Basic HTML Version
Chapter 3: Whose Turn is it? Phasing, Rings, and Barriers
85
One of the primary functions of a traffic signal is to provide a time separation between the conflicting
movements at an intersection. A stop sign accomplishes the same result but leaves much of the discretion
to the driver to decide “whose turn it is.” A traffic signal provides clear instructions to drivers traveling
through the intersection that it is their “turn to go” using an appropriate signal or display.
In Chapter 3, you will complete a set of activities to learn about the most commonly used method of
separating conflicting movements at an intersection. Known as NEMA (National Electrical Manufacturing
Association) phasing, this method assigns numbers to each of the four left turn and four throughmovements,
and provides a logical process through which each of the movements is served in turn. Each movement
is controlled by a phase (or “timing unit”), and eight phase numbers account for the basis of a NEMA
phasing plan. Left turn phasing is of particular importance, particularly the determination of whether a left
turn should be protected (indicated by a green arrow) or permitted (indicated by a flashing yellow arrow
or a green ball). Left turn phasing is covered in Chapter 8 of this book.
When you have completed the activities in this chapter, you will be able to
•
Describe NEMA phasing and the concept of rings and barriers
•
List the phase numbers at a standard intersection with eight movements
•
Draw and describe a ring barrier diagram in which there are two rings and eight phases
•
Determine the phasing pattern and sequence for a signalized intersection in the field
•
Begin to appreciate how theoretical information about phasing, rings, and barriers is used and applied
by professionals
This chapter begins with a
Reading
activity and ends with an
In Practice
activity. The reading content
describes the concept of the ring barrier diagram and how phases are sequenced. The In Practice activity
provides further information on these concepts. The chapter also includes two other activities. Activity
#14 is an exercise to practice what you’ve learned from the reading. In Activity #15, you will document
the phasing pattern for an intersection in the field using the NEMA phasing scheme.
Number and Title
Type
13 Phasing, Rings, and Barriers
Reading
14 What Do You Know About Phasing and Ring Barrier Diagrams?
Assessment
15 Verifying Ring Barrier Operation in the Field
Field
16 Phasing, Rings, and Barriers in Practice
In Practice
3
Whose Turn is it? Phasing, Rings, and Barriers