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The Delights and Evils of Traffic Signal Coordination

The Delights and Evils of Traffic Signal Coordination. Peter Furth Northeastern University NACTO Training “Street Design 301: Transit Friendly Signals” September, 2019. One-Way Coordination (Green Wave): Offset  Travel time from reference intersection.

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The Delights and Evils of Traffic Signal Coordination

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  1. The Delights and Evils of Traffic Signal Coordination Peter Furth Northeastern University NACTO Training “Street Design 301: Transit Friendly Signals” September, 2019

  2. One-Way Coordination (Green Wave): Offset  Travel time from reference intersection

  3. Draw green intervals, centered on offsets See the green wave

  4. 2-Way Coordination (ideal): Cycle length= Round Trip Travel Time between neighboring intersection Offset = one-way travel time from reference intersection (use A St) Coordinate for a bus route. Suppose stop delay = 30 s (delay for accel / decel, dwell, etc.) Cycle length for 2-way coordination is C = 15 + 15 + 30 = 60 s. Offset at A = 0 (reference intersection) Offset at B = travel time, A to B = 15 + 30 = 45 s A St B St travel time = 15 s Stop east Stop

  5. Mark offsets for 2 or 3 cycles, making A St the reference int’n A St B St Time (s) 0 45 60 45 +60 = 105 120

  6. Draw trajectories (time goes down) A St B St Time (s) 0 45 60 45 +60 = 105 120

  7. Draw green intervals centered on the offsets – say, 20 s at A and 26 s at B A St B St Time (s) 0 45 60 45 +60 = 105 120

  8. The Evils of Signal Coordination (as typically applied) 1. It usually comes with a Long Cycle (100 s, 120 s, 240 s) WHY: • Corridor will have a “Critical intersection” that needs a longer cycle for capacity; all intersections must then get that same cycle. • With irregular intersection spacing, long cycles enable better green waves OUTCOMES: • Long delay for all except arterial thru traffic (peds, transit, local traffic) • Poor compliance by pedestrians & cyclists • Renders transit signal priority ineffective • Speeding opportunities due to abundant excess green

  9. The Evils of Signal Coordination (cont’d) 2. It is inflexible • Can’t adapt to changes in traffic volume • You’re always running the right plan for the wrong time • Can’t provide “just-in-time” green for transit 3. Where intersection spacing is not ideal (and it never is), and where cars turn on and off the arterial (and they always do), coordination isn’t actually very effective!

  10. 2-way Coordination with Short Blocks: Ineffective, and Promotes Speeding Involves clusters of intersections with simultaneous green Larger clusters with • shorter blocks • longer cycle length • higher progression speed Only a small part of the green belongs to the green wave Works well only when traffic is light Large clusters have a speeding incentive Cluster 2 Cluster 1

  11. Alternatives to Traditional Coordination • Small coordination zones of 2 or 3 intersections, each with its own tailored cycle length • Fully actuated control (“run free”) – especially where signals are spaced 1000 ft (300 m) or more Mass. Ave., Boston’s South End (7 intersections) Simulation study results

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