Passages For Wildlife! How To Reduce The Dramatic Effect Of Roads On Biodiversity

By Margaux d’Ambly and Andréa Poiret

Landscape fragmentation is known to be one of the main causes for the decline of biodiversity worldwide. The multiplication of transport infrastructures over the last decades has changed the landscape drastically, heavily affecting natural ecosystems and wildlife. Highways are acting as physical barriers, which hampers wildlife movements and disrupts habitat connectivity. Additionally, collisions with vehicles are direct sources of wildlife mortality, which can be catastrophic to some animal populations.

The “barrier effect”

The survival of animal populations relies on their ability to move freely across the landscape. Indeed, animals are always searching for food, a shelter or a partner to reproduce with. The construction of transport infrastructures enhances a “barrier effect,” induced by the fencing of roads and/or the noise and light pollution generated by car traffic. Consequently, the landscape is divided into small isolated “islands,” where natural habitats and populations find themselves disconnected from each other’s. This fragmentation of the environment reduces the quantity of available habitats, resources and sexual partners. Population isolation increases consanguinity, which weakens animal populations and can possibly result in local extinction in the long-term.

Landscape fragmentation reduces animal territories, available habitats and sexual partners. Competition for resources increases, as well as consanguinity. (Reference: Norwex Movement).

Some species attempt crossing roads, when those are not fenced. Vehicle-wildlife collisions can cause a significant demographic deficit and increases the risk of extinction in the case of rare and endangered species. A study conducted in the US, has estimated 1 million road kills per day for vertebrate species.

The most affected species are:

  • not very mobile (batrachians) as they are slow and easily get run over by cars
  • frequenting the sides of the road, as it favors the risk of collisions
  • with low numbers and/or reproduction rates, as their population size is small and each death brings the population closer to extinction
  • with very large surface requirements, which requires them to cross many roads
The boreal lynx, a rare species present in the Alps is heavily affected by transport infrastructures, which fractures its ecosystem, create barrier effects and cause direct mortality of individuals by collisions. The species is protected by law in Europe, and appears on the IUCN red list of endangered species. (Picture reference: G. Keith Douce, University of Georgia,


Wildlife crossings

France was one of the first countries to build wildlife crossings in the 1960s. The creation of these structures together with the fencing of highways responded to two stakes: increasing driver safety and preserving biodiversity. These measures aimed to reduce the number of wildlife/car accidents by restricting wildlife access to heavily used road infrastructures. In addition to the safety aspects, they were also built on the rail network for economic reasons: collisions impact rolling stock and infrastructure, and each collision represents a delay and therefore a reimbursement. This is why, in France the rail network first started to take collisions into consideration. The barrier effect and biodiversity loss has only recently been taken into account.

Since the Natura 2000 European network and national environmental protection plans, the reasons for building these crossings have evolved, from concerns about road safety to responding to a global emergency: the erosion of biodiversity. It is in this ecological and legal context that wildlife crossings are now taken into account in development projects.

These passages are built exclusively for wildlife, to favor their safe passage across transport infrastructures. They are designed to be attractive for animals. The effective use of structures dedicated to wildlife passage is well documented and the scientific literature abounds in data indicating the positive role of these crossings on wildlife movements.

They are located at strategic locations and are still sparse due to the costly investment they represent. The location for the construction of a wildlife crossing is decided on the basis of a preliminary study identifying main wildlife movements (ecological corridors) and points of high collision rates. The construction of a crossing also depends on the level of fragmentation of the ecosystem in which the transport infrastructure is located, and on the presence of species of high conservation value. Species conservation value is understood in terms of the vulnerability of the species (e.g. the lynx, which is classified on the IUCN Red List).

A wildlife overpass. This type of structure is preferred by most species. They are built as large as possible, with a natural substrate and indigenous vegetation. Ponds can be created to attract biodiversity and wooden walls are placed on both sides of the bridge in order to reduce noise from car traffic. (Picture reference: Pinterest, David Amrine)


A wildlife underpass. Some species can be reluctant to use these passages due to the tunnel effect. (Picture taken by Tony Clevenger)


Non-wildlife passages

The excessive financial cost (several million euros) of those passages has led some ecologists to look at already existing non-wildlife passages crossing highways and whether these could be used or not by animals. These conventional under- and over-passes, meant for human use or for water courses are abundant along transportation networks. Camera traps were placed at those passages and revealed that some animals actually use them. Several species such as badgers, foxes, coyotes, bobcats, wild boars and deers were spotted crossing these passages.

Ecologists have been working to identify the factors influencing the use of these passages by wildlife. The dimensions, the type of substrate on the floor and the human activity in and around the passages are strong influencers. For instance, they observed that large mammals prefer large passages and small mammals don’t mind using small tunnels underneath a road. They observed that most species are reluctant to use passages with concrete on the ground whereas a natural substrate (sand or dirt) is more attractive. Additionally, the presence of suitable habitats (forest, shrubs, ponds) near the passage increases its use by wildlife.

Gathering such information is very valuable for developing recommendations to adapt already existing non-wildlife passages in order to facilitate their use by animals, or for the construction of mixed-use passages (fauna/human).

This type of work has already been conducted in France and actions consisted, among other things, in creating benches inside hydraulic structures, installing panels to block out noise from car traffic, softening the slopes of the banks, etc.

A forest road overpassing a high-speed rail line. The creation of a grassy strip on the left side of the road increases the attractiveness of the overpass and its capacity to facilitate wildlife movements. (Photo taken by B. Georgii)


A culvert undercrossing a highway in France. The creation of a bench on the left side aims at facilitating the use of this passage by small mammals such as otters, badgers, martens and foxes. (Picture reference: Andréa Poiret).

It is widely acknowledged that the fragmentation of natural habitats is one of the greatest pressures on biodiversity but there is an increasing interest to restore ecological corridors. Scientific studies show that the creation of mixed-use passages and the improvement of already existing conventional passages can significantly contribute to reconnecting wild populations and natural habitats. However, the construction of passages dedicated exclusively to wildlife remains the most effective way to reduce the barrier effect associated with transport infrastructure.

Preserving and restoring biodiversity also means maintaining the many services it provides us and which are necessary for our daily life and that of future generations (pollination, soil fertility, air and water quality, landscape beauty and recreational activities etc.).

Let us mobilize to convince elected officials and infrastructure managers of the need to develop wildlife passages and / or redevelop mixed passages to fight against the erosion of biodiversity.


Cover photo reference:—and-what-to-do-if-you-hit-one/

Carsignol J., Pauvert S., 2008, « TRAME VERTE ET BLEUE : Les suites du GRENELLE de l’environnement, Le rétablissement des perméabilités écologiques par des passages à faune », DIREN PACA, CETE de l’Est et CETE Méditerranée

Carsignol J., 2012, “Passages à faune, trame verte et bleue, statut de l’animal sauvage”, colloque Cohabitation hommes – faune sauvage, vendredi 27 janvier 2012, université Paul Verlaine de Metz

Dumont A.G., Berthoud G., Tripet  M., Schneider S., Dändliker G., Durand P., Ducommun A. Müller  S.  &  Tille  M., 2000, « Interactions  entre  les  réseaux de la faune et des voies de  circulation », Manuel, Département fédéral de  l’Environnement, des  Transports, de  l’Énergie et  de la  Communication, Office fédéral des  routes, 194 p., Lausanne

Glista D, DeVault T, J. DeWoody A (2009). A review of mitigation measures for reducing wildlife mortality on roadways. Landscape and Urban Planning 91, 1–7

Hosy C., Urbano S., Guerrero A., Oumhand A., 2012, « Biodiversité et grands  projets ferroviaires  intégrer les enjeux écologiques  dès le stade des études », Ed. FNE, RFF, en ligne URL :

Bhardwaj M, Olsson M, Seiler A (2020). Ungulate use of non-wildlife underpasses. Journal of Environmental Management 273 111095

Savouré-Soubelet A., Sordello R., Rogeon G., Haffner P., 2012, « Réflexion préliminaire concernant les impacts du réseau ferroviaire sur le Lynx boréal (Lynx lynx) », Muséum national d’Histoire naturelle – Service du patrimoine naturelle. 16 p.

Sétra, 2007, « Fragmentation des habitats due aux infrastructures de transport »,  Manuel européen d’identification  des conflits et de conception de solutions, 179 p., En ligne URL :