In earlier times, trees were planted on either side of the road to provide shade and shelter for road users, which were primarily pedestrians or slow moving vehicles. Roadside plantation of trees continues even now on all types of roads as these also improve the appearance of highway by adding variety and enhance the aesthetic appeal of surroundings because of their foliage, flowers and shape. Vegetation also protects slopes against erosion by rain & wind and checks the advancement of desert, which is a very important factor in arid and semi-arid regions. Planting of trees and shrubs along highways follow a variety of schemes, such as, avenue plantation in a single or multiple rows along the roadside; group plantation on plots of land adjoining the road or mixed plantation involving variety of species.
With the improvement of the condition of roads and vehicles, the speed of travel has increased considerably especially on State Highways and National Highways. Travel on foot and non-motorised vehicles, especially on highways has also declined over the years. If trees are planted too close to the carriageway, the erring vehicle may strike the solid trunk of the tree and cause damage to the vehicle and injury to the occupants. In the Ministry of Shipping, Road Transport and Highways guidelines of September 1998, it has been specified that the first row of trees should be placed at a minimum distance of 10-12 m from the centre line of the extreme traffic lane to provide recovery area for the vehicle that runs off the road. As per the guidelines the second row of tree 6 m further away is also desirable and planted wherever possible. Preferably the first row of trees should consist of species with thick shade and other rows of vertical type providing thin shade. Future expansion programme is also required to be kept in view while locating the trees.