Aggressive Signal Priority with Compensation: Maximizing the Transit Benefit Without Disrupting Traffic Peter G. Furth Northeastern 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 2

3 Overview of Ruggles Bus Terminal 13 different bus routes 96 buses enter and leave, AM peak At the busiest intersection Buses = 3% of vehicles Bus passengers = 37% of travelers Average bus entry + exit delay = 150 s Research Question: How much difference can priority make near a major terminal?

4 BUS TERMINAL Back Entrance Main Entrance Exit Ruggles- Busway Ruggles- Tremont-Whittier Tremont-Cass Cass-Columbus

5

6

1 Bus Delays with Incremental Priority Treatments, by Route

Passive Priority Operate without detecting buses Shorter cycles (shorter red shorter wait) Cycle splits and offsets that favor bus movements Diverting upstream traffic 8

9 9 Increasing EBL Split by 5 s: It only consumes 2.5 s Max Green = 16 seconds Proportion p (max-out) = 84.6% Avg bus delay = 98 s p (max-out) = 51.2% Avg bus delay = 67 s Max Green = 21 seconds Avg Green (EBL) = 15.3s Proportion 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) 10 Sketch 1

Upstream Detector, with travel time = maximum green extension Simplicity: Request = detection No need for priority request generator 11 Weaknesses: assumes constant speed no flexibility for updates, time of day settings not suitable for other priority tactics

What if Theres a Near-Side Stop? Detector located just after stop Disable optical signal until door closes (Portland, OR) 12

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 13

Priority to Buses in Mixed Traffic Electronic bulldozer Flushing the queue ahead of the bus = tracking queue length (Zurich) 14

In Mixed Traffic, Near Saturation Detectors & logic for queue management – Stopped cars, not moving cars, hinder buses 15 (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 16

17

18 Green Time Distribution for EBL No Priority Proportion Avg Green (EBL) = 17.8s Avg Green (WBT) = 30.3s p (max-out) = Avg Green (WBT) = 29.8s p (max-out) = p (extended) = Avg Green (EBL) = 18.1s Proportion With Green Extension Extended Green

Priority Push Extension Increment no priority, uniform arrivals R = effective red C = cycle length v = arrival rate s = discharge rate 19

with priority X = green extn Priority push! 20

Priority Push vs. Extension Increment (cycle length = 100 s, red time = 50 s, degree of saturation = 85%) 21

Priority Push vs. Red Time (cycle length = 100 s, extension increment = 15 s, degree of saturation = 85%) 22

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 23

Early Red Incompatible with typical coordination logic –custom programming Shorten bus streets current green to get faster return to green in the next cycle, or red light during stop for pedestrian safety 24

Phase Rotation and Insertion Dynamically change leading left to lagging left 25 Second realization on bus detection only

26

27 10 s inserted EBL phase: only consumes 2.5 s Green Extension Only Proportion Extended Green Avg Green (EBL) = 18.1s Avg Green (WBT) = 29.8s Avg Green (insertion) = 4.4s Avg Green (WBT) = 27.3s p (insertion) = Extended Green Primary Phase, when Phase Insertion is Programmed Avg Green (primary) = 14.7s Avg Green (total) = 19.1s Proportion 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 buss 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 29 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 30 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 31

Self-Organizing Coordination No fixed cycle length Each signals start of green becomes a request to downstream signal –Peer-to-peer communication between signals –upstream signals request has lower priority that bus request Result: spontaneous green wave Inherently interruptible 32 Simulated for San Juan, Puerto Rico

33

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 35

1 Bus Delays with Incremental Priority Treatments, by Route

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 37