5.1 Coordinated Signal Concepts and Terminology

1.1 Coordinated Signal Concepts & Terminology

Traffic signal coordination is the process of synchronizing multiple traffic signals along a corridor or within a grid network so that main-street vehicle platoons can travel continuously through successive intersections with minimal stops and delay. When traffic signals operate independently (isolated actuation), vehicles arriving at downstream intersections frequently encounter red lights, causing high fuel consumption, increased tailpipe emissions, and elevated rear-end collision risks. Coordinated signal systems establish a structured temporal relationship across adjacent intersections to group traffic into organized platoons and provide continuous progression.

To achieve effective coordination, field technicians and signal engineers must master five primary operational parameters: cycle length, split, offset, yield point, and system reference point.

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Background Cycle Length ($C$)

The background cycle length ($C$) is the total time in seconds required for a traffic signal controller to complete one full sequence of all phase indications (green, yellow change, and red clearance) across all rings.

The Common Cycle Length Rule

A fundamental law of traffic signal coordination is that all controllers operating within a coordinated group or subsystem must share the exact same background cycle length, or an exact sub-multiple (such as a half-cycle). If one controller operates on a 90-second cycle while an adjacent controller operates on a 100-second cycle, the time relationship between their green indications constantly shifts, making continuous green progression impossible.

Cycle Length Determination & Selection

Cycle length is dictated by the critical intersection—the intersection along the corridor requiring the greatest amount of green time to satisfy volume-to-capacity demands and pedestrian clearance requirements. Standard cycle lengths in North America typically range from 60 seconds to 180 seconds:

• Short Cycles (60–90 seconds): Ideal for low-to-moderate volume arterials, off-peak periods, and corridors with short intersection spacing. Short cycles minimize overall delay for side-street traffic and pedestrians.
• Medium Cycles (90–120 seconds): Standard for moderate-to-heavy arterial corridors during peak hours, balancing main-street platoon bandwidth with acceptable side-street waiting times.
• Long Cycles (120–180 seconds): Required for complex, high-volume intersections with multiple left-turn phases, heavy cross-street volumes, or wide pedestrian crossings. However, excessively long cycles increase average network delay and encourage pedestrian non-compliance.

$\text{Total Lost Time per Cycle } (L) = n \times t_L$

Where $n$ is the number of phase changes per cycle and $t_L$ is the start-up and clearance lost time per phase (typically 3 to 5 seconds).

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Phase Splits ($S$)

A phase split ($S$) is the portion of the background cycle length allocated to a specific traffic phase or movement, expressed either in seconds or as a percentage of the total cycle length:

$S_i\% = \left( \frac{S_i \text{ (seconds)}}{C} \right) \times 100\%$

Composition of a Phase Split

Every phase split consists of three distinct intervals:

1. Phase Green Interval ($g$): The time allocated for vehicles to proceed through the intersection.
2. Yellow Change Interval ($y$): The warning interval informing drivers that the green signal is terminating, typically timed between 3.0 and 6.0 seconds based on approach speed ($V_{approach}$).
3. Red Clearance Interval ($r_{cr}$): The all-red buffer interval providing safe intersection clearance before conflicting movements receive green, typically timed between 1.0 and 3.0 seconds.

$S_i = g_i + y_i + r_{cr\_i}$

Dual-Ring Split Constraints

In standard NEMA dual-ring controller architecture, the sum of splits for all non-overlapping phases along Ring 1 must equal the sum of splits along Ring 2, and both must equal 100% of the background cycle length (or $C$ seconds). Barriers within the dual-ring structure enforce synchronized phase transitions across conflicting movements.

Pedestrian Clearance Requirements

Phase splits assigned to movements with pedestrian crossings must equal or exceed the minimum required pedestrian timing:

$\text{Split}_{min} \ge \text{Walk} + \text{Flashing Don't Walk (FDW)} + \text{Yellow} + \text{Red Clearance}$

If an allocated split is less than the required pedestrian timing, a pedestrian call will force the controller to extend green beyond its allocated split, causing the controller to temporarily drop out of coordination (enter "transition mode").

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System Reference Point & Master Sync

The system reference point (also called system sync zero or master zero) is the absolute time origin ($t = 0$) established across a network of coordinated traffic signals.

Timebase Synchronization

Modern traffic signal networks use Distributed Time Base Control (TBC) synchronized via Global Positioning System (GPS) receivers using NMEA 0183 / NTP time protocols or central Advanced Traffic Management System (ATMS) software master clocks. In standard field deployments, the system reference point resets automatically every day at midnight (00:00:00). The local background cycle timer in each controller continuously counts up from 0 to $C$ seconds and rolls over, ensuring all controllers maintain a synchronized reference clock without requiring hardwired interconnect cables.

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Signal Offset ($O$)

The signal offset ($O$) is the time relationship, expressed in seconds or percentage of cycle length, between a designated point in the local intersection cycle and the system reference point ($t = 0$). Offset determines when main-street green begins relative to adjacent intersections, allowing vehicle platoons to move seamlessly along the corridor.

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Yield Point & Actuated Coordinated Mechanics

In actuated-coordinated signal controllers (such as NEMA TS 1, NEMA TS 2, Model 170, Model 2070, and ATC units), the main-street movements (typically Phase 2 and Phase 6) are designated as coordinated phases. These phases are generally non-actuated or operate in recall, holding green until recalled by side-street demand.

Operational Role of the Yield Point

The yield point is a specific point in the background cycle (typically occurring near the end of main-street green) where the controller is permitted to yield main-street green and transition to service active vehicle or pedestrian calls on non-coordinated phases (side streets and turn pockets).

• If calls are present: The controller yields green at the yield point and sequences through active non-coordinated phases subject to their configured force-off points.
• If no calls are present: The controller ignores the yield point and dwells (holds green) on the coordinated main-street phases, ensuring main-street green remains active until side-street demand appears.

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Parameter Summary Table