Wildlife & Traffic
A European Handbook for Identifying Conflicts and Designing Solutions
7 Solutions to reduce transport infrastructure impacts on wildlife
Updated version (2022). Produced in cooperation with the project Horizon 2020 BISON. ‘Biodiversity and infrastructure synergies and opportunities for European transport networks’.
7.3 Other measures to reduce mortality and disturbance
Disturbance caused by traffic, such as noise and light pollution, impacts wildlife in important ways identified by recent studies using new technology. To guarantee long term conservation of many species -invertebrates, bats and birds in particular- their habitats should be kept quiet and dark at night to guarantee the quality of the habitat and to avoid altering animal behaviour and movements relating to the infrastructure.
Disturbance can both repel animals from otherwise suitable habitats related to transport infrastructure or attract animals to them despite elevated risks of mortality, e.g. road lighting which attracts some invertebrates. Hence, measures which reduce disturbance effects often also reduce wildlife mortality.
Different types of solutions are described in this section: Section 7.3.1 – Adapting road verges, Section 7.3.2 – Noise screens, Section 7.3.3 – Earthen embankments, Section 7.3.4 – Adapting kerb and drains, Section 7.3.5 – Adapting road lighting and Section 7.3.6 – Carcasses removal to avoid attracting scavengers.
7.3.1 Adapting infrastructure verges
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 carriageway by attracting them elsewhere, others by influencing the behaviour of animals on the verge or by making them more visible to drivers.
Road verges have several functions related to landscaping, stabilisation or safety, that must be balanced. Those related to traffic safety and avoidance of negative effects on surrounding landscapes (flooding, forest risks, dispersal of invasive species, etc.) must always be prioritised.
Many actions described in this section could be developed by improving maintenance practices or by modifying elements of the infrastructure during its operation (see Chapter 10 – Maintenance of ecological assets on transport linear infrastructure).
- The clearing of bushes and trees within a 3-10 m strip alongside roads may reduce the attraction for large mammals such as moose and other deer, while at the same time improving the visibility of the animals to drivers. It may also reduce some bird species attraction. This measure is designed to reduce the number of collisions between large mammals and cars, and it is suitable for roads with low traffic density and for railway lines.
- The success of this measure depends entirely on regular and frequent vegetation control. If shrubs and young trees are allowed to grow, they quickly provide even better shelter and more attractive food resources than the original habitats in surrounding landscapes.
- In some areas, verges with short vegetation often have high densities of smaller mammals (rodents) and are therefore attractive to birds of prey, with a corresponding increase in the risk of collision with birds.
Choice of plant species
- The choice of the right plant species alongside carriageways can reduce the number of collisions between cars or trains and animals. While native plant species are advised, care should be taken to avoid plants which may attract animals to the road verges to forage, increasing the risk of collisions with cars.
- Bushes and trees, which are not attractive to browsing deer, should be planted far enough from the carriageway to keep wildlife separate from the traffic and avoid roadside hazards such as overhanging branches or trees which cars could strike in an accident.
- No bushes with attractive berries should be planted, particularly on the median. Fruiting bushes attract songbirds, mainly during migration.
- Forest fires often start adjacent to roads. Species that burn easily should not be planted on verges to reduce the risk of fires spreading to surrounding habitats.
- Hedges growing along the exterior of fences can help lead animals towards fauna passages.
- Tall tree hedges are positive elements in monotonous landscapes and enhance drivers’ attention but can sometimes be attractive for birds because they provide food, perching and nest sites, with an increased risk of collisions. However, they force birds in flight to rise above the traffic and reduce the risk of collision. In local hotspots of bird mortality, a mitigation measure could be to cut existing trees.
- The visibility and availability of food from the hedge must be considered before planting.
7.3.2 Noise screens
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. Even when not constructed to protect wildlife, these barriers have to be included here because they can do more to fragment a habitat than fences. When installed in densely built-up areas, noise barriers do not usually create such problems, however in more natural surroundings they can be complete barriers for all terrestrial animals.
- Noise barriers built of concrete, wood or other material are complete barriers for animals so, in natural environments they must be combined with wildlife passages. In such cases screens should be designed to act as guiding structures towards passages. This is also the case for low noise screens along railway lines, which may hinder the movement of small vertebrates like snakes, which would not have been greatly affected by the railway line had there been no barrier.
- 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 evidence exists as to the effects on the adjacent animal populations or on possible solutions to reduce the barrier effect, such as small openings at the base of the structures.
Transparent screens are used in areas where landscape issues or shading created by a non-transparent wall are an issue for people who live beside or who use the highway. These screens are also placed on bridges to allow drivers and passengers to see the surrounding landscape as they cross. There is a high mortality risk created by these screens for birds which can easily fly into them, particularly where natural vegetation can be seen through the glass or bushes and trees are reflected. This also happens at windows of houses and higher buildings. It has been shown that using appropriate markings, the number of collisions can be reduced substantially (Figure 7.3.1).
- No reflective material or transparent glass should be used in sensitive areas.
- It is possible to install materials that reduce the transparency of the screen. Uniform colour applied on transparent screens can be an efficient solution.
- Another solution is to apply silk-screened strips or other patterns to make transparent screens visible to birds. Vertical markings are recommended, although other pattern types may also be effective (Figure 7.3.2).
- 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 because they are more visible at twilight. A combination of white and orange strips is a good solution visible in different light conditions.
- Markings should be applied on the outer side of the wall (i.e. away from road) to avoid reflection.
- The strips do not have to be strictly parallel.
- Dot patterns must at least recover 25% of the screen surface
- A complementary solution to reduce the mirror effect can be the installation of material with a reflectivity as low as 15%.
To be avoided
- Silhouettes of birds of prey are not recommended. They are only effective at preventing collisions if put up at a very high density.
Points for special attention
- Wherever possible, transparent screens should not be erected. Non-transparent walls can be covered with bushes or climbing plants to make them visually acceptable.
- No trees or bushes should be planted in the vicinity of transparent noise barriers because this attracts in wildlife which can then collide with the screen. Where trees or bushes are planted as noise mitigation measures, transparent noise barriers should not be added.
7.3.3 Earthen mounds
Numerous studies indicate that roads or railways under embankments are less dangerous for flying vertebrates, particularly compared to infrastructures at the same level of their surroundings or on embankments. This is probably because linear infrastructure under embankments creates a bird flight corridor which is above the traffic (Figure 7.3.3). On level and embanked roads, an alternative measure to reduce hotspots of flying animals’ mortality is to raise earthen mounds on both verges to re-create a «buried road» under embankment effect.
- Earthen mounds can only be installed on wide verges: at least 12 to 15 m in width, but the width of mounds can be reduced by several metres using a retaining wall.
- The mounds must be a minimum height of at least 4m to direct birds above the traffic or the catenaries in railway infrastructure.
- Non fruiting shrubs – which are less attractive to birds – and herbaceous vegetation can be planted on the earth mound.
- No trees should be planted on earthen mounds to avoid creating attractive habitats for nesting, feeding and perching, increasing the risk of road mortality for many bird species.
- If the verges are not wide enough and if roads are embanked, another solution is to build high screens which match the height of the tallest vehicles in place of mounds which also direct bird flight over the traffic. The installation of acoustic screens or vegetated mounds (Figure 7.3.4) could also help to reduce noise.
- In places with narrow verges and where noise reduction is not necessary, standard screens, made from wood or any other material, are a potential alternative mitigation measure, being less expensive and as efficient as acoustic walls in reducing roadkill (Figure 7.3.4).
- When installed along protected natural areas that require low noise levels, such as wetlands where large bird populations concentrate, the noise barriers can advantageously prevent collision with flying vertebrate and also create natural areas with reduced noise and light pollution.
- In natural areas that are home to steppe-land birds, fences on the top of the earth mounds should be provided with white marks to avoid birds flying into them (Figure 7.2.34).
7.3.4 Adapting kerbs and drains
Vertical kerbstones are often too high for small amphibians, reptiles, mammals or invertebrates. If these animals cannot exit the carriageway, they become trapped and usually die.
- Gently sloping kerbs that allow small animals to access the verges are a cheap alternative to vertical kerbstones (Figure 7.3.5).
- A gap between vertical kerbstones can also provide the possibility of escape, especially if plants are allowed to grow between the kerbstones.
Escape ramps from drains
The gaps in metal covers of drains (and U-shape ditches), if they are installed, 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 amphibians, a dense wire mesh placed under the cover of the drain prevents animals from falling in. However, appropriate and periodic maintenance should be performed to avoid clogging. Amphibians may survive the journey from drains to water purification plants and therefore need purpose-built escape ramps at the plant to get out.
- The ramps should have a rough surface to provide 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.
- Ramps in U-shape ditches are placed on the verge side only.
7.3.5 Adapting road lighting
RATIONALE. Lighting management
The use of extensive artificial lighting is related to safety even if some studies have found little or no evidence of this relationship (CEDR, 2009; Steinbach et al., 2015; Marchant et al., 2020). Furthermore, it has been proven that light pollution has impacts on ecosystems, affecting flora and fauna, both diurnal and nocturnal species (Falcón et al., 2020; Touzot et al., 2021; Sanders et al., 2021; Sordello et al., 2021). These effects reach distances far beyond the source of emission affecting adjacent habitats and even protected areas (Jechow et al., 2020; Mu et al., 2021; Sordello et al., 2021).
Within linear transport infrastructure the presence of light, either from the vehicles or from the infrastructure itself, reinforces habitat fragmentation (Barré et al., 2020; Sordello et al., 2021). Several species of different groups have shown a general avoidance to light in the infrastructure (Degen et al., 2016; van Grunsven et al., 2017), including avoidance of lit underpasses (Bliss-Ketchum, et al., 2016; Barré et al., 2020; Bhardwaj et al., 2020b).
The need to establish ‘dark ecological networks’ has also been proposed (Challéat et al., 2021) as a measure to enhance ecological connectivity. The timing and location of lighting within these networks must be carefully considered in order to be truly effective.
Light on transport infrastructure, both from vehicles and street lighting, has different effects on wildlife. The most obvious can be the impact of exhaustion or predation on nocturnal insects that are attracted to the lights. But light may affect animal reproduction, navigation or communication. Different studies have further shown that road lighting affects the phenology of some species, changing the flowering period and modifying the plant species composition in favour of those which are light tolerant. Artificial light often attracts insects and consequently bat species, resulting in high mortality for insects and cascading effects within ecosystems. Moreover, a barrier effect can be created for several species such as bats, insects, birds and even some amphibian or fish species whose nocturnal movements can be disturbed or even stopped by artificial lights (see Chapter 3 – Effects of Infrastructure on Nature).
- The guiding principle for any artificial lighting should be: ‘as much as necessary and as little as possible’. The need to illuminate roads should therefore be balanced against the consequences for the surrounding nature. The installation of lights must be limited to those areas where it is strictly necessary due to the amount of traffic, weather conditions, existence of night-time accident hotspots, singular crossing points, etc. In those areas, the illumination period should also be limited as much as possible, both daily and seasonally.
- Different taxa of nocturnal animals have different sensitivity regarding light wavelength and, therefore, there is no solution which fits all. However, yellow/orange lights with wavelengths around 585 nm have been recommended as the most beneficial, especially for insects and mammals. This is problematic for LED lights, which have a strong peak at blue wavelengths (around 470 nm). A potential solution is the Amber LED lamps with Narrow Spectrum (ALNS, without blue emission), although these have lower illumination efficiency.
- The effects on wildlife by LED technologies which emit ‘nearly’ monochromatic light, still need to be evaluated.
- There are technological and management practices that provide solutions for different situations. A best practice process to evaluate and adapt a project to install road lighting is described below:
– The first step in any sensitive area being considered for artificial illumination is to question how useful the installation will be and weigh up the advantages it would provide to the local population against the impact that it would create for fauna and flora (and for human health) without any mitigation measures. The positioning of planned road lighting can be modified to generate less impact on areas where it would be less useful and/or where wildlife would be more impacted. Superfluous lighting can be effectively reduced by the correct choice of the required light-radiation geometry. Recommendations to avoid common mistakes in outdoor lighting are available on Table 7.3.1. To reduce the impact of vehicle lights installing screens or constructing earthen mounds on the carriageway verges are good mitigation measures, especially where the road traffic intensity is over 10 000 vehicles per day. Nevertheless, this measure will not completely avoid the impacts of skyglow and acoustic screens may require adaptation so as not to increase the barrier effect.
– The time periods when road lighting operates are crucial. The longer the lights are off the better it will be for local wildlife. Areas that are permanently illuminated must be as small as possible (Figure 7.3.6).
– The choice of different technologies for artificial lighting provides various solutions. The main categories of lamp which generate low impacts on wildlife mainly emitting yellow, orange or red coloured light and are listed below (Figure 7.3.7):
a. Low Pressure Sodium (LPS) lamps: a low impact on wildlife but no colour rendering. They have low energy consumption but is an old technology that will soon be obsolete.
b. High Pressure Sodium (HPS) lamps: a good balance between low wildlife impact and good colour rendering.
c. Metal Halide (MH) lamps: Strong illumination emitting in a yellow-green colour but also emit light at short wavelengths (blue), only for small areas use where a powerful lighting device (> 1 kW) is needed.
d. Amber LED lamps with Narrow Spectrum (ALNS, without blue emission): the only LED lamps recommended in sensitive natural areas, with medium efficiency of illumination compared to “white LED” (-30% to -70% efficiency).
– Finally, an evaluation of impacts after avoidance and reduction measures have been applied must be carried out and if required, provide a compensation for that part of the impact not reduced (for example: suppressing streetlighting to an equivalent coverage area to that of a natural zone with potential nocturnal animal corridors which has been impacted by lighting).
- At European level there is no specific regulation regarding lighting impacts on biodiversity. There are, however, international guidelines providing recommendations on light characteristics according to landscape zoning, from protected areas to city centres (Table 7.3.1).
- A local or regional ‘Nocturnal Greenways planning document’, where it exists, can provide complementary information during all these steps. Local (Figure 7.3.8) or national/federal regulations as well as urban zoning categories can also modify the parameters of different projects.
- The design of the lamp should direct the light towards the desired area (always focused downward), limiting the intrusive light and eliminating any emission of light above the horizontal plane. This can be achieved in different ways such as lowering the height of the light post as much as possible (when possible), setting the lamp at the proper angle and/or using shields or screens (Figure 7.3.9).
- An adaptive management approach should be implemented to monitor, correct, and improve any aspect of the lighting system affecting biodiversity. For example, biological surveys must be carried out to assess the effects of the lighting on road mortality or to evaluate if breeding or nesting periods for any sensitive species is being particularly affected. In these cases, further restrictions should be applied, always ensuring traffic safety. In the same way, maintenance monitoring must be performed to detect any lamp that is not properly installed or to fix/replace any mitigation measure that is not fulfilling its function.
7.3.6 Carcasses removal to avoid attracting scavengers
Animals killed by traffic can become an attraction to scavengers in turn increasing the risk of mortality. Therefore, a carcass removal procedure must be established, which also facilitates the recording and tracking of these accidents as well as appropriate carcass disposal. This carcass removal procedure should be:
- comprehensive, ensuring the checking of verges and other habitats adjacent to the infrastructure.
- periodically scheduled according to target species affected. When seasonality affects AVC rates, such as amphibian migrations, the frequency should be increased.
- carried out preferably in the mornings, since most of the roadkill usually happens at night.
- in compliance with national legislation and standards, particularly regarding sanitary and biosafety regulations
This carcass removal procedure could also serve to identify AVC hotspots and particularly vulnerable species in order to design specific mitigation measures. To do so, accurate data on location and species identification to the lowest taxonomic level possible should be gathered (see Chapter 10 – Management of road killed animals).