Roads and traffic exert a variety of direct and mostly detrimental effects on nature. The transport sector has acknowledged its responsibility to control these impacts and develop appropriate and cost-efficient mitigation. This cannot be done, however, without the involvement of and collaboration with other societal sectors. European environmental policy and law set the overarching goals and define if and where mitigation may be needed. While in some respects jurisdiction already provides a clear framework for mitigation (e.g. chemical pollution, CO2), there are other domains (e.g., habitat fragmentation, disturbance, mortality) where the broad goal yet has to be translated into measureable tasks.
The EU goals for biodiversity state that species should be kept at a favourable conservation status. This implies that we should seek to ensure that populations can maintain a certain (favourable) size and exert dynamics that do not lead to a steady decline. Population dynamics are driven by a few key factors such as i. fecundity of reproduction (which is directly linked to food and resource availability, age structure and genetic properties of a population); ii. survival of offspring and adults (that directly affects population size and thereby the risk for extinction); and iii. immigration (as a recovery mechanism that prevents local populations from getting extinct and maintains genetic exchange).
In many species, especially the larger wildlife, the transport sector has a special responsibility for the latter two key factors: Survival is directly linked the death toll wildlife pays due to road and railroad traffic. Millions of animals are killed annually as they enter or cross transport infrastructure facilities, but statistics are limited (often only to those that are of immediate traffic safety concern) and despite legal requirements on a monitoring system for incidental mortality (i.e. road kills), empirical data is scarce. Immigration, on the other hand, is dependent on the permeability of transport corridors for wildlife. Many if not most terrestrial species experience significant movement barriers in roads and railroads. This barrier effect often increases with traffic volume and is thus linked to mortality, but in many species it also contains behavioural components (avoidance) that prevent accidents but increase the barrier effect. Also here, empirical data is often scarce and the functional relationship with traffic or road design mostly theoretical.
Barrier effect and road mortality are rather different in their nature, but they are intrinsically linked in their effect on population dynamics (as mortality can partly be compensated by immigration and vice versa). They thus should be addressed within the same mitigation strategy for “defragmentation”.
Hence, the important question is: under which conditions is mitigation needed to reduce mortality and when is it more appropriate to secure permeability. Of course, this depends on the species (abundance, meta-population, behaviour, area requirements, mobility, etc), on the landscape (habitat diversity, fragmentation, human population, etc), environmental settings (climate, seasons, etc), as well as on the type of infrastructure, its traffic and its significance to the populations of interest.
It is clearly impractical (as many studies already exemplified) to develop a static set of technical rules that must always be applied regardless of the actual conditions. Local and regional deviations from the rules may be necessary and render the static system ineffective. Instead, it may be more successful to identify which general properties or qualities in landscape, population and infrastructure must be achieved to produce an outcome that meets with the overall environmental objectives – that is to ensure favourable conservation status. Such outcome-based specifications may thus produce very different practical solutions under different conditions, but this is in line with how future procurement of civil works will be managed. The essential drawback, however, is that here is still a lack of appropriate knowledge and applied theory that allows for a quick development of such specifications.