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 -
Print Friendly, PDF & Email

7.4 Avoiding and reducing animal mortality

Collisions between large mammals and cars are the effect of traffic on wildlife most obvious to the road user. However, many smaller animals are killed on roads as well. In addition, features connected with the transport infrastructure may cause the death of many animals too: birds of prey collide with overhead wires on railway lines, small mammals get caught in drains, insects die when attracted to the light of street lamps, mammals can't get out of canals with steep banks etc. This chapter presents a variety of measures designed to reduce the number of animals killed on or around transport infrastructure. It is far from complete, but aims also at drawing attention to the detection and avoidance of potential traps that may cause the unnecessary death of animals.

7.4.1 Fences

General description and targets

Fences are erected to prevent the access of animals onto roads or railway lines. They are mostly constructed to reduce accidents due to collisions between large mammals and cars, but also to reduce the number of smaller animals killed on the roads. The disadvantage of fences is that they increase the barrier effect. Where fences or other barriers are erected, it has to be ensured that the species concerned have enough opportunities to cross the road or railway line. In most cases, fences must therefore be combined with wildlife passages. In these cases they fulfil an important role in guiding animals to the crossing points. When traffic safety is not an issue fences should only be erected where animal mortality might threaten a population otherwise the barrier effect might have worse effects on the survival of the populations in the long term than the mortality due to traffic.

Figure 7.78 - Several types of wildlife fences in Europe: Top: a standard fence in Switzerland (Photo by V. Keller). Centre: A high fence with an extra wire at the top in Norway (Photo by B. Iuell). Bottom: fence with wooden poles in Hungary (Photo by J. Zsidakovits).


  • In general, wildlife fences should be erected only in places where the number of animals killed is high or where there is a high risk of accidents involving wildlife. This is mostly the case along high-speed roads and railway lines. On ordinary roads with low traffic density fences should only be erected at highrisk spots.
  • The surrounding landscape has to be checked with respect to other fences or barriers to animal movement: creating new traps between parallel fences has to be avoided, and the number of fence lines should be reduced wherever possible.
  • Fences should always be built on both sides of a road or railway line. The ends of the fences are danger points: animals may go round the end of the fence and get trapped on the road. Fences should therefore end at structures like bridges. Where only a stretch of the road is fenced in they should be extended 500 m or more beyond the danger area.
  • On roads with relatively little traffic, openings in fences can be provided at locations where animals can easily cross and where crossing animals are well visible to drivers.
  • In areas where natural habitats for animals have been reduced to small patches, any potential habitat should be made available to the animals. From the point of view of the animals, a fence should therefore be placed close to the road to reduce the amount of inaccessible area along the verge, thus allowing animals to use the verge as a habitat or movement corridor. However, the location of the fence in relation to the road has to also take into account aspects of traffic safety and road maintenance.
  • Where a road is built on an embankment with a wide slope it is preferable not to put the fences at the foot of the slope but at the top or halfway up, depending on the local situation. The same applies to cuttings. • Particular attention has to be paid to the placing of fences in relation to fauna passages and other possible crossing points for animals. Fences must not block entrances to passages nor provide traps, but they do have an important function to guide animals towards passages. (See also Sections 7.2.1 and 7.3.2)


Conventional wildlife fences consist of wire mesh fixed with poles. Height and mesh size depend on the target species. In order to be an effective barrier, a fence has to meet the following requirements:

  • The height should be such that animals cannot jump over it.
  • The wire mesh has to prevent animals from passing through the openings.
  • The mesh has to be fixed such that animals cannot pass under the fence. • Electric fences are expensive to run and need frequent checks and maintenance. They are not an option for long stretches of road, but may be considered locally where a high risk exists for endangered species, and can be used temporarily to train animals to change their habits after a new road is built. 


  • The height is determined by the occurrence of different ungulate species: Red deer, fallow deer, moose: minimum height: 2.2 m (preferably 2.6-2.8 m) Roe deer, wild boar: minimum height 1.5m (preferably 1.6-1.8 m).
  • The height has to be adjusted to the terrain and is measured on the side of the approach of the animals (see Figure 7.79). Where the approach of the animals is downhill, this adjustment is essential.
  • In areas with snow cover, the minimum height has to be guaranteed in winter as well.
Figure 7.79 - The minimum height of a fence is measured on the approach side of the animals. 1: If the road is on an embankment, the fence can be lower than in a flat situation (2). 3: If the road is in a cutting, the fence has to be higher than in a flat situation (2). (After Müller & Berthoud 1996)


  • For conventional wildlife fences, a smaller mesh size in the bottom half or third of the fence is recommended. Distance between horizontal wires: bottom: 50-150 mm, top: 150-200 mm. Distance between vertical wires: 150 mm. • Wires should have a diameter of at least 2.5 mm and should consist of rust-free material.
  • In areas with heavy snowfall, the top wire of the netting must be reinforced with a cable capable of bearing the weight of the snow settling on it.
  • The bottom wire should lie directly on the ground and be fixed to prevent animals from crawling under the fence. Burying the wire mesh 20-40 cm under ground may be necessary in areas with badgers or wild boar. Where the ground is uneven, it has to be levelled out to avoid gaps e.g. due to holes in the ground. Special care should be given to places where fences cross ditches.
  • The wire mesh should be fixed on the outside of the poles (i.e. away from the road) to prevent mesh from falling away from posts when large animals crash into fence.
Figure 7.80 - A smaller mesh size at the bottom of a wildlife fence is used to prevent smaller animals from passing through the fence.


  • Metal or wooden posts are both suitable.
  • Poles have to be strong enough to withstand the impact of an animal in flight running into the fence. End posts should have a diameter of 2-2.5" (steel) or 10 x 10 cm / 12 cm diameter (wood). Middle posts can be slightly thinner. Poles should be replaced when they are damaged.
  • All posts must be firmly embedded in the ground (approximately 70 cm or more depending on the ground).
  • For deer, the distance between posts should be 4-6 m (up to 10 m in flat areas), for wild boar 4 m maximum.


  • Where there is a danger that animals might get trapped on the road, i.e. particularly when the whole stretch is not fenced, exits should be provided to allow the escape of animals.
  • It is better to avoid the Dutch type of exit doors for badgers or any other mechanical doors. Experience has shown that they often stay open or become damaged.
Figure 7.81 - A simple exit made of tree stumps allows Iberian lynx in the south of Spain to cross a fence if they get trapped between fences. (Photo by H. Bekker)
Figure 7.82 - An exit in the same area in the south of Spain designed for different animal species (Photo by H. Bekker)
Figure 7.83 - A ramp allows mammals to jump across the fence when they get trapped on the road side of the fence.

Additional considerations for small animals

  • Fences to keep out small animals (amphibians, reptiles, small mammals) should only be erected in conjunction with passages. Otherwise, small animals should not be kept away from road verges completely, because these often provide suitable habitat and serve as movement corridors for these species. Only in cases with high road mortality is it justified to keep species like tortoises or lizards away from the road areas.
  • To keep out small animals an additional mesh can be fixed to the standard fence. Depending on the species mesh size should not be greater than 2 x 2 to 4 x 4 cm. Height: 40-60 cm. To prevent animals from climbing over the fence, the top part should be turned out- and downwards.
  • For amphibians opaque barriers instead of wire fences are recommended. These are discussed in Section 7.3.7.


  • Fences have to be checked in detail as part of ordinary road inspection at least once a year and more frequently during the first year.
  • Particular attention has to be paid to:
    - holes (which have to be repaired immediately),
    - attachment to the poles,
    - attachment to the ground,
    - trails and hollows which indicate the regular passage of animals under the fence. 
  • If fences are damaged by cars after accidents or after storms, they have to be repaired immediately.

Points for special attention

  • Fences are effective barriers for most, but not all species: wire fences effectively hinder the access of deer, wild boar, hares and other non-burrowing and nonclimbing species, but brown bears, lynx, martens and others are able to climb over most fence types. Burrowing animals like badgers and foxes may force under a fence if the fence is not fixed underground.
  • Where gates have to be provided to allow access to the road or railway line, their design has to be such that only gaps as small as possible exist between the gate and the ground and between the gate and fence.
  • A dense row of bushes planted close to the fence on its outside can prevent mammals from attempting to jump across a fence. No plant species attractive to foraging animals should be used.
  • All small bridges, underpasses, culverts and other possibilities for joint-use fauna passages across the highway have to be accessible by animals from the outside of the fence.
  • Where narrow access roads require an opening in the fence cattle grids may provide a barrier to larger animals. However, they are a danger for small animals that easily fall in. They should therefore be equipped with escape ramps. A hole in the side of the base of the cattle grid can also help to avoid victims.
Figure 7.84 - Cattle grids are usually used on agricultural tracks as in this example from Denmark. However, they can also be used where a side road enters a main road. (Photo by B. Wandall)

7.4.2 Artificial deterrents

Artificial deterrents aim at keeping mammals away from roads or railway lines in order to reduce the number of collisions. They are mainly targeted at deer. Various systems exist based on optical, acoustic or olfactory devices. Experience shows that the effectiveness of such measures is usually very limited. Reflectors/mirrors Wildlife warning reflectors are widespread.They consist of various types of metal strips placed around trees or other structures. The light of approaching vehicles is reflected towards the side of the road, which should warn animals and stop them from entering the road. These features are popular because they are cheap and easy to place. However, a thorough analysis of studies carried out over the last 40 years all over the world found little evidence for the effectiveness of wildlife warning reflectors. Reflectors also require a lot of maintenance.
Figure 7.85 - A wildlife warning reflector in Spain. There is little evidence of the effectiveness of this measure. (Photo by C. Rosell)
Acoustic deterrents Ultrasound devices emit acoustic signals that should deter mammals. Like other acoustic deterrents there is no evidence for their effectiveness. Olfactory repellents Olfactory repellents are a relatively new measure to prevent accidents, mainly involving deer. Natural or artificial substances, usually a mix of scents from humans, wolves and other predators, are injected into a foam as a carrier substance which is then applied to trees or posts in the vicinity of the roads. The first tests of these relatively new systems indicate that the number of collisions with cars is effectively reduced. Observations seemed to show that deer increase their attentiveness and thus may become more aware of approaching cars. When there were no cars around, the animals crossed the roads. Other observations indicated, however, that while deer crossed less in the treated stretches of the road, they shifted to adjacent untreated areas. To prevent habituation by animals olfactory repellents should only be placed during critical periods, e.g. during the migration period of deer. Further research is needed to show the effectiveness of these measures in the long term. More experience is needed with regard to maintenance needs. The possible impact on non-target species is unknown.
Figure 7.86 - Olfactory repellents are applied to poles along the road. (Photo by C. Rosell)

7.4.3 Warning signs

Warning signs aim at influencing the behaviour of drivers in order to reduce the number and severity of collisions between large mammals and cars. Standard traffic signals are placed in areas where collisions often occur. They also exist for amphibians, waterbirds and other animals. However, research has shown that drivers don't pay much attention to signals on their own and in particular don't reduce their speed. Therefore systems have been developed to increase their effectiveness.


  • Wildlife warning signs should be placed only in places where there is a high risk of collisions, because the more widespread they are, the less people pay attention to them.
  • Putting up signs only during critical seasons could make people more attentive to them.

Points for special attention

  • The combination of a wildlife warning sign with a speed limit is slightly more effective.
  • The effectiveness is further enhanced if signs are marked with flashing lights or a flashing speed limit sign, which are lit only during periods of high animal activity.

7.4.4 Wildlife warning systems with sensors

Wildlife warning systems combined with heat sensors have shown to be able to reduce the number of collisions. Heat sensors in the vicinity of the roads detect approaching mammals up to a distance of 250 m. The sensors trigger the fibre optic wildlife warning signs which are combined with speed reduction signs (30-40 km). Normally the signs appear dark and the light points are only visible when activated. The system can be powered by solar energy. Wildlife warning signs without speed reduction are less effective.

Points for special attention

  • People should be informed about the way the combined system works, because only if they know that an illuminated sign does not only indicate a potential danger, but also the actual presence of an animal, will they adapt their behaviour.
  • Warning signs should be combined with speed limits.
  • Like any other technical equipment, the combined system has to be checked regularly.
Figure 7.87 - A moose warning sign in Norway. Such warning signs are not very effective, because drivers get used to them. (Photo by S. Persson, Østlandets Blad)
Figure 7.88 - A wildlife warning system combined with heat sensors, used in an area in Switzerland where red deer regularly cross the road. (Photo by H. Bekker)

7.4.5 Adaptation of the habitat alongside the infrastructure

General description

Different ways of designing and managing habitats alongside roads and railway lines are used with the aim of reducing the number of collisions. Some are designed to prevent animals from moving onto the road surface by attracting animals elsewhere, others by influencing the behaviour of animals or by making animals more visible.

Cutting of vegetation

The cutting of bushes and trees within a 3-10 m strip alongside the road reduces the attractiveness for large mammals such as moose. At the same time the visibility of the animals to drivers is improved. The measure is designed to reduce the number of collisions between large mammals and cars. This measure is suitable for roads with low traffic load and for railway lines. Verges with short vegetation often have high densities of small mammals (rodents) and are therefore attractive to birds of prey. This may increase the risk of collisions with birds.

Figure 7.89 - Cutting high vegetation alongside the roads makes large mammals, e.g. moose like in this example from the E18, Akershus in Norway more visible to drivers, and removal of vegetation which is an attractive food-source will reduce the risk of having animals foraging along the road. (Photo by B. Iuell)

Choice of plant species

The choice of the right plant species to be planted alongside roads can reduce the number of collisions between cars or trains and animals. While it is advised to use native plant species, care should be taken to avoid plants which may attract animals to the road verges for foraging, increasing the risk of collisions with cars: 

  • Bushes and trees, which are not attractive to browsing deer, etc.
  • No bushes with attractive berries, in particular not in the central reservation. Berry bushes attract songbirds, mainly during migration.
  • Forest fires often start from roads. Plant species that burn easily should not be planted to reduce the risk of fires spreading to adjacent habitats.


  • Hedges along fences can lead animals towards fauna passages. A gap between the fence and the hedge facilitates maintenance work along the fences.
  • Bushes alongside the fence reduce the danger that large mammals try to jump the fence.
  • Tall tree hedges force birds to fly high. Thus they cross the road at a height where they don't collide with cars. On the other hand, hedges may attract birds to the vicinity of the road increasing the risk of collisions.
  • The planting of hedges has to consider the above-mentioned points of visibility and choice of food plants.

7.4.6 Adaptation of infrastructure

Noise barriers

Noise barriers are constructed close to human settlements to reduce noise emissions, although in certain situations they are erected to protect, for example, colonies of breeding birds from disturbance. However, even if not constructed for wildlife they have to be treated in this chapter because they can increase habitat fragmentation even more than fences. In densely built-up areas noise barriers do not usually provide any problems in this respect. In more natural surroundings they are complete barriers for all terrestrial animals.

Non-transparent screens

Noise barriers built of concrete, wood or other material are complete barriers for animals. In natural environments they must therefore be combined with fauna passages. This has to be considered also in cases of low noise screens along railway lines, which may hinder the movement of small vertebrates like snakes, which without barriers would not have been greatly affected by the railway line.

In combination with passages noise screens can function as guiding structures.

Noise screens are usually built on a solid concrete base. They thus completely isolate the road verges from the surrounding habitats. For small animals, especially invertebrates, they are therefore a more complete barrier than fences. No experience exists as to the effects on the animal populations or regarding possible solutions to reduce the barrier effects, such as small openings at the base of the structures.

Figure 7.90 - This noise barrier has openings at the bottom to reduce the barrier effect for small animals. (Photo by H. Bekker)

Transparent screens

Transparent screens are erected in areas where planners wish the drivers or passengers to be able to see the surrounding landscape. They entail a high risk of mostly fatal collisions for birds, which don't recognise the wall as an obstacle, in particular where natural vegetation can be seen through the glass or where the glass reflects bushes or trees. It has been shown that with appropriate markings the number of collisions can be reduced substantiall

Figure 7.91 - Transparent noise barrier along a motorway in Switzerland with vertical markings. (Photo by H. Schmid)




  • Vertical markings are recommended, although other types may also be effective.
  • Marking strips should be 2 cm wide with a distance between the strips of a maximum of 10 cm (or 1 cm wide, distance 5 cm).
  • Light colours are preferable to dark ones, because they are more visible in the twilight.
  • Markings should be applied on the outer side of the wall (i.e. away from road) to avoid reflection.
  • Silhouettes of birds of prey are not recommended. They are only effective to prevent collisions if put up at a very high density.
  • No reflective material or glass should be used.
Figure 7.92 - A few isolated silhouettes of birds of prey are ineffective to prevent collision by birds. (Photo by C. Rosell)
Figure 7.93 - The planting of bushes close to transparent screens should be avoided. (Photo by V. Hlaváˇc)

Points for special attention

  • Wherever possible, transparent screens should not be built. Non-transparent walls can be covered with bushes or climbing plants.
  • No trees or bushes should be planted in the vicinity of transparent noise barriers because this increases the risk of collisions. Where trees or bushes are planted as mitigation measures, no transparent noise barriers should be built.

Adaptation of the kerb

Vertical kerbstones are often too high for small amphibians, reptiles, mammals or invertebrates. If they don't find an exit, animals get trapped and usually die. Gently sloping kerbs are a cheap alternative. With a height above the road of a few millimetres at the bottom they are still detectable, e.g. by blind people using a guiding stick. A gap between vertical kerbstones can provide escape possibilities as well, especially if plants are allowed to grow between the stones.

Figure 7.94 - Hedgehog trapped by a kerbstone. (Photo by B. Iuell)

Escape ramps from drains

The gaps in metal covers of drains are often too big for small vertebrates and for invertebrates, which fall in and drown. Ramps offer the possibility of escape. In areas with spawning runs of amphibians a wire mesh placed under the cover of the drain prevents animals from falling in. Amphibians are the only animals to survive the way from drains to clarification plants and therefore need purpose-built escape ramps at the plant to get out.


  • The ramps should have a rough surface to provide a good grip.
  • The end of a ramp should be about 15 cm higher than the surrounding terrain.
  • The end of a ramp should be fitted with wire netting to prevent small predators from climbing onto the ramp. The mesh size should be about the same size as the gap in the metal cover.
Figure 7.95 - Escape ramps from drains each 25 m help to avoid the deadly trapping of small animals.

Road lighting

Road lights often attract insects and as a consequence bats or nocturnal birds which hunt them. This results in high mortality for the insects as well as for their predators. In sensitive areas the need to establish road lights should therefore be balanced against the consequences for nature. To prevent collisions of insects the use of sodium lights is recommended.

Fauna exits from waterways

General description

Canals and irrigation channels are often protected against erosion by (almost) vertical embankments. The waves by wind or by boats, the current in the river and the lack of space between the water and the adjacent area are often the reasons for bank protection by sheet piling.

Many animals drown when there is no opportunity to climb out of the water. This problem is increased when the sheet piling is 0.2 m or more above the waterline. It is a problem for all kind of mammals and even young waterbirds may be victims.

It is possible to prevent animals from drowning by offering exits. There are two main solutions:

  • The best solution is a complete new embankment with a natural slope that gives opportunities for vegetation growth. The embankment can function as a habitat and as a corridor.
  • When there is no room for this, the sheet piling can locally be lowered below the water level to allow animals to exit the water.


A concentration of drowned animals can be an indication where fauna exits are necessary. The exact location of the fauna exit is at the place where tracks come close to the water. The identification of the place of drowning is a problem because drowned animals may be displaced by the current or removed by scavengers.

Figure 7.96 - Different possibilities for fauna exits on waterways in the Netherlands. Exits at the water side of the sheet piling is only possible when there is no danger for ships. In use by badgers, roe deer, young ducks and hedgehogs. (Photos by H. Bekker)

Design of natural embankments

The number of possibilities is enormous, depending on the situation, material, forces of waves and current, etc. Projects in the Netherlands have shown there can be many benefits for nature. The issue is covered in other handbooks

Design of fauna exits

  • Several designs are possible; depending on the function and characteristics of the waterway: ships, currents, waves, species.
  • Exits can be placed behind or before the sheet piling.
  • Exits can be made from wood, steel or stone.
  • Dimensions: in canals a high number of small exits of 1 m width or fewer of 5-7 m width are recommended in the Netherlands.
  • Distances of 50 m between fauna exits are recommended in the Netherlands.
  • Vegetation around the fauna exit can help attract the animals so that they swim towards the fauna exit. This vegetation can be part of the whole landscape structure around the fauna exit.
  • Exits are required on both sides of the water. The distances different species can reach by swimming needs to be researched.

Accompanying measures

  • Around the exit should be vegetation which provides shelter for the animals. The vegetation should be integrated with the surroundings landscape structure.
  • Fauna exits can be connected to underpasses when a road parallel to the canal forms an obstacle.
  • Fences or hedges can be used as guidance from the land side.


  • Sheet piling is often damaged because of big boats with big stern waves. Waves are worse when there are sheet-piling defences at both sides of the water. The defence in a fauna exit gets these same waves, which increases the risk of destruction by stern waves.
  • The fauna exits are often places where floating waste gathers. 

Extra information: The Dutch Handbook (CUR) contains six volumes on ecological embankments covering both the technical and ecological issues. It is currently being translated into English.