed without accompanying measures (see also Section 7.4.1). Most measures described in Chapter 7 are also suitable for existing infrastructure or may be adapted accordingly (see in particular Section 7.1.6).

The principles for dealing with existing infrastructure can be summarised as follows:

  • Construction of new engineering works (passages, etc.) above or below existing roads may give the best results but is often more expensive.
  • Adaptation of existing engineering works that have been designed for other purposes (e.g. water, forestry) is often not an optimal solution, but is in general less expensive. A large number of adapted passages may, in some cases, give better results than constructing one new specific passage for the same price.
  • Modification of maintenance procedures (e.g. treatment of vegetation) may improve the situation

7.1.8 Maintenance and monitoring of mitigation measures

All mitigation measures have to be routinely inspected and maintained to ensure their longterm functionality. Maintenance considerations, including cost, have to be considered at the earliest possible stage, i.e. when a measure is designed. Planning should define the type and frequency of maintenance procedures and the organisation of and responsibility for maintenance. In most cases maintenance will be carried out by road maintenance teams, but giving a mandate to nature conservation organisations and farmers, etc., has proven to be a good alternative for certain types of measures. Specific maintenance aspects are dealt with in the sections on the different measures.

The maintenance and monitoring of measures are closely linked. Monitoring procedures are mainly designed to check whether a measure fulfils its purpose, but at the same time they can identify maintenance deficits and needs. Monitoring requires clear definition of the objectives of the measures, and programmes should be planned in parallel with the design of the measures themselves. Monitoring procedures are dealt with in Chapter 9.