ITE Journal - August 2020 - 48

Figure 4. Signal Phasing Sequence.
DDI

4 Phase, TTI Phasing
Sequence

4 Phase, Non-TTI Phasing FLDI (EB Lead/WB Lag)
Sequence

FLDI lead/lead lefts

Despite lack of demand on
EBL@NBSR, the phase can't
be terminated in order to
serve WBT@NBSR which has
demand. This is due to the
fact that the east bound
through movement at NBSR
conflicts with the next phase
(WBT @ NBSR) in sequence.

Despite lack of demand
on EBL@NBSR, it cannot
be terminated due to
phasing sequence. TTI
4-phasing sequence where
outside movements are
served sequentially and in
clockwise scheme is often
implemented to prevent
traffic gridlock between
service roads due to either
heavy left turn volume,
inadequate spacing
between service roads, or
both.

EBL@NBSR can be
terminated to begin serving
WBT @NBSR movement.
However duration of the
interval that these two
movements can simultaneously run together depends
on left-turn volumes, left
turn lanes storage capacity,
and spacing between
service roads. Excessive
left turn volume can spill
over into through lanes or
into intersection causing
gridlock.

FLDI flexibility allows for
signal sequence along
arterial to be changed
in order to optimize
throughput or improve
synchronization. As shown
in the sequence on this
column, both EB and
WB lefts are leading. The
sequence can easily change
to lag both left turns when
appropriate.

With FLDI, it is possible to
terminate EBL@NBSR and
proceed to serve WBT@
NBSR. Both eastbound
and westbound through
traffic can simultaneously
run together since storage
capacity of the left turns and
spacing between service
roads are not a concern. This
is because these movements
are controlled by dedicated
signal indications outside
service roads preventing
left turn traffic to queue up
between service roads. As
shown in this column, EB
left leads, and WB left lags.
However, the sequence can
easily be changed to lead
WB left turn and lag EB left if
preferred.

Protected/Permitted FYA indication
(1) - Advance green Intervals are only for a few seconds and provide additional green time before next conflicting movement arrives.
(2): Duration of this type of concurrent intervals depend on volume of left turning movements, spacing between service roads, and storage capacity of left turn lanes.

Au gust 2020

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ITE Journal - August 2020

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