25-06-2014, 04:29 PM
FULLY ACTUATED VS. SEMI- ACTUATED TRAFFIC SIGNAL SYSTEMS
[attachment=65338]
Introduction
ISTEA puts an emphasis on management
systems. Traffic signals are a good example of a
congestion management tool. The question arises
of how to get the most for the highway dollar. This
paper discusses the benefits and drawbacks of fully
actuated versus semi - actuated signals. For
comparison, pretimed signals with and without
pedestrian times are included.
Qualitative Comparison
Semi-actuated isolated signals have
detection on some or all movements except the
mainline. Non-detected phases are controlled on a
pretimed basis. Cycle length is allowed to change
by varying the detected phase lengths. Split
percentages for the mainline are then arrived at by
dividing the fixed mainline phase into the variable
cycle length. Sufficient green time (and therefore
capacity) for the mainline is not guaranteed without
additional delay to the other movements. Once the
mainline's minimum green has been served, the
non-coordinated phases can be served when a call
arrives (with clearance times). Without the
presence of a conflicting call, the signal normally
will rest in the mainline phase
Examples
Using a sixty second background cycle at a
two phase semi-actuated intersection, a side street
(non-coordinated phase) vehicle arriving one
second after the yield point will be delayed 59
additional seconds for missing the yield point. If
progression is good on the mainline, immediately
following the yield point there should be very little
traffic to delay the side street traffic. Yet, the side
street vehicle is delayed for what is ordinarily
considered an unacceptable amount of time
Qualitative Summary
The following list summarizes the benefits
of fully actuated over semi-actuated signals in a
coordinated system:
1. Variable mainline bandwidths to correspond to
traffic flows.
2. Dilemma zone protection for mainline traffic.
3. Reduced side street delay in periods of low
mainline demand.
4. Free-run operation with variable mainline
phase lengths.
The ability to change in the future is
difficult to value, but must still be considered.
Placing detection on the mainline during initial
construction allows future flexibility for full or
semi-actuated operation as well as for free-run
during off-peak hours. If loops are not placed
during initial construction, later installation will
involve construction under traffic, with additional
maintenance of traffic, and construction costs. It is
highly desirable that mainline detection be
constructed as part of other signal modifications or
roadway construction
Quantitative Comparison
While the qualitative comparison lends a
strong argument for using full detection in all cases,
the costs involved are substantial. A literature
review produced no quantitative research into the
Research
To address the issue of quantifying and
verifying the above discussion, research was
performed using Traf-Netsim, a microscopic
simulation tool. Version 3.05 was released early
with a warning that it was not official, and had
some problems to be corrected. During the
research, some problems that played directly in the
results of some test cases were encountered. Using
the two animation programs, Static Netsim and
Animated Netsim Graphics, several problems were
identified and worked around. No other output
format could replicate this information
Costs
These benefits must be weighed against the
additional cost. In Indiana, the additional costs
would be for approximately 650 lft of conduit, 2 - 6
loops, 2 - 4 detector housing, 2 - 4 hand holes, 2 - 6
loop amplifiers, 1000 ft of lead-in wire, and
maintenance costs. The initial construction costs
could vary from $10,000 to $25,000 per
intersection. This represents 20 - 30% of the total
signal construction costs. Maintenance costs can
vary widely and are not discussed here. There are
no significant differences in operating costs
Conclusion
This paper applies to the coordination
effects of the two types of detection. Pretimed
systems with and without pedestrian timings were
also analyzed for a basis of comparison to check for
errors in the coordination parameters. No attempt
was made at quantifying the non-coordinated
benefits. There are several other articles that
address full versus partial detection on an isolated
basis. Benefits noted in the qualitative comparison
provided a basis for the assumptions that started this
investigation.
While the data is not conclusive due to
insufficient quantity, fully actuated signals should
be considered for use in systems. The pretimed
systems without pedestrian timings results indicate
these systems also should be considered.
The real benefit to fully actuated systems
may be in their insensitivity to timing plans. While
pretimed systems can produce good results, they
should be updated regularly. Where funding is tight
for staff to maintain timings, it may be better to
have a more forgiving system. The issue of where
the funding for staff versus capital expenditure
comes from should be considered also
[attachment=65338]
Introduction
ISTEA puts an emphasis on management
systems. Traffic signals are a good example of a
congestion management tool. The question arises
of how to get the most for the highway dollar. This
paper discusses the benefits and drawbacks of fully
actuated versus semi - actuated signals. For
comparison, pretimed signals with and without
pedestrian times are included.
Qualitative Comparison
Semi-actuated isolated signals have
detection on some or all movements except the
mainline. Non-detected phases are controlled on a
pretimed basis. Cycle length is allowed to change
by varying the detected phase lengths. Split
percentages for the mainline are then arrived at by
dividing the fixed mainline phase into the variable
cycle length. Sufficient green time (and therefore
capacity) for the mainline is not guaranteed without
additional delay to the other movements. Once the
mainline's minimum green has been served, the
non-coordinated phases can be served when a call
arrives (with clearance times). Without the
presence of a conflicting call, the signal normally
will rest in the mainline phase
Examples
Using a sixty second background cycle at a
two phase semi-actuated intersection, a side street
(non-coordinated phase) vehicle arriving one
second after the yield point will be delayed 59
additional seconds for missing the yield point. If
progression is good on the mainline, immediately
following the yield point there should be very little
traffic to delay the side street traffic. Yet, the side
street vehicle is delayed for what is ordinarily
considered an unacceptable amount of time
Qualitative Summary
The following list summarizes the benefits
of fully actuated over semi-actuated signals in a
coordinated system:
1. Variable mainline bandwidths to correspond to
traffic flows.
2. Dilemma zone protection for mainline traffic.
3. Reduced side street delay in periods of low
mainline demand.
4. Free-run operation with variable mainline
phase lengths.
The ability to change in the future is
difficult to value, but must still be considered.
Placing detection on the mainline during initial
construction allows future flexibility for full or
semi-actuated operation as well as for free-run
during off-peak hours. If loops are not placed
during initial construction, later installation will
involve construction under traffic, with additional
maintenance of traffic, and construction costs. It is
highly desirable that mainline detection be
constructed as part of other signal modifications or
roadway construction
Quantitative Comparison
While the qualitative comparison lends a
strong argument for using full detection in all cases,
the costs involved are substantial. A literature
review produced no quantitative research into the
Research
To address the issue of quantifying and
verifying the above discussion, research was
performed using Traf-Netsim, a microscopic
simulation tool. Version 3.05 was released early
with a warning that it was not official, and had
some problems to be corrected. During the
research, some problems that played directly in the
results of some test cases were encountered. Using
the two animation programs, Static Netsim and
Animated Netsim Graphics, several problems were
identified and worked around. No other output
format could replicate this information
Costs
These benefits must be weighed against the
additional cost. In Indiana, the additional costs
would be for approximately 650 lft of conduit, 2 - 6
loops, 2 - 4 detector housing, 2 - 4 hand holes, 2 - 6
loop amplifiers, 1000 ft of lead-in wire, and
maintenance costs. The initial construction costs
could vary from $10,000 to $25,000 per
intersection. This represents 20 - 30% of the total
signal construction costs. Maintenance costs can
vary widely and are not discussed here. There are
no significant differences in operating costs
Conclusion
This paper applies to the coordination
effects of the two types of detection. Pretimed
systems with and without pedestrian timings were
also analyzed for a basis of comparison to check for
errors in the coordination parameters. No attempt
was made at quantifying the non-coordinated
benefits. There are several other articles that
address full versus partial detection on an isolated
basis. Benefits noted in the qualitative comparison
provided a basis for the assumptions that started this
investigation.
While the data is not conclusive due to
insufficient quantity, fully actuated signals should
be considered for use in systems. The pretimed
systems without pedestrian timings results indicate
these systems also should be considered.
The real benefit to fully actuated systems
may be in their insensitivity to timing plans. While
pretimed systems can produce good results, they
should be updated regularly. Where funding is tight
for staff to maintain timings, it may be better to
have a more forgiving system. The issue of where
the funding for staff versus capital expenditure
comes from should be considered also