Traffic signals have a greater impact on surface transportation
system operation than any other traffic control device. Signal operations
affect motorists' travel time, stops, delays, safety, fuel consumption, air
quality and frustration. Signal timing projects have been shown to have
benefit-cost ratios as high as 80:1, while ratios of 20:1 are common. These are
compelling reasons for highway agencies at every level of government to
emphasize high-quality signal system operation. Yet surveys of traffic signal
operation have consistently shown that these important devices are receiving
inadequate attention to ensure efficient traffic operation.
The public is becoming increasingly frustrated with this
situation. It is up to the traffic management and engineering profession to
raise the bar on signal operations.
Traffic signal systems, and to a lesser extent ramp meters
and lane control signals, are the only ITS devices that provide the means for
proactive control of traffic flow. Other ITS technologies provide advisory
information (dynamic message signs, highway advisory radio, websites, etc.),
improve response to incidents (service patrols, CCTV surveillance and improved
communication) or improve the efficiency with which payments are made
(electronic toll collection, e-commerce, etc.). The promise of ITS can only be
met with improved traffic controls, which in turn suggests the need for
increased emphasis on signal system operations.
Yet, it is ironic that many in the community continue to
debate whether traffic signals are a legitimate component of ITS. It is likely
that this results from the fact that signal systems have been used for so long
that they have lost their attraction as a new or advanced technology. Signal
systems are often operated and maintained
by field personnel with inadequate engineering and public administration
training to communicate the importance of these systems to management,
politicians and the public. Funding for signal operations must often be
obtained in competition with other municipal and state operations (schools,
police, fire, etc.), a difficult competition at best. The combined absence of
high-level management attention and inadequate funding has created a situation
in which few signal systems are operated with maximum efficiency. In other
words, traffic signal systems have become the Rodney Dangerfield of the
transportation industry—they just don't get no respect.
As if these difficulties were not enough, there are many
myths and misconceptions that stand in the way of high-quality system
performance. The following is a small sample of these difficulties separated
into categories of culture, signal timing and operations.
Cultural myths
Myth: It is not
possible to convince decision-makers that signal operations should receive a
higher priority and increased levels of resources.
Response: It is
difficult but not impossible to make the case that signal operation should
receive greater organizational emphasis. One of the greatest impediments is the
lack of understanding on the part of those responsible for allocation of
resources, regarding potential of improved timing and the steps that must be
taken to accomplish that goal. When discussing the issue of improved signal
timing, it is not unusual to hear the following comments:
* We've already invested millions of dollars in improved computers
and communications facilities. What do you mean we're not paying enough
attention to signals?
* I know that our signals may not be perfect, but how could
a small increase in green time help when this would only permit a few
additional cars to enter the intersection?
The first comment indicates confusion regarding the
difference between facilities and operations. The second statement demonstrates
a lack of understanding of the most basic principles of signal operations.
The speaker obviously does not realize that the two or three
additional cars must be multiplied by the number of lanes and the number of
cycles per hour, an increase in throughput that could exceed 1,000 vehicles per
hour. These comments demonstrate our failure to adequately educate managers and
politicians.
But, our failures don't stop with the basics. We have failed
to convince them of the potential payoffs of a well-timed system. The public is
becoming increasingly impatient with poor operations and could potentially
serve as a future source of support.
Myth: Automobile
travelers are the only beneficiaries of improved signal operations.
Response: This is
another example of our failure to communicate. Practitioners are well aware of
the fact that buses and commercial vehicles also benefit from efficient
operations. Practitioners also know that well-timed signals result in improved
emissions, which benefit the entire community. Outside of the immediate
community, these facts are often unknown.
Myth: Telephone
calls from the public are only a nuisance.
Response: The public is often overlooked as a
potential resource and ally. When receiving a citizen inquiry some agencies
spend time educating the caller about the operation of the intersection in
question. They follow up on inquiries to inform the caller of its disposition.
Perhaps most important, the caller is asked to take responsibility for future
monitoring of the intersection's performance and reporting any degraded
operation. The agencies taking this approach have developed a low-cost resource for educated monitoring of
intersection performance, while at the same time enlisting a new ally. This is
another path to supplementing existing support for signal system operations.
Myth: Improved
signal timing, like any other roadway improvements, only generates additional
traffic.
Response: One
overlooked aspect of signal timing changes is that, contrary to most roadway
improvements, they do not generate additional traffic. Thus while improved
timing provides significant system-wide benefits, they are too subtle at any
single location to have any effect on travel behavior. This is another benefit
of signal operation that should be communicated to managers and politicians.
Signal timing
Myth: Only three
timing plans are required: morning
peak, evening peak and off-peak.
Response: Using
analysis tools and simulation (such as the Synchro signal timing software,
Transyt 7F, Corsim and others), it can be demonstrated that typically five to
seven plans are required for normal weekday operation. Additional plans are
necessary for weekends, holidays and incidents. Generating these plans requires
an effective data collection capability and the use of software to assist in
the process.
Myth: Real-time
adaptive control does not work. It is an esoteric discipline whose benefits are
theoretical and are not realized in practice.
Response: The U.S.
is one of very few countries in the world that do not routinely implement
adaptive control for every new signal installation. Can the U.S. be right and
the rest of the world (including many third-world countries) be wrong?
It is more likely that the reason for the U.S. failure to
implement adaptive control is shortage of funding and lack of understanding
since careful evaluations in other countries have demonstrated its benefits.
Part of the confusion results from the fact that adaptive systems do not
outperform conventional, well-timed systems. But the adaptive systems excel
when unanticipated traffic conditions occur. Their performance improvements
also are noticeable with the passage of time and gradual degradation of
conventionally demonstrated plans.
Myth: Signal system
retiming can be expected to reduce motorist delays by as much as 40%.
Response: These
levels of improvement have been measured on several occasions. However, such
large improvements do not necessarily demonstrate the quality of the new
timing, so much as they provide an indication that the old signal timing it
replaced was badly out of date.
Improvements in the order of 10% to 15% are more common.
Myth: Heavier
traffic flows are always served best by longer cycle lengths.
Response: Control of
saturated or near-saturated traffic conditions has been studied extensively.
Yet few practitioners apply the results of this research and cycle lengths are
automatically increased when traffic demand nears saturation. As shown in the
table at left, the benefits of using increased cycle lengths are minimal.
Although not explicitly shown in the table, the disadvantages of longer cycle
lengths include increased stops and delays often outweigh the benefits of minor
capacity increases. Longer cycle lengths also might cause overflows of
left-turn bays and many other operational problems. Many alternatives to longer
cycle lengths are available for saturated intersections.
Operations myths
Myth: Systematic
system-wide signal timing upgrades are an unnecessary expense. Fine-tuning
individual problem intersections is an adequate alternative.
Response: This is a
dangerous misconception. Research has repeatedly shown that multiple localized
changes will eventually result in degraded system performance. System-wide
traffic flow can only be optimized through system-wide timing. System
performance must be periodically monitored to determine when new plans are
required. Statistical techniques are available to determine when system
performance has degraded to the point at which new system-wide timing plans are
needed; typically every three to five years.
Myth: Signal timing
is an art. It is not an engineering discipline.
Response: Signal
timing is a combination of art and engineering. Neither aspect of the process
should be ignored. Engineering is performed at the initial steps of the
system-wide signal retiming which include the collection of traffic demand data
and the application of signal-timing software. The art is applied when the
software outputs are reviewed both in the office and on the street, to ensure
that the subtleties of system operation and local needs are adequately
considered. Neither aspect of the process can be ignored.
Myth: There is no
need to develop signal-timing plans for incident conditions. The police will
take over control anyway.
Response: Nothing
could be further from the truth. Police may take over intersection operation
either because they do not understand the objectives of the signal timing or
the timing is unresponsive to incident conditions. Agencies can respond to
incident conditions through either
the addition of incident plans to the signal-timing plan database or the
manual modification of plans currently in effect. Another alternative is the
use of adaptive signal control software that will automatically adjust to
unusual traffic incident conditions.
Public understanding
The public is gradually developing a relatively
sophisticated understanding of the problems and potential of signal operations.
If approached properly, this understanding can be used as a source of support
for the allocation of increased resources for signal system operation.
In return for this support, it is incumbent upon
transportation agencies to provide a high level of service. Signal timing
should be monitored and systematically upgraded at regular scheduled intervals.
Whether automatically or manually implemented, signal timing should be
responsive to both recurring and non-recurring (incident) traffic conditions.
Both supervisors and front- line staff members must receive adequate training,
so that their responsibilities can be fulfilled with full knowledge of
available hardware, software and engineering technology. Public agencies should
insist on high quality products and services from the signal control industry
(manufacturers, system integrators, consultants). The traffic signal community
and the public will benefit from the implementation of these recommendations.
TME
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