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33
CHAPTER 4
CLASSIFICATION AND ASSESSMENT
OF TYPICAL MEDIAN OPENING DESIGNS
A key objective of this research is to identify and classify The second factor, degree of access served, not only deter-
those median opening designs typically used to accommo- mines what movements need to be accommodated at a median
date U-turn maneuvers at unsignalized locations. A classifi- opening, but also the number of potential conflict points a
cation system of this type is presented in this chapter. This median opening will have. For example, a median opening
classification system identifies how particular median open- that only serves U-turn maneuvers will have considerably
ings function and where they are located relative to other ele- fewer conflicting maneuvers than a median opening at a three-
ments of the highway system. This chapter presents the fac- or four-leg intersection, where U-turns use the same roadway
tors used in the classification process, provides an overview as left-turn and crossing maneuvers. Median openings can be
of typical median designs, identifies the factors that influence classified by whether access points are present on neither
the safety and operational performance of median openings, side, on one side, or on both sides of the roadway. Access
and presents typical combinations of median opening designs points at median openings may include either intersecting
used along an arterial. public roads or driveways.
A further objective of the research is to estimate the safety Figures 14 through 18 illustrate median openings with var-
and operational performance of particular types of median ious combinations of the first two classification factors: type
openings. This chapter of the report takes a step toward that of geometry and degree of access served.
objective by cataloging the advantages and disadvantages of The third factor used in the classification is whether or
particular types of median openings and establishing a frame- not a median opening has a left-turn lane. Median openings
work within which their relative performance can be deter- generally operate better when left-turn lanes are present to
mined. The relative safety of various median opening designs provide a deceleration and storage area for vehicles before
is assessed on the basis of potential traffic conflicts. The spe- they enter the median. In fact, the AASHTO Green Book
cific safety and operational performance of median openings specifically encourages the use of left-turn lanes at median
is presented in Chapter 6. openings to reduce or eliminate stopping on the through
lanes (12).
The final factor in classification of median openings is
FACTORS USED IN CLASSIFICATION
whether or not a median opening is accompanied by a loon.
OF MEDIAN OPENING DESIGNS
A loon is an expanded paved apron on the shoulder opposite
The four key factors used to classify or describe the design a median crossover. The purpose of loons is to provide addi-
of a median opening are tional space for larger vehicles (particularly trucks) to nego-
tiate turns, and thus, to allow the installation of conventional
· Type of geometry, or directional median openings along narrow medians. The
· Degree of access served, provision of loons to serve U-turns by large vehicles is a new
· Presence of left-turn lanes, and technique that formalizes past use of paved shoulders for the
· Presence of loons. same purpose. Initial results by highway agencies that have
used loons appear promising (55).
The first factor, type of geometry, determines which move- Based on the four factors discussed above, median open-
ments are possible at a median opening. Conventional median ings can be classified based on their design characteristics as
openings (sometimes referred to as "full median openings") follows:
typically permit all movements, while directional median
openings may restrict certain movements. Jughandles are an · Type of geometry (traffic movements permitted)
indirect left-turn treatment that enable drivers to make U-turn Conventional (all movements permitted),
and left-turn maneuvers efficiently on divided highways with Directional, and
relatively narrow medians. Jughandle;
