Purpose of highway planting is not only to provide shade but also to enhance the aesthetic value of the roads. Safety on highways, roads and streets is the important factor to contemplate when planning and designing for roadside plantations. Trees on roadsides are hazard objects and can cause accidents if not properly planned and designed. Special mitigation measures are necessary to avoid collisions with trees. The roadside plantations should be in accordance with the tree plantation guidelines of the Highways Department, if any exist. Following are a few general guidelines:
- Plant large evergreen trees in the outer row, followed by medium sized trees in the middle and flowering small trees, shrubs or bushes in the inner row on either side of the road.
- Highway planting should achieve a mass effect for the viewer traveling at the assigned speed of the highway. It should also achieve a well-balanced combination of both planted areas and open grassy spaces.
- The plantations will look more beautiful and achieve better aesthetics if same species is planted on both sides for at least one km, then another species.
- The species should be both functional and aesthetic, representative of the natural environment of the area.
- Native trees that promote bird and pollinator biodiversity may be planted in mixture with other trees.
- Tree form and shape should be used for harmony and contrast. Round headed or spreading plants form more desirable masses, while columnar or conical shapes add greater visual emphasis.
- The trees that grow up to 10 m should be planted at least 3m from road shoulder and 3 m apart. Trees that grow 10-15m tall should be planted 4m and the trees that grow more than 15m tall should be planted at least 4m from the shoulder, with similar spacing between the trees.
- Plant selection should emphasize the use of native plants to the greatest extent possible. Preference should be given to regionally native plants.
- Avoid planting trees under overhead utility lines unless the mature tree size is less than the height of the lines.
- Avoid planting bushes that may obstruct drivers’ view of pedestrians, livestock, and wildlife etc. crossing the roads.
The trees when grown up should not obstruct the view of the motorists. This is especially important at intersections, sharp bends, and similar places.