A tunnel may be the best design solution to protect high-value landscapes (see Chapter 5).

Though construction costs may be high the benefits to the natural environment will be incalculable (see Chapter 6). The scale of these benefits is dependent upon the method of tunnel construction. Bored tunnels allow sites of high nature conservation value to remain undisturbed and are least damaging environmentally. Cut-and-cover tunnels may be more appropriate for sites of lower conservation interest, but where the maintenance of connectivity between habitats is desirable. Methods of habitat restoration can be employed to provide safe crossing for a range of wildlife.

Siting of the tunnel portals, their landscape treatment, the alignment of the approach road and the design and siting of any ventilation shafts and control buildings are the major environmental design issues for any tunnels. They may intrude into habitats and cause disturbance and pollution locally for sensitive species.

The cut-and-cover tunnel

A cut-and-cover treatment may be a desirable alternative to the open cutting as it permits the landscape to be restored over the line of the infrastructure.

The landscape elements appropriate to the particular location should be carefully designed to be carried over the engineering structure, which must be capable if necessary of supporting large forest tree species. The reuse of the original soils should be considered if they can be stripped and stored in such a way as to minimise compaction and loss of structure.

The soil profile should be constructed to match the adjoining profile in order to reproduce the hydrological characteristics as well as the physical structure and chemical properties of the original substrates.

Where the cut-and-cover tunnel is to be used by a range of fauna, the natural vegetation type for the species’ habitat should be planted over the tunnel and on the approaches.

At the design stage it is important to understand the type of vegetation and species composition that is appropriate to the setting of the new transport infrastructure. Integration with the landscape, nature conservation benefit and passenger interest are key considerations. Where possible, species included in planting designs should be locally indigenous (especially in rural areas) and occur naturally on the soil type adjacent to the route. They should not require irrigation for successful establishment. Where suitable, natural regeneration should also be considered as an alternative method for vegetating new landscapes. Allowing vegetation to regenerate naturally will produce a habitat most suited to the local surroundings. Some Mediterranean regions have special legislation to regulate the use of vegetation on verges. The high number of forest fires that begin beside roads has obliged highway authorities for example Catalonia in northeastern Spain, to forbid the use of pyrophytic plants such as rock roses on road verges. In the same region there is a requirement to prevent continuity of canopies between trees and shrubs on the verges and the forest trees on adjacent land.

Retaining existing vegetation

• The conservation value of existing vegetation, especially mature vegetation, should be respected since it will have associated with it a mature ecology including lichens, invertebrates, birds and small mammals (see Chapter 9).
• Existing vegetation should be retained where it is likely to remain viable and contribute both to nature conservation and to the integration of the infrastructure.
• Retaining mature trees provides a habitat for many species.
• Where infrastructure dissects an existing woodland the newly exposed edge trees should be thinned/coppiced to provide a more attractive and stable edge.
• Woodland edge species, predominantly shrubs, can be planted to increase the ecological value of the woodland.

Screening Function

• High planted screens may be used to provide a barrier to certain bird species that need to be discouraged from hunting along road verges such as the barn owl. A tall screen will lift their flight path over the road or railway above the area of turbulence from traffic (see also Section 7.3.6). Care should be taken to ensure species planted as screens or in the central reserve are not attractive to birds as a food source.
• A minimum thickness of 10 m is required for a tree screen; 5 m for shrubs.
• Vegetation needs to be at least 4.5 m tall to screen heavy goods vehicles and large trees should be placed at a safe distance from the carriageway as determined by local or national regulations.
• Best practice design provides a screen of varied width and height while maintaining long distance views.
• In flat landscapes planting must be designed around existing features.
• Off-site planting may be useful for this purpose. (i.e. on land outside the immediate transport corridor.) Agreement with third party landowners will normally be required to establish and maintain the planting.

Establishing woodland

Woodlands are usually the product of a long period of management of self-generating trees and shrubs or of deliberate planting and do not have a natural distribution of species. New planting is an opportunity to create a more natural woodland type that will give a special character to the area and be of high wildlife interest.

Natural woodland structure is a mosaic of groups of the same species responding to local changes in soil, topography and drainage. In the Atlantic region for instance beech woodland, is almost entirely dominated by one species while ash and field maple woodland is more varied. Other trees may dominate over more confined areas in more specialised habitats such as willows and alders on wet ground and the elms on richer soils.

• Woodland structure and composition must fit in with any adjacent woodland.
• Species must be native to the locality.
• Correct planting distances are essential for good establishment.

Scrub and tree groups

Scrub communities can be as varied as woodland or they can comprise of large, uniform areas of common species such as hawthorn and blackthorn. Careful appraisal of local conditions is required and the arbitrary introduction of species should be avoided. For example, some Mediterranean countries have regulations or legislation that require a particular planting pattern to reduce fire risk.

• Scrub and small groups of trees are useful for softening the edges of woodlands and help to integrate the infrastructure into the landscape and enhance the wildlife interest.
• Intermittent planting of this kind is particularly important for landscapes like downland and wetland, where large-scale planting is usually inappropriate.

Hedges

Hedges should be provided where they are a feature in the landscape. They are important for nature conservation, especially as corridors for the movement of species such as bats, birds and small mammals. They should be sited in such a way that they can be accessed for maintenance. Hedgerow trees are an important component in the plant mix for hedgerows and should be incorporated in the design.

• Species composition should reflect that of neighbouring hedges and should be planted as a staggered double row of transplants.
• Fencing with light metal posts and stockproof wire will often be required whilst the hedge is establishing.

Grassland and heathland

Large-scale tree and shrub planting may not be the best landscape strategy for new infrastructure. Where screening is not an issue it may be appropriate to create grassland, heathland or scrub of nature conservation interest.

• Grassland should comprise of species of low maintenance requirement sown in moderate diversity into suitable soil conditions (preferably low fertility).
• Heathland, a rapidly dwindling resource, should be created where appropriate using locally indigenous plants or seed.
• A site-specific management regime is essential for ensuring the establishment of habitats of nature conservation value; frequency and height of cutting are key considerations.

The choice of seed for wildflower mixes is complex and should be undertaken with care to suit the site conditions and availability of seed. Where plants are used they should be of native origin. 

The management of all vegetation types is an essential prerequisite to meeting the desired objective of maturity. This must be considered at the design stage and the choice of plant size, planting distance, soil preparation and detailed maintenance requirements need to be set out in the specifications for the work.

Fences and walls may have serious barrier effects as well as a significant effect on the appearance of the road in the landscape. Their use should be restricted to locations where they are absolutely necessary. Technical details regarding these features are given in Chapter 7, but some general principles on integrating them into the landscape are set out below.

•  Appropriate styles and alignments should be used to blend them into their surroundings.
• Hedge planting is often complementary to fencing and will have high nature conservation benefits providing both a habitat and a linking feature for mobile species.
• Fencing at bridge abutments and junctions needs particular attention to avoid gaps.
• Fencing and walls should reflect local styles and materials. 
• Dominant fencing should be avoided wherever possible and it should not be located on the skyline.
• Fences need not follow property boundaries. Alignments which flow with the road and relate to the alignment of adjacent field boundaries are desirable. (See Section 7.4.1.) 

Environmental barriers are structures aimed at minimising the impact of the road on adjacent property, e.g. earth mounding, continuous fencing, brick walls, concrete barriers, etc. It is important to provide fauna crossings where there are long lengths of barrier otherwise these noise and visual screens become major sources of fragmentation.

Lighting should be designed for the minimum light spillage beyond the carriageway or railway to minimise its impact on fauna. For specific design details see Section 7.4.6.