Clear
zone distance: The distance required to provide an adequate clear
zone is based on three criteria: traffic volume, design speed and
slope next to a roadside. Clear zone distances can range from 7 feet
for low-volume, low-speed roadways with gentle side slopes to greater
than 30 feet for high-volume, high-speed roadways with steeper side
slopes (see the chart in Appendix B, outlining clear zones in relation
to the criteria). The clear zone distance is measured from the edge
of the driving lane, which is not necessarily the edge of pavement,
if on-street parking or bicycle lanes are present.
Curbs:
The Roadside Design Guide points out that “realistically, a non-mountable
curb has only limited redirectional capabilities, and only at low
speeds.” Consequently, curbs are not typically considered to be barriers.
Types
of vegetation that are allowed: The Roadside Design Guide notes
that any single tree with a trunk diameter of greater than 5 inches
is considered a fixed object. In addition, a group of shrubs or small
trees that are close together can be assumed to have the same effect
as a single larger tree.
Safety
barriers: There may be situations, particularly where there are
high design speeds, where unobtrusive barriers should be considered.
Discussion with local engineering departments and the Tennessee Department
of Transportation would need to occur to determine what would be acceptable
depending on the jurisdictional and functional classification of each
roadway. A barrier such as a guardrail should only be considered if
it will reduce the severity of potential accidents, since the barrier
itself can be considered a roadside hazard, particularly at its terminal
end. Aesthetically pleasing barriers should be used. Any proposed
barrier must have passed several Federal Highway Administration test
procedures in order to be considered crash-worthy. Potential barriers
that have aesthetic merit include: (1) the dual cable barriers used
along the interstate medians in South Carolina (their “transparency”
permits views of the landscaping); and (2) wooden barriers that are
used by the National Park Service along its parkways and by Connecticut
along the Merritt Parkway.
Considerations
in urban environments: The Roadside Design Guide has a separate
chapter that deals specifically with urban environments in which it
is recognized that roadside design is restricted due to limited right-of-way
or dense abutting development. This chapter points out that, while
the clear roadside is still the goal of the designer, compromises
may be necessary to address various constraints. Engineering judgement
often must be used, and determinations of appropriate roadside measures
need to be made on a case-by-case basis.
