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.
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