The construction of new infrastructure and in most cases the upgrading of existing infrastructure requires the consequences of habitat fragmentation to be taken into consideration, for example through the EIA. In contrast, there are no legislative rules to ensure that barrier problems in relation to existing infrastructure are solved.

Nevertheless, through the Habitat Directive (1992), EU member states are obliged to “establish supervision of unintentional catch and killing of species mentioned in annex IVa”.

This includes traffic casualties and the Directive is therefore relevant to existing roads. The annex states that for those species where unintentional killing is known to have a negative effect, preservation measures must be taken.

The purpose of barrier mapping is to identify points or sections where the existing road conflicts with natural structures (rivers, river valleys, forests, etc.) that are part of the main dispersal network or are locally important habitats or dispersal areas. Furthermore, the aim is to point out where and how to improve existing measures and where it is necessary to establish new measures to compensate for the negative consequences of the road.

With simple methodology it is possible to get an overview of the barrier effect of existing infrastructure. The method should comprise:

• Identification and mapping of conflict points.
• Survey and description of conflict points.
• Recommendations of measures to reduce barrier effects.
• Prioritising tasks.

The criteria for identifying conflict points are the same as mentioned in Section 5.4.3. The following must also be taken into consideration:

• Areas with a high concentration of traffic casualties.
• Existing over- or underpasses for transportation, recreational or agricultural crossings

Mapping should be followed by detailed descriptions of each of the surveyed localities and comprise descriptions of existing structures and features and their position, type and function. The condition of existing constructions, fencing, junctions, embankments and vegetation should be recorded. A visual assessment of the site should also be made to optimise the design for the specific situation and meet functional and aesthetic objectives.

The assessment must be followed by recommendations of which locations and sections to improve to reduce the barrier effect. The recommendations will typically include modifications to existing tunnels, installation of new or additional tunnels and plantings and changes in maintenance practices.

Establishing passages across existing barriers is much more expensive than building passages during the construction of new roads and railways. For a large part of the existing infrastructure, bridges, culverts and other constructions can be adapted to provide mitigation measures. By making small adjustments, existing human passages may also be suitable for adaptation to joint-use passages.

Recommendations should include:

• A description of which criteria were used to identify the location as a conflict point.
• A description of objectives and targets (which are the target species and other species likely to use it?).
• A description of recommended adjustments (construction, dimensions and materials).
• Alternative possibilities. • Rough estimates of costs.

For details about design and construction, see Chapter 7.

It is difficult to give general guidelines for prioritising tasks as regional planning and ecological conditions vary between countries. In the long term though, priority should be given to ensure the integrity of the overall ecological network. In some situations, nevertheless, it may be necessary to preserve local habitat for a single species. In other situations it can be practically impossible to improve conditions, for instance because of the road's position in the terrain. Furthermore, in most places it is appropriate to give priority to obvious, minor improvements of existing passages or to changes in maintenance routines.

As a basic guideline for the sections of infrastructure which require measures:

• Priority should be given to areas that meet several conflict-point criteria. 
• The long-term benefits of mitigation measures must be weighed against costs.

Traffic calming by reducing the dimensions of infrastructure may be an appropriate measure to reduce road kill and habitat fragmentation. In certain situations temporary or seasonal closure of roads is appropriate, for instance in order to avoid road kill of amphibians in their migration periods or to avoid large quantities of snow being removed from the road into peripheral areas. More details on traffic calming measures are given in Section 7.5.1.