Transcript Session 9 - TRB Traffic Signal Systems Committee

Isolated Actuation, plus
The Dilemma Zone Dilemma
Rick Denney
Iteris
TRB Signal Timing Best Practices Workshop 2005
Actuated Controller Settings include
Minimum Green (or Initial)
Extension (or Passage)
Volume-Density: Variable Initial
Volume-Density: Variable Extension
Maximum Green (or Maximum Extension)
Coordination Settings
TRB Signal Timing Best Practices Workshop 2005
Minimum Green Settings
 Clears queue from stop line to first detector
 Typical formula (variations do exist) is the sum
of the startup time plus the time needed for
each car in queue:
 distanceto first detector 
MinimumGreen  5 s  2 s/v eh

v ehiclelength


TRB Signal Timing Best Practices Workshop 2005
Volume Density: Variable Initial
Minimum initial
Added initial (added per actuation)
Maximum initial
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Variable Initial—Diagram
Controller Time Settings
Red
Each actuation
increases initial
Green
Yellow
Maximum Initial
Added Initial
Minimum Initial
Extension
Vehicles Detected
Vehicles Detected
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Variable Initial Settings
Used in absence of stop-line detector
Add to the minimum green for each car
that passes over loop
Typically “added-per-actuation” = ~ 2 sec.
Minimum initial = minimum green
Maximum initial is 2 times the number of
cars that will fit between stop line and loop
TRB Signal Timing Best Practices Workshop 2005
Extension
Maintains green when density is high
Typically long enough for extending
vehicle to clear dilemma zone
With dilemma-zone detection, extension is
part of the design
With presence detection, extension long
enough for expected gap:
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Extension
Extension = X – speed/(length of loop +
car), where X depends on your goals…
Low Flow Conditions
P(gap)
PDF of queue departure gaps
X
0
2
4
PDF of arrival gaps
6
Gap
8
10
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12
Extension
Extension value a trade-off in heavy
traffic…
High Flow Conditions
P(gap)
PDF of queue departure gaps
PDF of arrival gaps
0
2
4
X?
6
Gap
8
10
TRB Signal Timing Best Practices Workshop 2005
12
Volume Density: Variable Extension
Variable extension (gap reduction)
Time before reduction
Minimum gap
Time to reduce, or reduce by
TRB Signal Timing Best Practices Workshop 2005
Variable Extension—Diagram
Controller Time Settings
Green
Extension
Yellow
Time
Time to
Before Reduce, or
ReducReduce By
tion
Minimum Gap
Initial Green
First Opposing Call
Gap Out
Vehicles Detected
TRB Signal Timing Best Practices Workshop 2005
Variable Extension—Diagram
Controller Time Settings
Green
Extension
Yellow
Time
Time to
Before Reduce, or
ReducReduce By
tion
Minimum Gap
Queue Clearance
Initial Green
Queue Gone
First Opposing Call
Gap Out
Vehicles Detected
TRB Signal Timing Best Practices Workshop 2005
Variable Extension Settings
Extension
Generous for queue departure: ~2.5-3 sec.
Time Before Reduction
Long enough to clear typical queue
Time to Reduce
Typically 5-10 sec.
Minimum Extension
Short enough to only allow tight gaps: ~2 sec.
TRB Signal Timing Best Practices Workshop 2005
Controller Time Settings
Maximum Green—Diagram
Green
Yellow
Maximum Green
Maximum
Extension
Initial or
Minimum Green
Red
Phase timer reaches
Max, phase maxes
out and terminates
Extension
Vehicles Detected
TRB Signal Timing Best Practices Workshop 2005
Maximum Green
Doesn’t care if you have dilemma zone
protection
Doesn’t care if the queue is cleared
Doesn’t care if a platoon is arriving, but
Often the only thing preventing insane
cycle lengths
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Some Biased Questions
Does the dilemma zone really exist?
What is the real purpose of dilemma zone
protection?
Prevent accidents from cars going too fast, too
close?
Reward fast drivers with longer green?
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Is There A Better Way?
To improve safety without rewarding
speeders?
To keep cars from entering intersection
dangerously?
TRB Signal Timing Best Practices Workshop 2005
The Main Problem
Dilemma zone protection set up at a point
where cars should start to stop, but
It’s when they go that causes the problem
DZ makes the decision to extend the
green 6-9 seconds before the problem
Lots of detection infrastructure required,
and sluggish timing unavoidable
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A Solution? (aka Research Needs)
Delay the crossing green to prevent
accidents, but
Don’t extend the green to reward the
speeder
Ergo: A variable red clearance interval
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Variable Red Clearance: Advantages
Not concerned with stopping distance—
stopping cars are already stopping
Concerned with cars that are still going at
the end of the yellow
Measured close to the stop line, where
detection is cheap and easy
Detects problems where they exist instead
of projecting them 6-9 seconds back
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Disadvantages
Controllers don’t work that way
Needs research to discover hidden
disadvantages
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Summary (Yup, this is biased, too)
Old methods used fancy timing to work
with limited detection
New methods use fancy detection layouts
that add more and more green time to
serve less and less traffic
None solve the problem: Clearance
problems should affect clearance intervals,
not green time
TRB Signal Timing Best Practices Workshop 2005