11-10-2012, 03:44 PM
Traffic Signal Control Systems
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INTRODUCTION
Traffic-signal control systems coordinate individual traffic signals to achieve network-wide traffic operations
objectives. These systems consist of intersection traffic signals, a communications network to tie them
together, and a central computer or network of computers to manage the system. Coordination can be
implemented through a number of techniques including time-base and hardwired interconnection
methods. Coordination of traffic signals across agencies requires the development of data sharing and
traffic signal control agreements. Therefore, a critical institutional component of Traffic Signal Control is
the establishment of formal or informal arrangements to share traffic control information as well as
actual control of traffic signal operation across jurisdictions.
Signal coordination systems are installed to provide access. A traffic-signal system has no other purpose
than to deliver favorable signal timings to motorists. The system provides features that improve the traffic
engineer’s ability to achieve this goal. These are primarily access features. They provide access to the
intersection signal controller for maintenance and operations. The more complete and convenient the
access, the more efficient the operator will be and the more effective the system.
In addition to control of traffic signals, modern systems also provide wide-ranging surveillance capabilities,
including various kinds of traffic detection and video surveillance. They also provide more powerful
traffic-control algorithms, including the potential for adaptive control and predictive surveillance.
SURFACE STREET CONTROL
Surface street control systems provide the majority of traffic-signal system applications. They are intended
solely to provide control of networks of signal-controlled streets. When traffic-signal systems are integrated
with freeway-management systems, their objectives take on a larger perspective. Integrated
traffic-signal systems will be covered in a later section.
TECHNIQUES/STRATEGIES
Isolated Signals
Isolated traffic signals are the basic building blocks of signal systems. Traffic signals are installed when
traffic at an intersection becomes too heavy for motorists to efficiently or safely assign their own right of
way. Because a traffic signal removes the motorist’s ability to coordinate his turn at an intersection, it is
considered a more restrictive form of control than uncontrolled or stop-controlled intersections. For this
reason, traffic signals are generally regarded as a last resort for intersection traffic control.
The decision to install a traffic signal depends on the conditions at the intersection meeting one of a
series of warranting conditions, as defined in the Manual on Uniform Traffic Control Devices. These
warranting conditions are intended to define the common circumstances when a signal might be appropriate.
In spite of these specified warranting conditions, the decision to install a traffic signal must be
made by a qualified traffic engineer based on a careful study of the intersection and potentially lessrestrictive
alternatives.
Distributed Systems
Distributed systems are those in which, generally, the intersection controller is responsible for control
decisions at the intersection. Distributed systems come in many varieties, ranging from small closed-loop
systems (see below) to powerful large-scale systems.
When traffic signals are located close enough together that traffic remains in recognizable platoons from
one intersection to the next, traffic engineers will usually seek to coordinate their operation. Signal
coordination merely requires that the signal timings at multiple intersections be timed to meet networkwide
objectives for traffic flow. This usually requires all signals to operate at the same or a compatible
cycle length, with careful design of the time-space relationships between the intersections. The signals,
therefore, need some means of staying in step with each other.
Central systems have the following characteristics:
• They depend on reliable communications networks. Because real-time control commands are
transmitted from the central computer to the local intersection, any interruption in the communications
network forces the local controller to operate without that real-time control and revert to its backup
plan. In traditional centralized systems, the backup operation was usually isolated traffic actuation.
More recent systems revert to time-based coordination, but this still requires a transition from central
control to local control. During this transition, signal coordination is usually lost for a short period of
time. For this reason, communications networks for centralized systems usually include some form of
fixed communications, with most agencies preferring to own their infrastructures. These communications
media include twisted-pair copper wire and fiber-optic cable. The physical media typically
provide inherent reliability of 99.995 percent to 99.99995 percent, with downtime ranging from a
few seconds to a few minutes a year. In real systems, downtime is much higher because of physical
intrusion on the infrastructure, though some fiber network approaches even minimize the effects of
that danger.
• They depend on reliable central computers. Without the central computers, centrally controlled
systems do not make much sense. When the central computer is down the system has the same
problems as when the communications network is down, except that the problem affects all intersections,
not just the few on that communications branch. Traditional centralized systems ignored this
problem, and consequently earned a reputation for reliability problems. Newer centralized systems
are employing advanced computer reliability techniques. The most interesting (and expensive) of
these approaches is known as fault tolerance. Fault-tolerant systems employ two identical central
computers networked together with a high-speed network connection. They both contain the same
software, and share a joint operating system. Each computer runs in lock-step with the other, so that
they operate as twins. When one computer fails the system is operated by the other computer, and
the joint operating system arbitrates between the two systems during the onset of failure.
• They are often not easily expanded. Many traditional centralized systems are designed around a
maximum network size. Increasing the size of the network requires a significant investment in central
computer upgrades and often, upgrades in the software as well.