Coordinated Arterial with Progression Band Failure
A 2.4 km arterial with six signalized intersections is coordinated with a 90 s cycle and a 60 s green band (progression band) at 50 km/h. The offsets were set for the original 50 km/h progression. The city has since lowered the speed limit to 40 km/h and added a mid-block pedestrian crossing with a signal that is not coordinated. Field observation shows platoons arriving at intersections 3 and 4 during the red, causing stop-and-go conditions and increased delay. The engineer is asked to restore progression without changing the cycle length.
1. Explain why the progression band has failed and how the uncoordinated pedestrian signal affects it.
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The progression band was designed for 50 km/h, but the speed limit is now 40 km/h. The offset between intersections is the travel time at the design speed. At 50 km/h (13.9 m/s), the travel time over a 400 m block is 400/13.9 ≈ 28.8 s. At 40 km/h (11.1 m/s), the travel time is 400/11.1 ≈ 36 s. The difference of ~7 s per block accumulates over six intersections, so platoons arrive progressively later than the green band, eventually arriving during red at intersections 3 and 4.
The uncoordinated pedestrian signal is a random interruption: it stops platoons for a pedestrian phase that is not synchronized with the arterial offsets. Each stop adds delay and breaks the platoon, so even if the offsets were correct, the pedestrian signal would disrupt progression.
Key point: the offset must match the actual travel time at the operating speed, and any uncoordinated signal in the corridor breaks the progression band.
2. Recommend how to restore progression, including offset recalculation and treatment of the pedestrian signal.
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Recommendations:
- Recalculate offsets for 40 km/h. For each block, offset = travel time at 40 km/h (distance / 11.1 m/s). If blocks are 400 m, the offset is ~36 s. Use a time-space diagram to verify the band.
- Coordinate the pedestrian signal with the arterial: either bring it into the coordination plan (assign it a phase in the cycle) or convert it to a pedestrian-actuated signal with a maximum recall that fits the cycle. If it remains uncoordinated, it will continue to break platoons.
- Consider a shorter cycle if the 90 s cycle is too long for the corridor; but the problem states the cycle cannot change, so focus on offsets.
- Verify the band width: with a 90 s cycle and 60 s band, the band is 67% of the cycle. Recompute the band at 40 km/h to ensure it still provides progression.
Verification: collect travel time runs before and after, and check arrival-on-green at each intersection. The goal is to have platoons arrive during green at all six intersections.