7.2 Arterial Road Roundabouts
In addition to the above requirements, the grading and landscaping on arterial road roundabouts must be designed to ensure the achievement of sight distance requirements as set out in Section 3, and to avoid obstructing the visibility of signs. Otherwise, trees and other high landscaping features may be positioned in the inner area of the central island, provided it is large enough to ensure that sight lines are not impeded and clear zone requirements are met.
In addition, the landscaping of the central island should:
- clearly indicate to drivers that they cannot pass straight through the intersection; this is usually achieved by continuous kerbing and enhanced by mounding of the topsoil, appropriate planting and hazard markers, etc.
- ideally discourage the passage of pedestrians across to it (seats or similar attractions should not be provided in the central island)
- prevent parking or other vehicular access (except for maintenance purposes), unless the island is intended to be mounted by large design vehicles.
Generally, unless splitter islands are very large as in the case with wide medians, they should not be used to accommodate anything (e.g. trees, planter boxes, rigid lighting or power poles) that would adversely affect roadside safety or sight lines for vehicles approaching or entering roundabouts. As the splitter islands are located within critical sight triangles, care should be taken with landscaping to avoid obstructing sight distance.
Local authorities often provide landscaping within the central islands of roundabouts in order to enhance visual amenity. The relevant authority should ensure that roundabout central islands are not used to accommodate anything that would adversely affect roadside safety or sight lines for vehicles approaching or entering roundabouts.
On very large roundabouts planting, including trees, can be provided in the central island, whether it is raised or depressed. Figure 7.1 shows a very large roundabout where a large tree has been retained at an appropriate set-back from the central island kerb, with low planting in order to maintain sight triangles for entering and circulating traffic.
Note: Large tree and artwork in centre set well back from kerb. Grass and low-height ground cover to maintain sight triangles.
On arterial road roundabouts that have smaller central islands (nominally less than 20–25 m in diameter) it is not usually possible to provide substantial planting and comply with sight distance and clear zone requirements. In these cases, low ground cover can be planted and the island raised at its centre by no more than 200 mm to facilitate drainage.
Landscaping within the central island will need to be kept outside the sight triangles as outlined in Section 3, unless low-growth vegetation is provided. The maximum mature height of this vegetation must be below the sight lines. The current and likely future maintenance regime must also be considered. Vegetation within the central island should preferably contrast with vegetation on the outside of the roundabout to help increase driver recognition of the central island. Large trees (i.e. > 100 mm diameter when mature) should not be planted in central islands of smaller arterial road roundabouts.