An important aspect of roadside landscape design is the treatment of trees. Single-vehicle collisions with trees account for nearly 25 percent of all fixed-object fatal accidents annually and result in the deaths of approximately 3,000 people each year. This problem is most apparent on roads that have existing trees, where designers do not have direct control over placement. For landscape projects, where the type and location of trees and other vegetation can be carefully chosen, the potential risks can be minimized. Integrating trees into the design of a facility has many advantages. Trees provide a visual “edge” to the roadway that helps guide motorists. Trees also add to the aesthetic quality of a highway. In urban and suburban areas, trees soften the edges of arterial and collector streets. If sight distance is a concern, taller trees with lower branches that are trimmed or low-growing (shorter than 1 m [3 ft]) herbaceous and woody plants can be another option along both the roadway edge and in raised medians.
It is important to select the appropriate species of tree for the highway environment. In particular, trees need to be chosen that can survive poor air quality, infertile and compacted soils, and extreme temperature fluctuations. Remember that maintenance, particularly during the first year after installation, is essential to the long-term health and viability of trees and other plants. Utilize the skills and knowledge of the city or town urban forester or arborist, the local agricultural extension service, or a landscape architect to identify the plant material that will be best suited for the location. In addition to selecting a type of tree for its hardiness, the size and placement of trees is another important consideration. Generally, a tree with a trunk diameter greater than 100 mm (4 in) measured 100 mm (4 in) above the ground line is considered a “fixed object” along the roadway. Because most trees grow larger than this, their placement along the roadway needs to be carefully considered. Factors that affect this decision include the design speed, traffic volume, roadway cross section, and placement of guardrail. Trees should not be placed in the clear zone for any new construction or major reconstruction, nor should they be considered safe because they are placed just outside the clear zone. The safe placement of trees to prevent errant drivers from hitting them should be made in conjunction with a highway designer who is knowledgeable about safety.
Trees are an important aspect of community identity and carry a great deal of emotional ties with the residents. As such, the decision to create a clear zone that requires the removal of existing trees is an issue that should be presented to the public and addressed by the multidisciplinary team early on. If communities consider existing trees a valuable resource, alternatives to complete eradication should be pursued. These include installation of traffic barriers, lowering of the design speed, or even complete redesign of the facility to incorporate the trees. It is not unusual for a community to value one specific tree and desire to preserve it. In general, transportation designers must balance safety with other community values when considering facility design and tree preservation.
Source: Flexibility in Highway Design p. 91-92
Published: 1997
