ITE Journal - August 2020 - 47

Figure 3. Proposed Roadway Geometric Layout After Modification: The proposed intersection layout and specifically alignment of the eastbound and
westbound left-turn lanes allows for running of left-turn movements simultaneously or lead-lag operation; and also independent from their adjustment
movements thus providing opportunity to both enhance the capacity and improve synchronization along the arterial due to increased number of sequences.
#1, and one at the far-side intersection indicated as #2. Left-turn
traffic would only proceed to go through the nearside intersection
once indication #1 turns green to allow moving forward toward the
far-side intersection, where they are given the ROW by displaying a
protected green arrow or a left-turn flashing yellow arrow, allowing
the turn under permitted condition.
This design concept shows about a 40 percent reduction in stops
and delays for this specific intersection, since both arterial through
movements have light through volume and left-turn movements
are relatively heavy and equal, thus allowing running the left-turn
movements simultaneously, where this would not be possible under
the current geometric layout of the intersection.
The beauty of this concept is that the proposed improvement
can be done within the existing ROW, without the need of having
additional ROW or building additional lanes.
Flipped Left Diamond Interchange provides an operational
efficiency that might not possible by other diamond interchange
configurations. FLDI provides this operational efficiency by
maximum utilization of arterial green intervals, which is possible by
termination of a companion movement in absence of demand and
allowing to proceed and to serve the next compatible phase in the
sequence where there is demand. This maneuverability might not
be possible with other diamond interchange configurations because
of either geometric layout, traffic pattern, signal phasing sequence/
ring structure, or all. However, just like it is possible at conventional
4-legged intersections, FLDI allows termination of a companion

movement with no demand, being either the left-turn or through
movement, and then continues to serve the next compatible phase
in the sequence-where this might be problematic at conventional
diamond interchanges, or DDI. For example, consider signal
operation at a diamond interchange under three different types of
conventional diamond interchange, DDI, and FLDI.
As shown in Figure 4, for both conventional (TTI 4-phase
sequence) and DDI interchanges, left-turn movement (EBLT@NBSR),
which is referred to as companion to eastbound through movement
at southbound service road (EBT@SBSR) will remain green as long as
there is demand on EBT@SBSR. In other words, the duration of green
interval for EBLT@NBSR movement is dictated by demand on EBT@
SBSR movement, and not by demand on EBLT@NBSR. Obviously,
a holding green arrow with no demand will result in increased
unnecessary delay for WBT@NBSR movement. However, the FLDI
allows termination of EBLT@NBSR movement in absence of demand
and then would allow proceeding to serve the next compatible
movement (WBT@NBSR) in the sequence where demand exists.
Another potential improvement possible with FLDI is
utilization of protected/permitted left-turn display. Due to safety
concerns associated with limited sight distance, often left-turn
displays at intersections with wide medians are "protected only"
type displays, where left-turn movements are allowed only during
display of a green arrow, where FDLI makes it easier to implement
"protected/permissive" mode left-turn display such as FYA (flashing
yellow arrow) due to improved sight distance and reduced number
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ITE Journal - August 2020

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