7.4 Recall Modes, Volume-Density Control & Parameter Diagnostics

7.4 Recall Modes, Volume-Density Control & Parameter Diagnostics

1. Controller Recall Modes & Operating Characteristics

Recall settings force the controller to service specific phases regardless of real-time detector inputs. Modern ATC and NEMA TS2 controllers support five distinct recall parameters:

In-Depth Analysis: Soft Recall Mechanics

Soft Recall is unique because it introduces zero delay penalties to conflicting movements:

• When all conflicting calls have been serviced and side streets gap out, the controller returns right-of-way to the Soft Recall phase (typically Phases 2 and 6) and rests in green.
• Unlike Minimum or Maximum Recall, Soft Recall does not place an active demand in the background. If a side-street vehicle arrives on Phase 4 while the controller is resting in Phase 2 green, the controller does not require an actuation to clear—it can immediately time yellow and red clearance and hand over control without waiting for a minimum green extension.

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2. Volume-Density Control: Variable Initial & Dynamic Gap Reduction

On high-speed arterial approaches (speeds $\ge 40\text{ mph}$ / $65\text{ km/h}$), advance detectors must be placed 250 to 500 feet upstream to provide dilemma zone protection. Standard fixed actuated timing breaks down under two scenarios:

1. Queues formed between the stop line and the advance detector during red cannot be cleared by a brief fixed minimum green.
2. A long fixed passage time (e.g., $4.5\text{ seconds}$ required to cover 300 feet) causes the phase to hold green during sluggish traffic, inflicting massive, unnecessary delay on waiting side-street queues.

Volume-density control introduces dynamic timing features to solve both challenges.

+-----------------------------------------------------------------------------+
|                   DYNAMIC GAP REDUCTION TIMING CURVE                        |
+-----------------------------------------------------------------------------+
| Passage Gap (s)                                                             |
|    |                                                                        |
| Max Gap (e.g., 4.5s) ---------\                                             |
|    |                           \  <- Linear Reduction Rate:                 |
|    |                            \    (Max Gap - Min Gap) / TTR              |
|    |                             \                                          |
| Min Gap (e.g., 1.8s) -------------+---------------------------->            |
|    |                              |                            |            |
|    +------------------------------+----------------------------+----------> |
|    0                     Time Before Reduction        Time to Reduce  Time  |
|                                 (TBR)                      (TTR)            |
|    <--- Conflicting Call Begins                                             |
+-----------------------------------------------------------------------------+

1. Variable Initial Timing (Queue Clearance Mechanics)

Variable Initial dynamically calculates the required initial green duration based on the actual number of vehicles stored between the stop line and the advance detector:

• Minimum Initial ($G_{\min_init}$): The baseline minimum green duration (typically 8 to 10 seconds).
• Added Initial ($t_{\text{added}}$): The increment of green time added for each vehicle actuation received while the phase displays YELLOW or RED (typically 1.5 to 2.0 seconds per actuation).
• Maximum Initial ($G_{\max_init}$): The absolute maximum duration to which the initial interval can expand (typically 25 to 35 seconds).

The Computed Initial Algorithm

The controller monitors actuations during the red interval and evaluates: $G_{\text{computed}} = N_{\text{red\_actuations}} \times t_{\text{added}}$ $G_{\text{initial}} = \min\left(G_{\max\_init}, \max\left(G_{\min\_init}, G_{\text{computed}}\right)\right)$

• Field Calculation: If $G_{\min_init} = 8\text{ s}$, $t_{\text{added}} = 1.8\text{ s/veh}$, and $G_{\max_init} = 30\text{ s}$. During a long red phase, 12 vehicles cross the advance detector: $G_{\text{computed}} = 12 \times 1.8\text{ s} = 21.6\text{ seconds}$ Since $8 < 21.6 < 30$, the controller automatically allocates an initial green of 21.6 seconds before passage timer countdown commences, completely clearing the standing queue!

2. Dynamic Gap Reduction Mechanics

Instead of retaining a wide passage gap indefinitely, Gap Reduction dynamically lowers the extension threshold as waiting drivers on conflicting approaches experience mounting delay:

• Passage Time (Max Gap): A generous initial gap (typically 3.5 to 5.0 seconds) that guarantees dilemma zone protection for initial vehicle platoons.
• Time Before Reduction (TBR): The delay buffer (typically 10 to 20 seconds) initiated the instant a conflicting call is registered. During TBR, the allowable gap remains locked at Max Gap.
• Time to Reduce (TTR): The duration (typically 15 to 30 seconds) over which the controller linearly decrements the allowable passage gap from Max Gap down to Min Gap.
• Minimum Gap: The lowest allowable headway threshold (typically 1.5 to 2.0 seconds), representing dense, saturated flow. If the time between vehicles exceeds Min Gap after TTR has elapsed, the phase immediately gaps out.

Mathematical Formulation of Dynamic Gap

For any elapsed time $t$ after the registration of a conflicting call: $\text{Gap}(t) = PT_{\max} \quad \text{for } t \le TBR$ $\text{Gap}(t) = PT_{\max} - \left(\frac{PT_{\max} - PT_{\min}}{TTR}\right) \cdot (t - TBR) \quad \text{for } TBR < t < (TBR + TTR)$ $\text{Gap}(t) = PT_{\min} \quad \text{for } t \ge (TBR + TTR)$

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3. Controller Parameter Configuration & Field Diagnostics Table

Improper controller parameter configuration degrades capacity and introduces severe safety hazards: