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Chapter 2: Modeling What We’ve Observed: Queuing Systems
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The purpose of this chapter is for you to learn how to apply queuing theory to the operation of a signalized
intersection.
When you have completed the activities in this chapter, you will be able to
•
Describe the components of a queuing model and how a queuing model can represent a signalized
intersection
•
Connect your observation of traffic flow at a signalized intersection with a model framework
•
Represent and interpret queuing diagrams for a range of traffic flow and control conditions
•
Connect a theoretical queuing model with real world conditions
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Represent and interpret queue accumulation polygons for a range of traffic flow and control conditions
•
Compare and contrast the traffic flow representations used in the
Traffic Signal Timing Manual
with
those that you studied in the activities in this chapter
Amodel is a representation of reality. It includes only those elements or features of the real world (“reality”)
that are important for understanding a system or a process, in this case the traffic signal control system. In
Chapter 2, you will learn about queuing models and how to connect the theory included in these models
with what you observe in the real world. A queue is a line waiting for service: people in a check-out line
at a grocery store, vehicles at a stop sign waiting to safely enter the intersection, or pedestrians waiting for
the Walk indication at a signalized intersection. In each case, we need to specify the pattern of arrivals,
the manner in which service is provided, and the discipline within the queue. For example, vehicles may
arrive at a traffic signal in a random manner (arrival pattern), be delayed during red and be served during
green (service pattern), in a first-come, first-served manner (queue discipline).
This chapter begins with a
Reading
activity (Activity #8) on the modeling of traffic flow at a signalized
intersection using queuing theory. In Activity #9, you will be asked to complete several tasks to validate
your understanding of queuing models by answering questions about and preparing sketches of queuing
models that represent various traffic flow conditions. In Activity #10 you will link this queuing model
with the real world, using a high resolution data set of field observations from a signalized intersection.
Your ability to link theory with what you observe in the field is an important skill. In Activity #11, you
will enhance your understanding of queuing models by observing traffic flow in the field and collecting
data that you will use to link to the models. Finally, in Activity #12, you will learn how the
Traffic Signal
Timing Manual
represents traffic flow at a signalized intersection and compare this representation with the
descriptions in this chapter.
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Modeling What We’ve Observed: Queuing Systems