ITE Journal – December 2019 - 37

CASE STUDY

Dynamic Flashing
Yellow Arrow Operations

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Shutterstock/Patthana Nirangkul

By Justin R. Effinger, P.E. (M)
here is a need for more consideration when it comes to low-cost alternate
methods of reducing vehicle delay and utilizing minimal infrastructure costs,
complex features of existing traffic signal controllers, and flashing yellow arrows
at signalized dual left turn lanes or high volume single left turn lanes, especially

with the development of connected vehicle technology. The use of flashing yellow arrows for
signalized dual left turn lanes or high volume single left turn lanes can potentially reduce delay,
while not compromising the safety of the left turning vehicles or opposing through vehicles, by
utilizing two different left turn scenarios, including: protected only left turn operations by time
of day, or demand/queue driven protected only left turn operations.
In dual left turns, there are inherent safety issues in which two
vehicles are trying to see opposing traffic at the same time, and
there is potential for the opposing left turn traffic to block the line
of sight. This was overcome in this case by selecting an intersection
that had a recent geometric reconstruction to modern safety
standards, along with adding set back detection to keep the left turn
visibility high.
For the most reduction in delay, the goal of this research is for
the flashing yellow arrow to operate based on the conditions near
the intersection (demand/queue driven) rather than at scheduled
times (time of day driven). In this context, when northbound or
southbound traffic is too high, the signal will not operate with
a flashing yellow arrow (protected only operation), making the
vehicles wait until the next green arrow indication. When the
conditions near the intersection are below the calibrated thresholds,
then the flashing yellow arrow will operate (hybrid of protected/
permitted operation and permitted only operation).

The purpose of this research is to identify a demand/queue
driven left turn operation approach to accomplish the following
criteria:
 Reduce left turn delay;
 Utilize existing controller logic/coding features;
 Keeping the safety theory of protected only operations when
conditions are satisfied; and
 Minimize infrastructure implementation cost.

Background on Traditional Left Turn Operations
Prior to the use of the flashing yellow arrow, a green ball has been
used for permissive left turns. One of the biggest operational
advantages to using the flashing yellow arrow is to lag a left turn,
avoiding the "yellow trap."1 In corridors with unequal intersection
spacing or complex geometry, it can provide bigger green bands, in
which the mainline platoons can get through the majority, if not the
entire corridor, without stopping.
w w w .i t e.or g

D e cember 2019

37
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ITE Journal – December 2019

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