Vertical Visual Cues
One of the most important reasons to provide landscape plants for roundabouts is to provide vertical visual cues for approaching drivers. Lighting is another essential visual cue for roundabouts as a warning at night. Traffic flow for roundabouts is a new concept for American drivers. Roundabouts are often located at awkward intersections with odd angles of approach. Without a visual warning, someone confused by the new traffic patterns might drive right through the central island. Vertical visual cues are needed for roundabouts, especially for high speed approach settings.
The landscape in the picture has been providing decades of delight for locals, directing traffic with soft vegetation, and preventing accidents by providing a block to errand drivers that might over run the traffic separator. It is fairly self-sustaining in design. There used to be a low, evergreen hedge of Yaupon Hollies hiding a lot of the leaf debris that fell from the small-scale trees, with the leaf litter from the trees providing mulch inside the ribbon of evergreens to suppress weeds. The low hedge was removed during construction and reconfiguration of the approaching roads. Because the small trees, which have been there for over thirty years, were important to the local community, the city government took great care to preserve them during the changes.
These trees are dwarf Acoma Crapemyrtles, with small, multi-stem trunks. The trees in most roundabouts need to be small for a small roundabout island. Trunks that generally stay less than four inches in diameter are more likely to break away if hit by a vehicle. That’s good. You don’t want to place solid structures and fixed objects within the stopping distance path of automobiles that run off the road. Choosing the right tree species and cultivar for roundabouts is an important safety consideration.