Analysis and Design of the Traffic Control System/Introduction
Introduction | Users | Detectors | Controllers | Displays
1. The Importance of the Traffic Signal Control System
“When I was in Mrs. Lavender’s kindergarten class in 1953, in a suburban public school in Los Angeles, we were visited one day by a local police officer. The officer brought a traffic signal, mounted on a short pole. The signal had displays that could be seen by two lines of students, as if we were vehicles waiting to cross an intersection. We were instructed to form two lines and to follow directions: walk when it was green and stop when it was red.” We learn the lessons of traffic control early in life, and with good reason. There are nearly 300,000 traffic signals today in the United States. Each traffic signal performs this same task of regulating whose turn it is to go and who must wait. Some signal systems, known as fixed time systems, provide the same amount of time to serve each group of users and in the same sequence. Other systems respond to the volume of vehicles and pedestrians present at the intersection and provide varying amounts of time to serve these users. Some signals operate independently and respond to the traffic demands at that intersection alone, while others operate together in a system so that traffic can be moved with as few stops or as little delay as possible.
The traffic signal system is probably the most important kind of transportation facility in operation today, considering the perspectives of both safety and efficiency. Two-thirds of all miles driven each year in the U.S. are on roadways controlled by traffic signals. In some urban areas, signals at busy intersections control the movement of more than 100,000 users per day. The signal system also has a great impact on energy and the environment. The more times a vehicle stops, the larger the level of pollutants that it emits. And, twenty percent of the oil used by automobiles traveling along urban arterials is consumed while waiting at a red light at a signalized intersection. According to a 2007 report from the National Highway Traffic Safety Administration, 20 percent of all motor vehicle fatalities in the United States each year occur at an intersection. Between 1997 and 2004, this figure represented 76,162 lost lives. In addition, tens of thousands of drivers, cyclists and pedestrians are injured each year in traffic accidents at intersections.
A traffic signal system at its core has two major tasks: move as many users through the intersection as possible doing this with as little conflict between these users as possible. The first task relates to efficiency and capacity while the second relates to safety. Both tasks are performed by first clearly defining which group of users has the right of way at a given time and second by determining how long the group has the right of way. Despite the importance of traffic signal systems, a recent national report card gave the nation’s traffic signal systems poor grades. [add information from Tarnoff report] While there are a number of reasons for this poor assessment, we believe that there are three major contributing factors. First, there is a lack of high quality and comprehensive references defining good practice. While many states and local jurisdictions do have standards that guide their signal timing design practices, these standards are often not based on good science or sound theory that allow the standards to be transferrable to new situations or conditions. Second, university textbooks do not cover traffic signal systems in a comprehensive and realistic manner. Too often, the systems are assumed to be fixed time (rarely the case in the field) while the traffic controller itself is not covered at all. Third, traffic engineers often have little direct experience with traffic controllers since their university experience is often limited to using models that often poorly emulate the operation of a traffic controller. This results in a problematic dichotomy. Signal engineers design the signal system and timing plan but the implementation of the timing plan (and the important timing details) are left to the technician. The former understands how the system should work while the latter understands how the traffic controller actually works but without the same broad perspective that the engineer brings to the problem.
So, how do we overcome these problems and provide systems of learning that will produce transportation engineers who understand how traffic control systems work and have the ability to design the components of these systems? Happily, there are signs that things are changing in the right direction. The Federal Highway Administration has produced a new traffic signal timing manual that brings together a broad array of information that can be used by traffic engineers to design traffic signal systems. FHWA has also produced a new guidebook on intersections, both signalized and unsignalized, that provides basic guidance on intersection design and operation. We offer this textbook to fill another gap: the provision of an environment in which both students and professionals can follow a logical process to learn how traffic signal systems operate and how to design the components of these systems.
2. Our Approach
Our motivation in writing this book is to provide a learning environment and the necessary materials for a graduate level university course in the design and operation of one important part of the traffic signal control system: the isolated intersection. A future book would cover coordinated traffic control systems. Some of the material in this book may also be appropriate for portions of advanced undergraduate classes in transportation engineering.
We have established two objectives in preparing the material for this book that will guide how we present material, the process for using this material, and how you approach learning this material. First, it is important to understand the traffic control system and its components, how they work together, and how each of these components functions individually. Second, it is important to be able to design the components of the system to be able to operate in a defined environment.
To meet these objectives, we have made six assumptions that have guided the preparation of the material in this book. These assumptions are described in the following pages: • You must understand the traffic control system and it component parts. • It is critical that you have knowledge of the traffic controller. • You must know how to solve messy and complex problems. • You need to have an understanding of the design process and gain experience in the application of this process. • Learning environment • We believe in three fundamental guiding principles that should govern signal timing design
Understanding the traffic signal control system
We take the view that the traffic control system includes four interrelated subsystems or components: the user, the detector, the controller, and the display. Each component directly affects another component: for example, the detector responds to the user, while the controller responds to the detector. Further, we will provide you with a set of visualized tools that will allow you to see these relationships and more thoroughly understand them. The system and these components are more fully described and illustrated in section 3 of this chapter.
Knowledge of the traffic controller
It has been our experience that you (engineering students) have, in recent years, gained considerable experience with simulation models of various systems, including transportation systems. Simulation allows engineers to test and observe the performance of a system under a wide variety of conditions, without disturbing the operation of an actual system. However, at the same time, you are getting less experience with the fundamental devices and equipment that are the basis for the operation of many transportation systems. This is certainly the case with the traffic controller, the most ubiquitous and fundamental device of today’s urban transportation system. We believe that in order for an engineer to design and operate a traffic system, understanding the operation of a traffic controller, and how its various settings affect the flow of traffic at an intersection, is critical. It is the task of the engineer, not the technician, to establish the policy and guidelines for the operation of city streets and rural highways, and the control of these streets and highways must be based on the engineer’s knowledge of the controller itself, how it functions, and how its various settings result in varying levels of performance at an intersection.
Complex and messy problems
It is also our experience that many transportation engineering classes are based on simplistic and clean problems, ones with a single solution path, and one “right” solution. Simplistic problems tend to give you a biased and inaccurate view of practice and often do not provide the complexity and challenge that most engineering students look for. In practice, however, problems are messy and complex, often with multiple solutions. These problems include the challenges that stimulate you, providing you with a greater understanding of what engineers do in practice, with all of the uncertainty that this entails. We believe that this book addresses these two important issues by providing the details of the operation of traffic controllers with a set of applications that provide a more realistic setting for the development and design of traffic signal control parameters. While we believe that the use of software tools is an integral part of the design and analysis of transportation systems, learning to use these tools is not the function of this book. Rather, we have provided an extensive set of input and output data sets (generated from commonly used analysis and simulation models) that you can use as you develop your design and make the trade-offs needed in the determination of the appropriate signal timing parameters. Your instructor may decide to use software tools as part of this class, and in this case the input data sets can be used for this purpose. It is more important that you learn how to judiciously use the data generated from software tools than to learn to use the software itself.
We do, however, make extensive use of spreadsheets as the basis for computations, graphing, and programming. The sample spreadsheets provided here were generated with Microsoft Excel, the most commonly used spreadsheet application today. Where appropriate, we will provide example links to professional practice, showing examples of how agencies [do something or apply something].
The design process
There is a logical design process that should be followed when preparing a signal timing design. This book is divided into two parts. The first part deals with basic concepts of traffic control systems at isolated intersections. The second part provides a set of field problems in which you will apply the concepts that you have learned in the first part of the book. These field problems provide you with a context and motivation for applying the concepts presented in the first part of the book and further developed in the second part. Each of the chapters in the second part of the book is based on two pedagogic or learning components: performance and assessment. We believe that learning occurs in an optimal manner if there is a balance between the presentation of concepts (as from part 1) and the chance to apply the concepts that you learned (the subject of part 2) and to receive feedback on what you did right and what part of your work needs improvement. Provide an environment in which students can design components of the system and then test the design using a realistic simulation environment. If you learn to play the cello, you learn music theory. But most of your time is spent playing the cello: practicing or performing. And, your cello instructor provides periodic (sometimes constant!) assessment of how you were doing: holding the bow correctly, placing your hand on the finger board, and listening to the quality of the tone that you produce. We attempt to provide this same framework as you complete the field problems in part 2 of this book. You will be given a set of design requirements and asked to produce a design that meets these requirements. You will be provided a means to assess the quality of your design, working with your instructor. The feedback that you get from this assessment is a powerful means to improve the quality of your work as well as to insure that you have mastered the principles that we present.
The learning environment
Three guiding principles
We will use three overriding principles in the design of traffic signal control plans. First, we don’t want users to stop at the intersection. Second, if they do have to stop, we want to make this stop as short as possible. Third, users will be served based on a set of priorities that have been established for the control of traffic at the intersection. Since we focus here exclusively on isolated intersections, we can’t directly affect whether a vehicle stops or not when it arrives at the intersection. Why? Since the timing of any upstream intersection operates independently of our subject intersection, we can’t control when these vehicles arrive. In a coordinated system, however, when the timing parameters at a set of intersections are interdependent, a platoon traveling from one intersection can be timed to arrive at the next downstream intersection just as the signal indication changes to green, thus minimizing or eliminating all vehicle stops. But while we can’t control when the vehicles arrive at an isolated intersection (and thus can’t control whether they will stop or not), we can make sure that vehicles stop for as short a time as possible.