aggressive signal priority with compensation: maximizing the transit benefit without disrupting...
TRANSCRIPT
Aggressive Signal Priority with
Compensation: Maximizing the Transit Benefit
Without Disrupting Traffic
Peter G. FurthNortheastern University
Transit Signal Priority – Hype or Help?
• Zurich: nearly zero delay for trams and buses, normal traffic delays for autos
• Portland, OR: route level changes between 0% and 12%
• Many US applications: < 3 s savings per intersection , or no measurement at all
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3
Overview of Ruggles Bus Terminal
• 13 different bus routes
• 96 buses enter and leave, AM peak
• At the busiest intersectionBuses = 3% of vehiclesBus passengers = 37% of travelers
• Average bus entry + exit delay = 150 s
• Research Question: How much difference can priority make near a major terminal?
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BUS
TERMINAL
Back Entrance
1
2
3
4
Main Entrance
ExitRuggles-Busway
Ruggles-Tremont-Whittier
Tremont-Cass
Cass-Columbus
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6
1Bus Delays with
Incremental Priority Treatments, by Route
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“Passive Priority” Operate without detecting buses
• Shorter cycles (shorter red shorter wait)
• Cycle splits and offsets that favor bus movements
• Diverting upstream traffic
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9 9
Increasing EBL Split by 5 s: It only consumes 2.5 s
Max Green = 16 seconds
Pro
por
tion
p (max-out) = 84.6%
Avg bus delay = 98 s
p (max-out) = 51.2%
Avg bus delay = 67 s
Max Green = 21 seconds
0.000.100.200.300.400.500.600.700.800.90
8 9 10 11 12 13 14 15 16
Green Time ( s )
Avg Green (EBL) = 15.3s
Pro
por
tion
0.000.100.200.300.400.500.600.700.800.90
8 9 10 11 12 13 14 15 16 17 18 19 20 21
Green Time ( s )
Pro
por
tion
Avg Green (EBL) = 17.8s
Detection
• Check-in detector location– Early enough to allow time to respond
– Late enough to estimate bus arrival time
• Checkout detector to cancel request– Avoid wasted green
– Performance measurement
• In-ground vs. overhead
• Optical signal with calibrated sensitivity
• Continuous detection (short-range radio and GPS)
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Sketch 1
Upstream Detector, withtravel time = maximum green extension
Simplicity:
• Request = detection
• No need for “priority request generator”
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Weaknesses:
• assumes constant speed
• no flexibility for updates, time of day settings
• not suitable for other priority tactics
What if There’s a Near-Side Stop?
• Detector located just after stop• Disable optical signal until door closes
(Portland, OR)
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Advanced (Upstream) Detection
• “Predictive priority”
– Checkout loop 1 communicates to signal 4
– Logic needed to predict arrival time, generate priority request, choose appropriate priority action
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Priority to Buses in Mixed Traffic
• Electronic bulldozer
• Flushing the queue ahead of the bus = tracking queue length(Zurich)
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In Mixed Traffic, Near Saturation
• Detectors & logic for queue management– Stopped cars, not moving cars, hinder buses
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(Zurich)
(Eindhoven)
• Traffic metering
Green Extension• Built-in logic in modern controllers• Large benefit to a few buses
– Little disruption to traffic
• Extension increment is often fixed– Wastes green
• Is extra time “borrowed” or “stolen”?– Uncoordinated phase: often borrowed
– Coordinated phase: usually stolen
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Green Time Distribution for EBL
0.00
0.10
0.20
0.30
0.40
0.50
0.60
8 9 10 11 12 13 14 15 16 17 18 19 20 21
Green Time ( s )
No Priority
Pro
por
tion
Avg Green (EBL) = 17.8s
Avg Green (WBT) = 30.3s
p (max-out) = 0.512
Avg Green (WBT) = 29.8s
p (max-out) = 0.247
p (extended) = 0.213
0.00
0.10
0.20
0.30
0.40
0.50
0.60
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Green Time ( s )
Avg Green (EBL) = 18.1sP
rop
orti
on
With Green Extension
Extended Green
Priority Push Extension Increment
no priority,uniform arrivals
R = effective redC = cycle lengthv = arrival rates = discharge rate
svC
RdelayE
1
1
2][
2
cumulative vehicles
red green time
vs
bus delay
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with priority
X = green ext’n
)1(
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1
2][
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svC
X
C
RX
svC
RdelayE
Priority push!
cumulative vehicles
green time
1 s
X
bus delay
normal red
v 1
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Priority Push vs. Extension Increment(cycle length = 100 s, red time = 50 s, degree of
saturation = 85%)
0. 0
2. 0
4. 0
6. 0
8. 0
10. 0
12. 0
5 10 15 20
Green Extensi on ( s)
Push (s)
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Priority Push vs. Red Time(cycle length = 100 s, extension increment = 15 s,
degree of saturation = 85%)
0. 0
2. 04. 0
6. 08. 0
10. 0
12. 014. 0
16. 0
30 50 70 90
Red Ti me (s)
Push (s)
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Early Green
• How aggressive?– How much to shorten competing phases?
– Skip competing phases?
– Compensation to shortened / skipped phases?
– For buses arriving in early part of green?• Requires tracking queue length
• Smaller benefit to large number of buses– Hard to implement when bus frequency is high
Truncate and possibly skip preceding phases
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Early Red
• Incompatible with typical coordination logic– custom programming
Shorten bus street’s current green to get • faster return to green in the next cycle, or • red light during stop for pedestrian safety
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Phase Rotation and Insertion
• Dynamically change leading left to lagging left
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• Second realization on bus detection only
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10 s inserted EBL phase: only consumes 2.5 s
0.00
0.05
0.10
0.15
0.20
0.25
0.30
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Green Time ( s )
Green Extension Only
Pro
por
tion
Extended Green
Avg Green (EBL) = 18.1s
Avg Green (WBT) = 29.8s
Avg Green (insertion) = 4.4s
Avg Green (WBT) = 27.3s
p (insertion) = 0.386
0.00
0.05
0.10
0.15
0.20
0.25
0.30
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Green Time ( s )
Extended Green
Primary Phase , when Phase Insertion is Programmed
Avg Green (primary) = 14.7s
Avg Green (total) = 19.1s
Pro
por
tion
Bus delay = 55 s Bus delay = 33 s
Flush-and-Return
• Green extension (if needed) to clear queue from bus stop
• Force signal to red during stop– Minimizes bus’s
impact on road capacity
• Return to green as quickly as possible 28
Early green tactic for Near-Side Stops
bus
Predictive Priority
• Predict bus arrival time based on detection 2-3 minutes ahead
• Adjust cycle lengths so that bus will arrive on green
• Immediate priority as backup• Adaptive (learning) prediction algorithm
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Used for light rail in Houston & Salt Lake City; simulated for Boston
Conditional Priority
• Less interference with traffic (Eindhoven)• Push-pull means of operational control (Einhoven)• What is “Late:” 15 s or 3 minutes?• Demands fine-tuned schedule• Headway-based priority for short-headway service
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Priority to Late Buses
Recovery to Arterial Coordination
• Fixed background cycle: long way / short way– How “holy” is arterial coordination?
• Dynamic coordination (Zurich) – Small zones (1-3 intersections)– Shape green waves through the zone around bus– Zone boundaries are segments that offer storage buffer
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Self-Organizing Coordination
• No fixed cycle length• Each signal’s start of green becomes a request to
downstream signal– Peer-to-peer communication between signals
– upstream signal’s request has lower priority that bus request
• Result: spontaneous green wave• Inherently interruptible
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Simulated for San Juan, Puerto Rico
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1 68 81
2 229 205
3 53 63
realizations per cycleNBTL
(used by bus)WBL
average realizations per cycle
1.90 phases/cycle 1.89 phases/cycle
Free Actuation within a Cycle (Back Entrance)
Delay reduction for buses = 14 s (from 21 s to 7 s)
Delay reduction for general traffic = 7 s
Compensating Interrupted Streams
• Compensation logic is rare– Result: traffic engineers limit priority
• Actuation can provide automatic compensation• BUT, with typical arterial coordination, all the
slack goes to the main movement, preventing compensation to minor movements
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1
Bus Delays with
Incremental Priority
Treatments, by Route
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Six Keys to Performance
1. Aim for near-zero delay (Yes, we can!)
2. Multiple and intelligent tactics
3. Aggressive tactics, with compensation
4. Alternatives to rigid coordination
5. Advanced prediction with gradual cycle adjustments
6. Custom programming and continual improvement
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