Wildlife & Traffic
A European Handbook for Identifying Conflicts and Designing Solutions
7.2 Reducing the barrier effect: overpasses
Overpasses include all fauna passages that cross roads or railway lines above the level of the traffic. In general, the term 'wildlife overpass' refers to connections at the population/meta-population level and 'landscape bridge' refers to connections at the landscape/ecosystem level. Even though there is a continuum between these two extremes, the different terminology helps to distinguish between the different levels.
The main difference between the two types of overpasses is the width, because the larger an overpass the more functions it encompasses. The distinction between the two types is artificial and is defined by a recommended width. Deviations above or below the recommended width should be justified.
Landscape bridges and wildlife overpasses can be designed in various ways, for example as cut-and-cover tunnels or by adapting the design of bridges for traffic. Bored tunnels often have the same function as landscape bridges. They avoid habitat fragmentation by keeping the natural habitat intact. They are therefore not dealt with in this chapter (see Chapter 6).
7.2.1 Wildlife overpasses and landscape bridges
General description and targets
Wildlife overpasses and landscape bridges are purpose-built bridges, usually built over a road with several lanes and/or high-density and fast-driving traffic, over high-speed railway lines or over a combination of both. They are a costly but effective means for minimising, at least locally, the fragmentation effect of transport infrastructure for all terrestrial groups of animals. Several techniques have already been applied and these are described below.
Width, design and vegetation depend largely on the target species, which are usually ungulates or smaller mammals although invertebrates, reptiles and amphibians are also possible target species. For small animals, a bridge must be wide enough at its narrowest point to function as a habitat corridor. For larger mammals, the width and location of an overpass are more critical than the design details, substrate or vegetation. Overpasses have also been shown to act as guiding lines for birds, bats and butterflies, both enhancing the movements of flying animals that may be reluctant to cross open surfaces and reducing mortality.
However, costly constructions like overpasses should not be built just for one or two target species. In most cases the aim should be to connect habitats at an ecosystem level. This requires the simulation of the habitats on either side of the infrastructure on the overpass, taking into account vegetation and environmental factors such as soil type, humidity, temperature and light. This means, for example, that the connection between forests requires at least elements of similar forest habitat on the overpass.
The location of overpasses must be oriented to the occurrence and behaviour of the target species (see also 7.1.5)
- For large mammals, an overpass should be located along paths traditionally used by them. The paths can be determined with the help of fieldwork, e.g. mapping tracks in the snow or on marble dust, night censuses using spotlights, census of road kills, or by asking locals using specific questionnaires.
- Avoid areas where human activity causes disturbance.
- Avoid sections with large differences in level or embankments.
- Choose the location in relation to other crossing possibilities for animals.
- Where target species rely on a particular habitat type, the overpass and habitat must both be within reach of the animals.
The width of an overpass is given here from the perspective of the user of the overpass. Road constructors usually call this the length, i.e. the stretch of the road/railway line that is covered by the overpass.
- In general, larger mammals require wider overpasses than small vertebrates. On the other hand, small vertebrates and invertebrates rely more on the provision of special habitat features, which can only be provided on relatively wide passages.
- A standard width of 40-50 m (between the fences) is recommended. This width can be lowered to a minimum of 20 m if the aim is only to provide a movement corridor for not very sensitive species such as roe deer, or where the topography has a channelling effect leading the animals directly onto the crossing.
- A width below 20 m is not recommended. Experience with mammals has shown that individuals used to the local situation may use narrower overpasses, but frequency of use is generally lower than on wider overpasses. It is also not known how inexperienced animals react to narrower overpasses, e.g. young individuals during dispersal. In some cases, funnel-shaped overpasses with a minimum width below 20 m but a width at the entrance of c. 40 m have been shown to be used, for example by roe deer.
- The required width increases with the length of the overpass, i.e. an overpass across a six-lane motorway has to be wider than one over a two-rail high-speed railway line. The minimum width to length ratio should be greater than 0.8.
The recommended width for landscape bridges is >80 m. This enables the establishment of different habitats to provide a connection at the ecosystem level (Figure 7.4). The optimum width depends on the diversity and conservation importance of the habitats that have to be connected. In areas of high importance a landscape bridge may need to be several hundred metres wide to preserve the connectivity of the landscape.
- The aim is to guide the target species and a variety of other animals across the overpass.
- The vegetation on the overpass should reflect the habitats situated on either side of the infrastructure.
- Only plant species native to the local area should be used.
- Sowing grass/herb vegetation is not always necessary. Spontaneous establishment can lead to good results.
- An alternative to using expensive seed mixtures may be the transfer of seed bank material (hay, topsoil) from areas adjacent to the overpass.
- Hedge-like structures across the bridge provide a guiding line, cover and protection from light and noise from the road, especially for larger mammal species.
- Where small vertebrates and invertebrates are concerned, the vegetation is designed to resemble as much as possible that adjacent to the bridge, forming a suitable habitat corridor.
- Plant species which are preferred food sources can be used to attract herbivores to the overpass.
- Roots of trees can create maintenance problems on an overpass. The choice of suitable tree species should take maintenance and traffic safety into account.
- Soil is a prerequisite for vegetation and depth depends on the habitat types.
- Recommended topsoil depths: Grass/herbs: 0.3 m Bushes/shrubs: 0.6 m Trees: 1.5 m
- Topsoil or special mixtures can be used.
- Depending on the type of vegetation to be favoured, soil depth can be varied, giving a varied micro-relief and lowering costs.
Screening aims to reduce the disturbance of animals by light or noise. Artificial screens are more important on relatively narrow overpasses. On overpasses of 50 m upwards, hedges on either side, preferably on small mounds, are sufficient.
- The height of side screens should reach about 2 m. In that case, no fences are needed on the overpass.
- On overpasses <20 m wide (recommended only in special situations, see paragraph on width) high screens should be avoided as they may create a negative tunnel effect for animals.
- Screens are probably more important in areas where the only light emissions are from the infrastructure the overpass crosses than in areas where there are other emissions in the vicinity.
- To maximise the width that can be used by the animals, the screens are better placed at the outer edge of the construction.
- Screens have to be properly connected to provisions like noise screens along the road.
- Earth mounds at the outer edge of the overpass and extending along the transport infrastructure make good screens. They are particularly suitable for wide overpasses and landscape bridges. • Dense hedges used as screens are best placed on a low earth mound.
Fences are needed to guide animals to an appropriate fauna passage. Design and specifications are given in detail in Section 7.4.1.
- Fences are essential on the outer edge of an overpass if no screens are constructed.
- When screens are built as solid walls, a fence is not necessary.
- Fences on the overpass need a tight connection to fences alongside the infrastructure.
There are many construction types available. The choice depends mainly on topography, subsoil stability, cost, aesthetics and local design traditions. The following examples are chosen to provide ideas to the construction engineer. They are not intended to provide all the technical details, but to highlight features which are important to ensure the effectiveness for wildlife.
Construction principles relevant for wildlife:
- Leading the infrastructure through a natural or artificial cutting allows an overpass to be built on the level of the adjacent land. See also Chapter 6.
- Where the level of the overpass is higher than that of the adjacent land, the access ramps should not be too steep and should be well embedded in the adjacent landscape. So far there is little knowledge on the maximum gradient tolerated by different animals. In hilly areas steeper gradients may be more acceptable than in flat regions. Some existing overpasses that are used by animals have gradients from 16% in a flat landscape (Hungary) to 25% or more in mountainous regions.
- Shape and materials should ensure that the necessary features (soil cover, vegetation) and the connection to the adjacent land can be achieved. • On existing roads the use of prefabricated arches reduces construction time at the site.
New design alternatives
Even though the costs of a wildlife overpass usually make up only a small part of the total cost of a road or railway development project, they belong to the more expensive nature conservation measures in a planning scheme. The development of alternative less expensive designs should therefore be encouraged. Some ideas for new designs are presented here.
Points for special attention
- Overpasses are meant to be in use for a long time. Engineering works are developed for a period of 50 to 100 or more years. Safeguarding a corridor which allows access to the overpass has to follow a similar time frame and should be part of spatial planning at local and regional scales. A proper maintenance plan should be developed.
- In particular, no development (housing, local roads, industrial areas) should be permitted that reduces the functioning of the overpass.
- Hunting should be forbidden on the overpass and in its surroundings. There is little experience of the size of the nohunting zone required, but a distance of 0.5 to 2 km may be appropriate depending on the local situation.
- Specific overpasses (for the exclusive use by wildlife), are recommended as a general rule and especially if important daily or seasonal migrations of larger mammals have to be restored.
- The use of an overpass by vehicles or walkers has to be planned carefully. Jointuse overpasses are discussed in Section 7.2.2.
- Where access by walkers is foreseen, it is preferable to provide a narrow path, which concentrates their movements, than to provide no path which can lead to them using the whole width of the passage.
- Extra shelter at the overpass can be important for a wide variety of species. When it takes time to establish tall vegetation on the overpass, tree-stumps, a heap of branches and stones can provide shelter.
- Sand beds created to monitor tracks of animals leave a gap in the continuity of the vegetation and may pose an obstacle for invertebrates. They should only be left for a limited period of time while monitoring takes place.
- Roads and forestry tracks which run parallel to the infrastructure may obstruct access to the overpass. They should be routed so as not to block access for small animals, in particular invertebrates.
- The responsibility for maintenance has to be organised during the planning phase. Where maintenance is handed over to persons or organisations that were not involved in the planning process (e.g. farmers, foresters, nature conservation organisations), a close collaboration with the people responsible for road maintenance has to be ensured.
- The people responsible for maintenance have to be properly instructed. They have to be aware of the purpose of the overpass and a maintenance procedure has to be developed together with them.
- Maintenance procedures in the first two to three years should be planned during the construction phase of the overpass. Later it should be decided on an annual basis depending on the results of the inspection.
- Regular inspection of the structure and seal and drainage system is essential and should be part of ordinary infrastructure maintenance procedures which should also dictate the frequency of inspection.
- Vegetation should be maintained in accordance with the original targets of the overpass.
- Care should be taken that maintenance of vegetation does not damage the technical functioning of the bridge.
- Particular attention has to be paid to any misuse of the overpass and its surroundings that may hinder its functioning as a wildlife passage (e.g. fences on adjacent land, recreational installations, etc.).
7.2.2 Modified bridges over infrastructure: multifunctional overpasses
General description and targets
There are large numbers of bridges for local roads, forestry or agricultural tracks. They are usually covered with concrete, asphalt or tarmac and are hardly used by animals. With the simple addition of an earth-covered strip an improvement can be achieved. Such earthcovered or vegetated strips are used by invertebrates, small vertebrates, carnivores and occasionally by ungulates. They favour the dispersal of animals. They are no alternative for specific wildlife overpasses, but an additional measure to improve the general permeability of infrastructure barriers. If all local bridges outside built-up areas were equipped with an earth-covered strip, this would contribute to a mitigation of the barrier effect at little additional cost. Wider overpasses can be combined with local roads or forestry tracks as long as traffic intensity is low.
Cut-and-cover tunnels which are constructed for example for aesthetic reasons to preserve the original aspect of the landscape (see Chapter 6) can often be adapted to function as wildlife passages at the same time.
Road bridges with vegetated strip
- For the vegetated strip a minimum width of 1 m is recommended.
- Soil cover does not have to be deep (0.3 m).
- In most cases spontaneous vegetation is sufficient and no seeding is required.
- The road surface on lightly-used bridges should not be tarmacked.
- The modification of bridges with strips is recommended only when traffic intensity on the bridge is low.
- Roads, cycle paths and forestry tracks, etc. should only be combined with a wildlife overpass if traffic intensity is low.
- The width of any road on an overpass has to be added to the width required for the fauna passage, i.e. joint-use passages in general have to be wider than specific overpasses.
- Any paths or forestry tracks should be placed towards one of the outer edges of the overpass to ensure a maximum width of vegetated and undisturbed area (Figure 7.31).
- Access for the animals onto the overpass must not be hindered by roads at the entrance to the overpass (see also Figure 7.29).
- On landscape bridges, a lateral road that is likely to be the source of disturbance may be separated from the vegetated part of the overpass by an earth wall. Where a lateral road is used very lightly separation is not necessary.
7.2.3 Tree-top overpasses
General description and targets
For climbing mammals special types of passages may be needed. Squirrels or pine and stone martens readily cross roads and railway lines and fences are not obstacles. Where traffic is heavy this may result in high traffic mortality. Edible and garden dormouse on the other hand rarely descend to the ground and prefer to cross roads at points where the branches of trees get close to each other.
Wildlife overpasses will be readily used by squirrels and martens, whereas they may only be suitable for dormice when there is adequate tree cover. However, passages designed or adapted to allow climbing animals to cross the infrastructure above the traffic may be a good alternative to reduce the number of traffic victims. In a few countries these treetop overpasses have recently been constructed or are planned. So far there has been limited research and clear recommendations cannot yet be given. The first indications are, however, that these passages are indeed used by squirrels and dormice and in other parts of the world by monkeys or possums.
Tree-top overpasses should be considered:
- In wooded areas with important populations of dormice, red squirrels and pine martens.
- Where traffic mortality of target species is concentrated.
- In large parks in towns and cities where traffic mortality of squirrels is high.
- Tight enough for animals to walk on.
- Safe from predators.
- Places for small animals to hide.
- Good connections to trees and bushes on either side of the infrastructure.
- Safe in relation to road users.
The design of the tree-top overpasses depends on the type of road. On minor local roads the crowns of trees are often close enough together to enable climbing animals to move from tree to tree. When the distance is too big, a rope, rope ladder or other walkway can provide a connection. On wider roads and in other situations where the distance between tree crowns is too big, the connection needs more stability. Ropes and also constructions of steel cables with a small pathway in between have been implemented. These provisions have to be wide enough for animals to walk on.
- Squirrels will use ropes with a diameter of 4-10 cm.
- Rope ladders with a width of 30 cm have been installed in some locations.
- Walkways of two steel cables with a net between (20-30 cm) have also been implemented.
- Planting of trees and shrubs and additional ropes and planks can facilitate access to the overpasses for the animals.
- On broad motorways the installations for traffic signs over the road can be adapted with a wooden walkway, shelters and hiding places.
Point for special attention
- Protection from predators is an important accompanying measure. On an open rope or walkway an additional thin rope above the passage can prevent attacks by birds of prey.