Posted by Ray Dorsey
Recently, a friend of mine was taking his wife to the airport. They were running behind due to traffic on the Interstate. Upon taking the airport exit they pulled up to the stop sign at the top of the ramp. As they tried to enter the roadway it was difficult for them to see oncoming traffic due to juniper bushes that extended over the guardrail on either side of the ramp exit. Since they were running late and his wife did not want to miss her flight, he eased his car onto the roadway and they collided with a delivery truck.
Luckily, the delivery truck driver had reduced his speed coming into this interchange that resulted in minor damage to both vehicles. No one was hurt, but this could have been much worse.
During the accident investigation, it was noted that this incident was caused by the obstructed view of the driver easing onto the roadway due to the overgrown juniper bushes on either side of the ramp exit. Vegetation can be a real hindrance when it comes to sight distance.
When driving your vehicle, your line of sight is one of the most important factors in keeping you safe. State transportation officials often refer to this as sight distance. Sight distance is defined as the length of a roadway visible to a driver. Whether you are leaving your driveway or preparing to turn at an intersection, your sight may be obstructed by buildings, fences, signs, lamp posts, mailboxes, pedestrians, and vegetation.
Vegetation that obstructs sight distance comes in many forms. Large trees, shrubs, hedges, vines, tall growing weeds, and grasses can deter a driver’s sight. Mature tree species such as oaks, maples, beech, and pines can reach up to three feet in diameter at breast height. Shrubs can reach 10- to 15-feet tall with some species reaching that in width. Evergreen shrubs, such as American holly or cherry laurel cause year-round sight issues. Hedges pose a greater sight distance issue due to their length combined with width. Vines such as kudzu, English ivy, and wisteria can grow extremely fast, obstructing a driver’s view. Weeds such as marestail, burnweed, dogfennel, and lespedeza, and grasses such as Johnsongrass, Miscanthus, Phragmites, bushy bluestem, Chinese silvergrass, and bamboo can reach in excess of 20-feet tall with 2- to 5-foot widths.
Dealing with these vegetation sight distance obstructions brings many challenges to transportation officials. Certainly, planning and design play a role in reducing and or preventing vegetation from being a factor in sight distance. Transportation designers all use the setback rule. Setbacks are defined as the minimum distance from the edge of the travel lane, including interchanges where landscape material such as trees or shrubs can be planted. Designers usually place low growing trees at set back locations where view zones are not obstructed. Shrub species are selected that provide clear sight in the area between 2 and 6 feet above roadway elevations. Low growing woody plants include redwoods, dogwoods, crabapples, and dwarf species of shrubs.
Vegetation planning works well for new and redesigned highway construction but in most cases vegetation sight obstruction is basically an ongoing problem that requires constant maintenance. Existing large trees and shrubs may need to be removed or significantly pruned to improve sight distance. In many cases, however, this may create more problems—especially in urban roadsides where removal or extensive pruning can create an aesthetic void or damage historical significance. A possible solution in this situation would be mediation with the adjacent landowner(s) to remove the vegetation and replant low growing tree and/or shrub species.
Tall-growing grasses and weeds and fast-growing vines do create sight distance problems that can be managed easily with an Integrated Vegetation Management (IVM) plan. Mowing and herbicide applications can dramatically reduce or eliminate any sight distance issues. Roadside mowing performed at least 3-4 times per year reduces plant size and is aesthetically pleasing. Selective herbicide applications performed once or twice per year can either reduce, control or eradicate invasive weeds, grasses and vine species. Using an IVM plan, transportation officials can reduce accidents and provide a clear line of sight to the traveling public.
If you want to talk about improving driving sight distance and safety, contact me directly. I worked at Georgia Department of Transportation for more than 20 years and now serve as a consultant at Pine City Consulting (PCC). At PCC, we help you develop and improve your approach to vegetation management—right-of-ways, roadsides, transportation, and more.