2.7 Automated Traffic Signal Performance Measures (ATSPMs)
Key Takeaways
- Automated Traffic Signal Performance Measures (ATSPMs) replace costly, periodic manual counts with continuous, high-resolution controller data logging that enumerates every phase and detector state transition at 100-millisecond (0.1 second) precision.
- Standardized NEMA TS2 v03 and ATC 5201 controller event logging enumerates specific operational events, including Event 0 (Phase Begins Green), Event 7 (Phase Begins Yellow), Event 10 (Phase Begins Red), Event 81 (Detector On), and Event 82 (Detector Off).
- Purdue Coordination Diagrams (PCD) plot vehicle arrival actuations against cycle progression over time of day, visualizing the percentage of Arrivals on Green (AOG) and enabling precise offset adjustments without field drive-runs.
- The Platoon Ratio (Rp = AOG / [g/C]) quantifies coordination effectiveness: values greater than 1.0 indicate favorable platoon arrivals during the green interval, while values below 1.0 signify adverse arrivals during red.
- Purdue Split Failure metrics identify unserved phase demand when a phase registers high detector occupancy during the green interval (Green Occupancy Ratio > 0.80) coupled with high occupancy during the first 5 seconds of the subsequent red interval (Red Occupancy Ratio > 0.80).
2.7 Automated Traffic Signal Performance Measures (ATSPMs)
[!NOTE] IMSA Level III Examination Focus: Senior technicians must master the engineering principles of Automated Traffic Signal Performance Measures (ATSPMs). This includes understanding the 100-millisecond controller enumeration engine, mapping critical Indiana DOT/Purdue event codes (0, 7, 10, 81, 82), analyzing Purdue Coordination Diagrams (PCDs), calculating Platoon Ratios, and diagnosing field detector failures via automated analytics.
Historically, evaluating traffic signal timing required manual floating-car travel time studies, turning movement counts recorded by technicians sitting with count boards, and citizen complaint investigations. These manual methods were labor-intensive, expensive, and captured only a tiny snapshot of intersection operations. In response, the transportation industry—spearheaded by the Indiana Department of Transportation (INDOT), Purdue University, and the Utah Department of Transportation (UDOT)—developed Automated Traffic Signal Performance Measures (ATSPMs).
ATSPMs transform the traffic signal controller from a simple phase-switching device into a continuous, high-resolution data logger that records every operational event in real time, delivering objective analytics on safety, mobility, and physical hardware health.
High-Resolution Controller Data Logging Engine
At the core of ATSPM technology is the High-Resolution Controller Data Logging Engine, standardized in NEMA TS 2 (current edition, TS 2-2021) v03 and ATC 5201:
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| 100-MILLISECOND EVENT ENUMERATION ENGINE |
+-----------------------------------------------------------------------------+
| |
| Every internal state change generates a 4-Byte Binary Log Entry: |
| |
| [ 32-Bit Unsigned Integer Timestamp ] --> Precision: Exactly 100 ms |
| [ 16-Bit Event Code ] --> Action (e.g., Phase Green) |
| [ 16-Bit Event Parameter ] --> Identifier (e.g., Phase 2) |
| |
| Internal Controller FIFO Memory Buffer: |
| - Retains 50,000 to 500,000 chronological event records |
| - Automatically pushed via SFTP / Syslog to Central ATSPM Database Hourly |
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The 100-Millisecond Resolution Standard
Traditional central signal systems polled controllers every 1 to 5 seconds via low-bandwidth serial modems. This coarse sampling missed individual vehicle actuations and exact clearance interval transitions. The ATSPM standard mandates a timestamp precision of exactly 100 milliseconds (0.1 seconds). Every time a phase changes state, a detector is actuated, or a pedestrian presses a button, the controller generates a structured binary event log.
Standardized Controller Event Codes
The standardized enumeration schema assigns numeric codes to specific operational actions. Senior technicians must be intimately familiar with the core event enumerations:
| Event Code | Event Description | Parameter Meaning | Operational Significance |
|---|---|---|---|
| 0 | Phase Begins Green | Phase Number (1–16) | Marks start of green interval for split and coordination analysis |
| 1 | Phase Min Complete | Phase Number (1–16) | Minimum green timer has expired; phase is now eligible to gap out |
| 4 | Phase Gap Out | Phase Number (1–16) | Vehicle headway exceeded passage time; phase terminated due to lack of demand |
| 5 | Phase Max Out | Phase Number (1–16) | Phase reached maximum green limit under continuous demand; split failure indicator |
| 6 | Phase Force Off | Phase Number (1–16) | Coordinated phase forced to yield by master coordination background timer |
| 7 | Phase Begins Yellow | Phase Number (1–16) | Marks start of yellow change clearance interval |
| 10 | Phase Begins Red | Phase Number (1–16) | Marks start of red clearance / red dwell interval |
| 21 | Pedestrian Walk Begins | Phase Number (1–16) | Pedestrian Walk interval active |
| 22 | Ped Clearance Begins | Phase Number (1–16) | Flashing Don't Walk interval active |
| 45 | Pedestrian Call Registered | Phase Number (1–16) | Pushbutton pressed by pedestrian; call placed in controller |
| 81 | Detector On (Actuation) | Detector Channel (1–64) | Vehicle enters detection zone; start of presence actuation |
| 82 | Detector Off (Release) | Detector Channel (1–64) | Vehicle departs detection zone; end of presence actuation |
| 102 | Preempt State Begins | Preempt Channel (1–10) | Emergency vehicle or railroad preemption call active |
Purdue Coordination Diagrams (PCD)
The Purdue Coordination Diagram (PCD) is the cornerstone ATSPM metric for evaluating and optimizing coordinated signal progression along arterial corridors.
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| PURDUE COORDINATION DIAGRAM (PCD) |
+-----------------------------------------------------------------------------+
| |
| Cycle Time (Seconds) |
| 120 | |
| | . . . . . . . . . . . . . . . . . <- Vehicle Arrivals on |
| 100 | . . . . . . . . . . . . . . . . . . . . . RED (Early Platoon) |
| |------------------------------------------ (Start of Red Interval) |
| 80 | |
| | |
| 60 | . . . . . . . . . . . . . . . . . . . . <- Vehicle Arrivals on |
| | . . . . . . . . . . . . . . . . . . . . . GREEN (Ideal Coor) |
| 40 |------------------------------------------ (Start of Green Interval)|
| | |
| 20 | |
| | |
| 0 +------------------------------------------ |
| 06:00 07:00 08:00 09:00 10:00 (Time of Day) |
| |
| DETECTOR DOT OVERLAY: |
| - Every dot represents an individual vehicle passing the advance detector.|
| - Horizontal axis: Time of day across the peak period. |
| - Vertical axis: Time elapsed within the local cycle (0 to Cycle Length C)|
| - Shaded Bands: Green interval (40s to 80s), Yellow/Red (80s to 120s). |
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Mechanics of the Purdue Coordination Diagram
- Data Source: PCDs require an Advance Detector (typically an inductive loop, radar zone, or video detector) placed 300 to 500 feet (90 to 150 m) upstream of the stop bar. This distance is vital: the detector must capture vehicle arrivals before they join the back of a standing queue.
- Graph Construction: The horizontal axis (X-axis) spans the time of day (e.g., 06:00 to 10:00 AM). The vertical axis (Y-axis) spans the coordinated cycle length (e.g., 0 to 120 seconds). For every cycle, the controller plots the start of green, yellow, and red intervals as continuous horizontal bands.
- Vehicle Arrival Mapping: When a vehicle crosses the advance detector (Event 81), a single point is plotted at that exact time of day and cycle second.
- Visual Progression Analysis:
- Optimal Progression: The dense band of vehicle arrival dots falls entirely within the green interval band. Platoons arrive without stopping, maximizing corridor throughput.
- Adverse Progression (Early Arrival): The platoon dots cluster in the upper red band immediately prior to green. Vehicles arrive too early, hitting a red signal and forming a queue that degrades flow.
- Adverse Progression (Late Arrival): The platoon arrives toward the end of green and spills into the yellow and red intervals, causing mid-platoon cutoffs.
Quantitative ATSPM Metrics & Calculations
ATSPMs convert raw graphical plots into rigorous mathematical performance indices:
1. Arrivals on Green (AOG)
Arrivals on Green measures the percentage of total approaching vehicles that arrive during the green signal display:
Where $V_g$ is the volume of vehicles detected during the green interval, and $V_{\text{total}}$ is total approach volume across the cycle. Arterial coordination goals typically target an AOG exceeding 70% to 80% during peak commuter periods.
2. Platoon Ratio ($R_p$)
The Platoon Ratio ($R_p$), standardized in the Highway Capacity Manual (HCM), normalizes the proportion of vehicles arriving on green relative to the green-to-cycle ratio ($g/C$):
Where:
- $P$ is the proportion of vehicles arriving during the green interval ($V_g / V_{\text{total}}$).
- $g$ is the effective green time in seconds.
- $C$ is the cycle length in seconds.
+-----------------------------------------------------------------------------+
| PLATOON RATIO (Rp) INTERPRETATION |
+-----------------------------------------------------------------------------+
| Platoon Ratio (Rp) | HCM Arrival Type | Progression Quality |
|----------------------+------------------+-----------------------------------|
| Rp < 0.50 | Arrival Type 1 | Very Poor (Platoon arrives on red)|
| 0.50 <= Rp < 0.85 | Arrival Type 2 | Unfavorable (Poor coordination) |
| 0.85 <= Rp <= 1.15 | Arrival Type 3 | Random Arrivals (Isolated signal) |
| 1.15 < Rp <= 1.50 | Arrival Type 4 | Favorable (Good platoon on green) |
| 1.50 < Rp <= 2.00 | Arrival Type 5 | Highly Favorable (Exceptional) |
| Rp > 2.00 | Arrival Type 6 | Exceptional (Dense platoon/green) |
+-----------------------------------------------------------------------------+
- $R_p > 1.0$: Indicates favorable coordination. A higher percentage of vehicles arrive on green than would occur under purely random traffic conditions.
- $R_p = 1.0$: Represents random arrivals ($P = g/C$), typical of isolated, uncoordinated actuated intersections.
- $R_p < 1.0$: Indicates adverse coordination. The arrival platoon is arriving predominantly during the red interval, demonstrating poor offset timing.
3. Purdue Split Failure Metric
A Split Failure (or phase failure) occurs when traffic demand on an approach exceeds the green phase capacity, leaving residual queued vehicles stranded at the stop bar when the signal turns red.
+-----------------------------------------------------------------------------+
| PURDUE SPLIT FAILURE MEASUREMENT LOGIC |
+-----------------------------------------------------------------------------+
| |
| Stop Bar Presence Detector Actuation Analysis: |
| |
| 1. Green Occupancy Ratio (GOR): |
| GOR = (Total Time Detector Occupied during Green) / (Green Duration) |
| |
| 2. Red Occupancy Ratio in First 5 Seconds of Red (ROR_5): |
| ROR_5 = (Time Detector Occupied during First 5s of Red) / (5.0 Seconds) |
| |
| SPLIT FAILURE OCCURS WHEN BOTH CONDITIONS ARE MET: |
| [ GOR > 0.80 (80%) ] AND [ ROR_5 > 0.80 (80%) ] |
| |
| Interpretation: |
| The green phase was fully saturated (GOR > 80%), AND when the light |
| turned red, vehicles were still standing on the stop bar detector |
| during the first 5 seconds of red (ROR_5 > 80%), proving unserved queue. |
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If an approach consistently logs split failures (>20% of cycles) during peak hours, signal technicians must evaluate increasing the phase split duration, reallocating unneeded green time from minor phases, or implementing dynamic split management.
Automated Hardware Health & Maintenance Analytics
Beyond timing optimization, ATSPM platforms serve as continuous automated diagnostic monitors for physical cabinet hardware, alerting technicians to failures before citizens report them.
+-----------------------------------------------------------------------------+
| AUTOMATED ATSPM DETECTOR HEALTH ANALYTICS |
+-----------------------------------------------------------------------------+
| |
| 1. STUCK / LOCKED DETECTOR ALARM: |
| - Event 81 (Detector On) with no Event 82 (Off) for > 30 minutes. |
| - Cause: Failed inductive loop amplifier, severed lead-in cable. |
| - Controller Impact: Phase maxes out every cycle, wasting green time. |
| |
| 2. CHATTERING / OSCILLATING DETECTOR ALARM: |
| - Detector logs > 60 actuations/min (> 1,000/hr) during off-peak hours. |
| - Cause: Moisture in conduit, poor splices, cross-talk between loops. |
| - Controller Impact: False calls prevent phase gap-out. |
| |
| 3. DROPOUT / INACTIVE DETECTOR ALARM: |
| - Advance detector records 0 actuations during peak commuter hours |
| while stop bar detector records high volume. |
| - Cause: Blown surge arrestor, cut pavement loop from road milling. |
| |
| 4. PEDESTRIAN BUTTON FAILURE ALARM: |
| - Event 45 (Ped Call) continuously active across 10 consecutive cycles |
| OR zero ped calls recorded over 7 consecutive days. |
| - Cause: Jammed mechanical button, frozen switch contacts. |
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Preventive Maintenance Workflow
Prior to ATSPMs, a failed advance loop detector often went unnoticed for months, quietly forcing the controller into recall or max-out mode and degrading corridor progression. With ATSPM automated alerts:
- The central server runs nightly automated health audit scripts across all intersection event logs.
- When a detector channel exhibits stuck presence, chattering oscillations, or zero counts, the system automatically generates an alert ticket with the exact cabinet ID, detector channel, and assigned NEMA phase.
- Technicians arrive at the cabinet with the exact diagnostic tools needed (e.g., megohmmeter, loop analyzer, or replacement detector card), dramatically reducing mean time to repair (MTTR).
What is the standardized event logging timestamp resolution required in modern Advanced Transportation Controllers for ATSPM data collection?
In ATSPM split monitoring analytics, which quantitative condition indicates that a phase experienced a "split failure"?
How is a Platoon Ratio (Rp) greater than 1.0 interpreted on a Purdue Coordination Diagram?