12.4 Automated Traffic Signal Performance Measures (ATSPMs) & High-Resolution Data

12.4 Automated Traffic Signal Performance Measures (ATSPMs) & High-Resolution Data

PTOE Exam Focus: ATSPMs and high-resolution controller data represent a major modern advancement in Domain 4. Candidates must understand high-resolution controller event logging (100 ms / 10 Hz timestamps), Purdue Coordination Diagrams (PCDs), Platoon Ratio calculations ($R_p = \frac{\text{PAG}}{g/C}$), Purdue Split Failure (PSF) metrics (GOR and $\text{ROR}_5$), phase termination diagnostics (Gap-Out, Max-Out, Force-Off), and the FHWA Every Day Counts (EDC-4) framework.

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1. High-Resolution Controller Event Logging & The Indiana Protocol

Traditionally, traffic signal operations were evaluated using infrequent, labor-intensive manual floating-car travel time studies or isolated 24-hour turning movement counts. Automated Traffic Signal Performance Measures (ATSPMs)—developed by Purdue University and the Indiana Department of Transportation (INDOT), and championed under FHWA's Every Day Counts (EDC-4) initiative—revolutionized signal operations.

Modern traffic signal controllers log internal state transitions and external detector actuations at 100-millisecond ($10\text{ Hz}$) resolution. Each logged entry contains:

1. Timestamp: Exact date and time to the nearest $0.1\text{ second}$.
2. Event Code: Standardized integer enumeration representing the controller state change.
3. Event Parameter: Phase number, detector channel, or overlap ID associated with the event.

+-----------------------------------------------------------------------------------+
|                SAMPLE HIGH-RESOLUTION CONTROLLER EVENT LOG (100 ms)               |
|                                                                                   |
|  Timestamp                Event Code    Parameter    Description                  |
|  -------------------------------------------------------------------------------  |
|  2026-08-14 07:15:02.100  1             2            Phase 2 Green ON             |
|  2026-08-14 07:15:04.300  82            12           Detector Channel 12 ON (Veh) |
|  2026-08-14 07:15:04.700  81            12           Detector Channel 12 OFF      |
|  2026-08-14 07:15:32.400  7             2            Phase 2 Green Term (Gap-Out) |
|  2026-08-14 07:15:32.400  8             2            Phase 2 Yellow ON            |
|  2026-08-14 07:15:36.900  11            2            Phase 2 Red ON               |
+-----------------------------------------------------------------------------------+

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2. Purdue Coordination Diagrams (PCDs)

The Purdue Coordination Diagram (PCD) is the foundational ATSPM visualization for evaluating signal progression and coordination quality. A PCD plots individual vehicle arrivals (recorded at advance setback detectors, typically $300\text{ to }500\text{ ft}$ upstream) relative to the start of the coordinated phase green interval for every cycle over a $24\text{-hour}$ period.

  Cycle Time (s) ^
             100 | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (Red Interval)
                 |   .   .   .  .   .  .   .  .   .  .   .  .   .  .   .  .   .  .
              60 |----------------------------------------------------------------- (Yellow / Red Onset)
                 |       * * * * * * * * * * * * * * * * * * * * *                 
                 |     * * * * * * * * * * * * * * * * * * * * * * *   <--- DENSE PLATOON ON GREEN
              20 |---* * * * * * * * * * * * * * * * * * * * * * * * -------------- (Green Start)
                 | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (Red Arrivals)
               0 +-----------------------------------------------------------------> Time of Day (24h)
                 00:00        06:00        12:00        18:00        24:00

Key Information Extracted from PCDs:

1. Arrivals on Green (AoG) vs. Arrivals on Red (AoR): Dots falling between the green onset line and yellow onset line represent vehicles arriving during green; dots outside represent arrivals during red.
2. Platoon Cohesion & Offset Quality: A tight cluster of vehicle arrivals starting immediately after green onset indicates an optimal progression offset. A platoon arriving before green onset reveals an offset that is "too late," forcing the platoon to stop and clear as a standing queue.

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3. Platoon Ratio ($R_p$) & Arrivals on Green (AoG)

The Platoon Ratio ($R_p$)—defined in the Highway Capacity Manual (HCM)—quantifies the degree of vehicle platoon clustering arriving during the green interval:

$$R_p = \frac{\text{PAG}}{g/C}$$

Where:

• $\text{PAG}$ = Percent of vehicles Arriving on Green ($\text{PAG} = \frac{\text{Arrivals on Green}}{\text{Total Cycle Arrivals}}$)
• $g/C$ = Effective green ratio of the coordinated phase (effective green time $g$ divided by cycle length $C$)

HCM Arrival Type Classification Matrix:

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4. Purdue Split Failure (PSF) Metric

A Split Failure occurs when a signal phase does not provide sufficient green time to clear the standing queue, causing unserved vehicles to wait across multiple red cycles. ATSPMs detect split failures using stop-bar detector occupancy data:

$$\text{Green Occupancy Ratio (GOR)} = \frac{\text{Time Detector Occupied During Green}}{\text{Total Green Duration}}$$ $$\text{Red Occupancy Ratio in First 5s } (\text{ROR}_5) = \frac{\text{Time Detector Occupied in First 5s of Red}}{5.0\text{ seconds}}$$

  Green Occupancy Ratio (GOR) ^
                         100% |-------------------------------+-------------------------------+
                              |  QUADRANT II:                 |  QUADRANT I:                  |
                              |  Heavy Green Demand,          |  TRUE SPLIT FAILURE           |
                              |  Queue Cleared at Yellow      |  GOR >= 80% AND ROR_5 >= 80%  |
                          80% |===============================+===============================|
                              |  QUADRANT III:                |  QUADRANT IV:                 |
                              |  Undersaturated Demand,       |  Slow Clear / Trapped Veh     |
                              |  Low Occupancy                |  (False Split Failure)        |
                           0% +-------------------------------+------------------------------->
                              0%                             80%                            100%
                                      Red Occupancy Ratio in First 5s (ROR_5)

Split Failure Engineering Criterion:

A phase experiences a Purdue Split Failure if and only if:

$$\text{GOR} \ge 80\% \quad \text{AND} \quad \text{ROR}_5 \ge 80\%$$

• If $\text{GOR} \ge 80\%$ but $\text{ROR}_5 < 80\%$, the phase was fully utilized but successfully dissipated the queue at the end of green (no split failure).
• Persistent split failures during peak hours indicate inadequate green split allocation or downstream queue spillback blockage.

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5. Purdue Phase Diagrams (PPDs) & Phase Termination Diagnostics

Purdue Phase Diagrams track the duration and termination type of every phase green interval over time. Controllers log three distinct termination modes:

1. Gap-Out (Code 7): Green terminated normally when vehicle headways exceeded the passage time timer. High gap-out % indicates sufficient capacity.
2. Max-Out (Code 6): Green reached its $G_{\max}$ limit while vehicle calls were still actively extending the phase. High max-out % indicates oversaturation or split deficit.
3. Force-Off (Code 9): Coordinated controller forcibly terminated an actuated non-coordinated phase at its scheduled cycle ring boundary.

Indiana / Purdue High-Resolution Controller Event Enumeration Codes Reference

6. Worked Calculation Example: Platoon Ratio & Split Failure Diagnostic

Problem Statement:

A coordinated arterial intersection operates on a cycle length of $C = 100\text{ seconds}$. The coordinated Phase 2 through movement receives an effective green time of $g = 40\text{ seconds}$. High-resolution ATSPM advance detector data records $1,200\text{ vehicles}$ arriving on the approach during the peak hour, of which $780\text{ vehicles}$ arrive while the green indication is active.

Simultaneously, ATSPM stop-bar detector analysis on minor cross-street Phase 4 during a specific cycle shows:

• Phase 4 Green duration: $G = 15\text{ seconds}$
• Stop-bar detector occupied during green: $13.2\text{ seconds}$
• Stop-bar detector occupied during first 5 seconds of red: $4.4\text{ seconds}$

1. Calculate the green ratio ($g/C$) and the Percent of Arrivals on Green ($\text{PAG}$).
2. Compute the Platoon Ratio ($R_p$) and identify the corresponding HCM Arrival Type.
3. Compute the Green Occupancy Ratio ($\text{GOR}$) and Red Occupancy Ratio in the first 5 seconds ($\text{ROR}_5$) for Phase 4, and determine if a Purdue Split Failure occurred.

Step-by-Step Solution:

1. Green Ratio ($g/C$) and Percent Arrivals on Green ($\text{PAG}$):

   $$\frac{g}{C} = \frac{40\text{ s}}{100\text{ s}} = 0.40$$ $$\text{PAG} = \frac{\text{Arrivals on Green}}{\text{Total Arrivals}} = \frac{780\text{ veh}}{1200\text{ veh}} = 0.65\text{ (or }65.0\%\text{)}$$

2. Platoon Ratio ($R_p$):

   $$R_p = \frac{\text{PAG}}{g/C} = \frac{0.65}{0.40} = 1.625$$

   Assessment: Since $1.50 < R_p \le 2.00$, the approach operates at HCM Arrival Type 5 (Highly Favorable Progression with dense platoon arrival on green).

3. Phase 4 Split Failure Evaluation:

   $$\text{GOR} = \frac{13.2\text{ s}}{15.0\text{ s}} = 0.88 = 88.0\%$$ $$\text{ROR}_5 = \frac{4.4\text{ s}}{5.0\text{ s}} = 0.88 = 88.0\%$$

   Diagnostic: Since both $\text{GOR} \ge 80\%$ ($88\%$) and $\text{ROR}_5 \ge 80\%$ ($88\%$), Phase 4 experienced a verified Purdue Split Failure.