2.4 Controller-to-RSU Interface, CV Applications & MAP Field Verification

2.4 Controller-to-RSU Interface, CV Applications & MAP Field Verification

Controller-to-RSU Interface: NEMA ATC 5201/5301 Standards

In modern deployments, the traffic signal controller itself serves as the authoritative source of SPaT data, streaming status packets directly to the RSU across the cabinet Ethernet network.

+-----------------------------------------------------------------------------+
|                 CONTROLLER-TO-RSU DATA PIPELINE FLOW                        |
+-----------------------------------------------------------------------------+
|                                                                             |
|  [NEMA TS2 / ATC Controller]                                                |
|  - Linux-based NEMA ATC 5201 / 5301 Engine                                  |
|  - Executes signal timing logic every 100 milliseconds                      |
|  - Generates raw SPaT data stream (Internal phase states & countdowns)      |
|                            |                                                |
|                            v (Local Dedicated Ethernet / Cat6A)             |
|  [UDP / IP Socket Stream]                                                   |
|  - Transmitted via Controller Port ETH2 (Protected ITS Subnet)              |
|  - Destination: RSU IP Address on UDP Port 53738 (or 23000)                 |
|  - Transmission Rate: Exactly 10 Hz (every 100 ms ± 2 ms)                   |
|                            |                                                |
|                            v                                                |
|  [Roadside Unit (RSU)]                                                      |
|  - Ingests raw controller UDP packets                                       |
|  - Validates internal 1 PPS GNSS timestamp                                  |
|  - Encodes payload into SAE J2735 UPER ASN.1 format                         |
|  - Wraps packet in IEEE 1609.2 Security Envelope                            |
|  - Attaches ECDSA P-256 digital signature via internal HSM                  |
|                            |                                                |
|                            v                                                |
|  [5.9 GHz C-V2X Radio Broadcast]                                            |
|  - Broadcasts Signed SPaT (10 Hz) and Signed MAP (1 Hz) to all OBUs         |
+-----------------------------------------------------------------------------+

The NEMA ATC Architecture

Under the NEMA ATC 5201 Standard (Advanced Transportation Controller) and ATC 5301 (Cabinet Standard), controllers run an open Linux operating system capable of executing concurrent software applications. Modern ATC controller firmware includes an embedded V2X engine that queries the controller's active phase status, internal interval timers, and coordination engine every 100 milliseconds.

Technicians configure the controller's V2X output parameters:

• Destination IP: The static IP assigned to the RSU on the cabinet local subnet (e.g., 192.168.1.50).
• Destination UDP Port: Standardized connected vehicle port (commonly UDP port 53738 or vendor-specified port 23000).
• Phase-to-SignalGroup Cross-Reference Table: Field technicians must verify that the controller's phase numbering scheme accurately maps to the RSU's J2735 configuration file. For instance, if an overlap phase serves a dedicated right-turn movement, the cross-reference table must assign that overlap to the identical SignalGroupID defined in the intersection's MAP file.

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Connected Vehicle Applications Enabled by SPaT & MAP

The real-time synthesis of SPaT timing and MAP geometry inside vehicle On-Board Units powers revolutionary active safety and mobility applications:

+-----------------------------------------------------------------------------+
|               RED LIGHT VIOLATION WARNING (RLVW) TRAJECTORY                 |
+-----------------------------------------------------------------------------+
|                                                                             |
|   Vehicle Approaching Stop Bar at Velocity V_0                             |
|   [ OBU ] ─────────────────────────>  | Stop Bar                            |
|      \                                |                                     |
|       \                               |                                     |
|        v (GNSS Position + MAP Matching)                                     |
|        Calculates Distance-to-Stop-Line: d_stopline                         |
|                                                                             |
|        Reads SPaT: Current Phase = Yellow                                   |
|        Reads SPaT MinEndTime: Yellow expires in 1.8 seconds                 |
|                                                                             |
|        Evaluates Stopping Kinematics:                                       |
|        d_required = (V_0 * t_reaction) + [ (V_0)^2 / (2 * a_decel) ]        |
|                                                                             |
|        IF d_required > d_stopline AND Arrival_Time > 1.8 s:                 |
|        ───> TRIGGER CRITICAL AUDIBLE & VISUAL COCKPIT WARNING:             |
|             "RED LIGHT VIOLATION IMMINENT - APPLY BRAKES"                   |
+-----------------------------------------------------------------------------+

1. Red Light Violation Warning (RLVW)

RLVW is a high-priority crash mitigation safety application designed to eliminate right-angle (T-bone) collisions:

• The vehicle OBU matches its real-time GNSS location and heading to an ingress lane defined in the MAP message.
• The OBU reads the SPaT broadcast for the corresponding SignalGroupID to extract the active phase state and countdown timers (MinEndTime and MaxEndTime).
• The vehicle calculates its kinematic stopping trajectory based on current speed ($v_0$), driver reaction time ($t_{pr} \approx 1.5\text{ s}$), and maximum comfortable deceleration rate ($a \approx 3.4\text{ m/s}^2$ or $11.2\text{ ft/s}^2$):

$d_{\text{stop}} = (v_0 \cdot t_{pr}) + \frac{(v_0)^2}{2 \cdot a_{\text{decel}}}$

• If the calculation reveals that the vehicle cannot stop comfortably prior to the stop bar before the phase transitions to red, and current speed indicates the vehicle will enter the intersection on red, the OBU immediately issues an urgent visual, auditory, or haptic warning to the driver.

2. Green Light Optimized Speed Advisory (GLOSA)

GLOSA is an eco-mobility application designed to smooth traffic flow, eliminate unnecessary stops, and reduce fuel consumption and greenhouse gas emissions:

• As a vehicle approaches a signalized corridor, the OBU computes the distance remaining to the stop bar ($d_{\text{stopline}}$).
• It reads the SPaT message to determine the remaining green time ($t_{\text{green_remaining}}$) or the time until the next green phase begins ($t_{\text{red_remaining}}$).
• The algorithm determines a dynamic target speed window $[V_{\text{min}}, V_{\text{max}}]$:

$V_{\text{advisory}} = \frac{d_{\text{stopline}}}{t_{\text{target}}}$

• The in-vehicle display advises the driver to cruise at a specific speed (e.g., "Maintain 32 mph to clear next signal on green"), allowing the vehicle to glide through the corridor without stopping.

3. Pedestrian in Crosswalk Warning

When pedestrian pushbuttons or automated pedestrian thermal/radar detectors actuate a pedestrian phase, the controller updates the SPaT message to reflect the active Walk or Flashing Don't Walk interval for that specific crosswalk movement. If a turning vehicle attempts to execute a left or right turn across that crosswalk, the in-vehicle OBU detects the geometric conflict via MAP lane connection data and alerts the driver to yield to the pedestrian.

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MAP Creation, Verification & Field Diagnostics

Because MAP messages represent ground truth for vehicle navigation computers, an inaccurate MAP file introduces severe safety hazards.

Verification Procedures for Field Technicians

1. Stop Bar Accuracy: Technicians must verify that the stop bar node in the MAP file aligns with the physical painted pavement marking within ±0.5 meters (1.6 ft). If resurfacing shifts the stop bar, the MAP file must be resurveyed and updated.
2. Phase Mapping Audits: Using field diagnostic software (such as an RSU vendor client or connected vehicle drive-test tool), technicians observe live phase cycling while parked on each approach. If Phase 2 is green but the vehicle diagnostic tool displays a green indication for the left-turn lane (Phase 1), an error exists in the MAP ConnectsTo table or controller cross-reference configuration.
3. 10 Hz Rate Verification: Using a network tap and packet analyzer, verify that SPaT messages maintain a steady 10 Hz (100 ms ± 5 ms) broadcast cadence. Jitter exceeding 20 milliseconds indicates processor overloading in the ATC controller or bandwidth congestion on the cabinet Ethernet switch.