The nation's rights-of-way provide a major conduit for the flow of goods and services vital to the American economy. Rights-of-way include lands set aside for roadside, electric utility, railroad, and pipelines, and total more than 20 million acres across the country. Since rights-of-way are related to supplies and services for people, most right-of-way acres are located in the heavily populated eastern United States.

Right-of-way reliability and public safety are major concerns across all rights-of-way for controlling selected types of vegetation. The right-of-way must allow workers to perform their function without hazards to those that use and depend on them. Management of nearly all rights-of-way share common objectives, including motorist and worker safety, reduced fire hazard, and the ability to perform inspections. There are many issues unique to each type of rights-of-way.

Most rights-of-way managers are confronted with "noxious weeds." Many states have declared certain plants to be "noxious weeds" because they have a detrimental effect on public health, agricultural crops, natural ecosystem function, or animal production. State law may require their control. If the agency or company responsible for the property does not control these weeds, local governments can contract the work to an outside group and charge the negligent party. If a state has established a list of noxious weeds, applicators should know these weeds and be able to identify them at various stages of growth.

Maintaining the nation's roadside rights-of-way, the areas between the edge of the pavement and the outside right-of-way boundary, focuses on safe travel for the motoring public. These areas may be either constructed or natural, and they may include the median strips on multilane highways and interchange areas. Roadside vegetation is controlled for 1) safety concerns, 2) road structure maintenance, and 3) appearance or beauty (aesthetics). Safety requirements include maintaining a safe recovery or clear zone, sign visibility, and sight distance. Vegetation control improves drainage and snowdrift control, slows roadbed degradation, reduces erosion and fire hazard, and prolongs the life of roadside hardware. Vegetation control also enhances the beauty of roadsides.

Vehicles need a safe recovery or clear zone to stop when they accidentally leave the road. This area is called the safety recovery zone or clear zone. Roads designed for higher traffic speed and density have wider safety recovery zones than roads designed for slower speeds and less traffic. Cut slopes require narrower safety recovery zones than do flat areas. Steep fills require the widest safety recovery zones (Figure 1). Since many vehicles continue on to the bottom of steep slopes, a clear area beyond the edge of the slope is desirable. A wider recovery zone is necessary at curves because cars are likely to leave the road at a sharper angle.

Trees are the major vegetation problem in the safety recovery zone, because they are the single greatest cause of fatalities from vehicle collisions with fixed objects. Individual state policies differ on the size and height of trees permitted in the recovery zone, ranging from no trees allowed at all to trees allowed that are less than four inches in diameter at one foot above ground. Shrubs that do not block visibility are usually acceptable. A vehicle leaving the road can be funneled along the ditch bottom, or onto the backslope. Therefore, trees should not be located in or near the bottom of a ditch or on the backslope near the ditch.

Signs along roadsides must be visible regardless of season, time of day, or weather conditions. Signs convey warnings, guidance, and information. They are an important part of road safety, and can prevent accidents and driver confusion. Hidden signs area safety hazard because their message is obscured (Figure 2). Undesired vegetation can be near the sign or between the sign and the point where the driver should see the sign.

The length of road visible to the driver is the sight distance. Clear sight distance is needed for stopping and for making decisions about turning, changing lanes, passing, and entering the roadway. As speed increases, greater sight distance is needed. Vegetation has its greatest affect on sight distance on two-lane, rural highways. Sight distance is often limited by hills and curves. Vegetation can block sight distance at intersections and across curves (Figure 3). During ice and snowstorms, overhanging limbs become heavier and reduce sight distance more than usual.

Depth perception on the right side of the vehicle is difficult for many drivers. Vegetation encroaching from the shoulders often forces or subtly directs drivers to the center of the road on two-lane highways. This can reduce driver safety. Encroaching vegetation affects driver behavior most when weeds occur in short, isolated sections. Long, continuous stretches of vegetation have less affect on drivers. Tall vegetation and overgrown shrubs should be kept far enough from the roadside so as not to influence driver behavior.

Vegetation growing on the road shoulder and in ditches can prevent water from draining off the road rapidly (Figure 4). Water on the road surface can cause automobile hydroplaning or become ice patches in winter. Water can speed the degradation of pavement and road surfaces, soften and reduce the supporting ability of subgrades and shoulders, and deposit sediment in ditches and drainage structures. Cut plant material left after mowing can block drainage inlets, particularly during heavy rains when drainage is most critical. Weeds growing around or over the inlets can slow water drainage. The area around drainage inlets should be kept free of weeds.

Tall and thick growing plants such as trees, ragweeds, and cattails reduce water flow in drainage ditches by catching and holding debris. Low-growing grass is desirable because it controls erosion, and it minimizing interference with flowing water. Grass buffers can also act as biofilters by trapping sediment. Vegetation control promotes rapid drainage and often improves the appearance of the ditch. Shoulders should be maintained weed free to prevent drainage problems and damage to the roadway pavement because of shoulder deterioration. Weeds in the shoulder restrict subgrade drainage. This causes the soil to become "soft." The internal pumping action causes sorting of the aggregate and the road looses its weight-bearing capability. Gravel shoulders need to be maintained weed free so the shoulder is clearly defined to the motorist.

Weeds can destroy paved shoulders by growing through cracks and accelerating deterioration. Once established in the crack where the shoulder meets the road surface, roots or creeping rhizomes can punch through the road surface. The surface of roads shaded by trees dries slower, and ice and snow take longer to melt. Water reduces the life of the surface, especially asphalt.

Snowdrifts occur on the downwind side of any obstacle that slows the wind. Tall vegetation, either trees or weeds, can cause drifts as the snow piles up behind the vegetation. Fences, barriers, median plantings, and guide rails clogged with weeds or overgrown with vines can cause snow to drift. Narrow bands of vegetation upwind from the road and downwind from wide-open spaces increase snowdrifts on the road. Since an obstacle has an influence distance of 15-20 times its height, vegetation some distance from the roadside may require control in areas susceptible to snow drifting.

Fires can occur when vehicles pull off the road. Hot catalytic converters and mufflers can ignite dry vegetation. In regions where plants complete their life cycle with the onset of drought, particularly annual grasses such as wild oats and cheatgrass in western U.S, weed- free areas wider than the normal road shoulders are maintained so vehicles can pull off the road without risk of igniting a fire (Figure 5).

Soil stability is important to the maintenance of roadbeds and surfaces. Dense vegetation is effective in reducing erosion. However, vegetation growing at the road edge can prevent water from draining off the road after a rainfall event. Water that does not run off the road often collects or runs down the pavement. The flowing water gathers force and eventually breaks over the edge. Increased water pressure causes erosion of the shoulder at that point. Weed dams are a particular problem under guide rails where machine grading is not possible.