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Chapter 8: Left Turn Phasing—Permitted, Protected, or Both
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The safe separation of conflicting traffic movements was first discussed in Chapter 3. In Chapter 8, you
will learn about the various ways in which left turns are served and some of the ways in which conflicts
with the opposing traffic can be eliminated or reduced. Drivers can safely complete a left turn movement
without waiting when a green arrow display is shown. Or, when a green ball or flashing yellow arrow is
displayed, there is another level of instruction to the driver: you may proceed, but first you must judge that
it is safe to do so, with a large enough gap in the opposing traffic that allows you to safely complete your
left turn maneuver. You will complete a set of activities that describes the most commonly used methods
of serving left turn movements, particularly the determination of whether a left turn should be protected
(served by a green arrow) or permitted (served by a flashing yellow arrow or a green ball).
When you have completed the activities in this chapter, you will be able to
•
Describe the methods of left turn phasing
•
Describe the basic concepts of left turn phasing
•
Determine the efficiency of permitted left turn operations under various opposing through traffic volumes
•
Show that protected left turn phasing is more efficient than permitted left turn phasing under some
conditions
•
Describe the trade-offs and relative efficiencies between protected/permitted and protected left turn
phasing
•
Select optimal left turn phasing treatment based on analysis of performance data and observation of
simulation conditions
This chapter begins with a
Reading
(Activity #45) on left turn phasing. The chapter also includes five
activities, including an assessment of your understanding of left turn phasing (Activity #46) and three
activities (Activities #47, #48, and #49) in which you observe the operation and performance of various
left turn phasing options. In a design activity (Activity #50) you will test various left turn options for your
intersection and select the best option for the conditions that you have observed. The chapter concludes
with an
In Practice
activity (Activity #51) in which you will compare your left turn phasing plan with
information used in practice from the
Traffic Signal Timing Manual
.
Number and Title
Type
45 Left Turn Phasing
Reading
46 What Do You Know About Left Turn Phasing?
Assessment
8
Left Turn Phasing
—
Permitted, Protected, or Both