2.3 SPaT and MAP Message Structure under SAE J2735

2.3 SPaT and MAP Message Structure under SAE J2735

[!NOTE] IMSA Level III Examination Focus: Mastery of SAE J2735 message sets is essential for advanced signal technicians. You must know how the Advanced Transportation Controller (ATC) communicates with the Roadside Unit (RSU), how NEMA signal phases map to J2735 SignalGroupID values, how geometric node points define intersection approaches, and how in-vehicle safety algorithms process phase countdowns.

Connected intersection operations rely on standardized, machine-readable data streams that broadcast the physical layout and immediate operational state of traffic signals. The Society of Automotive Engineers (SAE) codified these data structures in the SAE J2735 Dedicated Short Range Communications (DSRC) / V2X Message Set Dictionary. Among the various messages defined in J2735, the two most critical infrastructure-generated payloads are Signal Phase and Timing (SPaT) and MAP (Intersection Geometry).

---

The SAE J2735 Message Family Overview

The SAE J2735 standard establishes a shared syntactic and semantic vocabulary for all connected vehicle participants. Messages are formatted using Abstract Syntax Notation One (ASN.1) and encoded using Unaligned Packed Encoding Rules (UPER) to minimize packet size over the air.

---

SPaT Message Internal Structure & Data Elements

The SPaT message describes the real-time operational status of every signal phase and crosswalk movement associated with an intersection. It answers the question: "What color is each movement displaying right now, and when is it expected to change?"

+-----------------------------------------------------------------------------+
|                     SPaT MESSAGE ASN.1 HIERARCHICAL TREE                    |
+-----------------------------------------------------------------------------+
| SPaT (Root)                                                                 |
|  └── intersections: IntersectionStateList                                  |
|       └── IntersectionState                                                 |
|            ├── id: IntersectionReferenceID (Region ID + Intersection ID)   |
|            ├── revision: MsgCount (0 - 127 roll-over counter)               |
|            ├── status: IntersectionStatusObject                             |
|            |    ├── manualControlIsEnabled                                 |
|            |    ├── stopTimeIsActivated                                    |
|            |    ├── failureFlash / conflictFlash                           |
|            |    └── preemptIsActive / tspIsActive                          |
|            ├── moy: MinuteOfTheYear (0 - 527,040)                           |
|            ├── timeStamp: DSecond (0 - 59,999 milliseconds in the minute)  |
|            └── states: MovementList                                         |
|                 └── MovementState (Repeated for each NEMA Phase/Overlap)   |
|                      ├── movementName: Descriptive Text                     |
|                      ├── signalGroup: SignalGroupID (1 - 255)               |
|                      └── state-time-speed: MovementEventList                |
|                           └── MovementEvent                                 |
|                                ├── eventState: MovementPhaseState           |
|                                └── timing: TimeChangeDetails                |
|                                     ├── minEndTime: TimeMark                |
|                                     ├── maxEndTime: TimeMark                |
|                                     ├── likelyTime: TimeMark                |
|                                     └── confidence: TimeIntervalConfidence  |
+-----------------------------------------------------------------------------+

Critical SPaT Data Fields

• IntersectionReferenceID: Uniquely identifies the physical intersection across regional networks using a combination of a 16-bit RoadRegulatorID (jurisdiction) and a 16-bit IntersectionID.
• IntersectionStatusObject: A 16-bit bitmap reporting real-time cabinet status flags, including whether the intersection is running under time-of-day coordination, manual police control, preemption override, or MMU conflict flash.
• SignalGroupID: An 8-bit integer (1 to 255) representing a distinct signalized movement. In standard North American deployments, SignalGroup IDs 1 through 8 map directly to standard NEMA TS2 Phases 1 through 8, while IDs 9 through 16 designate pedestrian phases or phase overlaps (e.g., SignalGroup 10 = Phase 2 Pedestrian; SignalGroup 14 = Overlap A).
• MovementPhaseState: An enumeration defining the exact instantaneous optical display:
  • protected-Movement-Allowed: Solid Green ball or arrow.
  • movement-Clearance: Solid Yellow change interval.
  • stop-And-Remain: Solid Red display.
  • permissive-Movement-Allowed: Flashing Yellow Arrow (FYA) or permissive green ball with yielding rules.
  • caution-Conflicting-Traffic: Yellow flashing operation.

The Mechanics of SPaT Timing Parameters

The timing block within each MovementEvent provides the critical predictive countdown values, encoded as a TimeMark in units of tenths of a second (100 ms) from the top of the current hour (0 to 35,999 tenths of a second):

$\text{Current Hour Second} = \frac{\text{TimeMark}}{10}$

1. MinEndTime: The earliest possible time at which the current phase state can terminate. For a green phase, this represents the expiration of the programmed Minimum Green plus any active pedestrian service intervals. For a yellow phase, it represents the exact end of the fixed yellow change clearance.
2. MaxEndTime: The latest possible time at which the current phase state can terminate under continuous vehicle extension. In actuated operations, this correlates to the phase Maximum Green (Max 1 or Max 2) or the coordination Force-Off point.
3. LikelyTime: An advanced controller-generated prediction of when the phase will terminate based on historical gap-out behavior, volume-density settings, or adaptive progression algorithms.
4. Confidence: A statistical probability score (0 to 15 scale) reflecting how reliably the controller expects the phase to terminate at the LikelyTime.

---

MAP Message Internal Structure & Geometric Modeling

While SPaT describes dynamic signal timing, the MAP message provides the static physical canvas upon which those timing displays operate. A vehicle cannot interpret a SPaT message without having the intersection's MAP message loaded into memory.

+-----------------------------------------------------------------------------+
|                     MAP MESSAGE GEOMETRIC TOPOLOGY                          |
+-----------------------------------------------------------------------------+
|                                                                             |
|   Approach 1 (Ingress)                                Approach 2 (Egress)   |
|   [Lane 1: Left Turn]  ──> ConnectsTo (SignalGroup 1) ──> [Lane 20]         |
|   [Lane 2: Thru Lane]  ──> ConnectsTo (SignalGroup 2) ──> [Lane 21]         |
|   [Lane 3: Right Turn] ──> ConnectsTo (SignalGroup 2) ──> [Lane 22]         |
|                                                                             |
|             \                      Stop Line Bar                     /      |
|              \======================================================/       |
|               \                                                    /        |
|                \             (Intersection Reference Point)       /         |
|                 \               Anchor: Lat / Long / Elev         /          |
|                  \                       (+)                     /           |
|                   \                                             /            |
|                    \===========================================/             |
|                                   Pedestrian Crosswalk                       |
|                                   (SignalGroup 10)                           |
+-----------------------------------------------------------------------------+

Core MAP Data Components

• Intersection Reference Point (Anchor Point): The geodetic focal point of the intersection, defined using the WGS-84 coordinate reference system. Latitude and longitude are specified to a resolution of one-tenth of a microdegree (0.0000001°), providing a horizontal geographic precision of approximately 1.1 centimeters. Elevation is defined with a precision of 10 centimeters above sea level.
• Approach & Lane Definitions: Roadways entering the intersection are designated as Ingress Approaches, while departing roadways are Egress Approaches. Each travel path within an approach is assigned a unique 8-bit LaneID (1 to 255).
• Lane Nodes and Delta Offsets: Rather than storing full 64-bit geographic coordinates for every point along a lane, MAP messages define lanes using a series of sequential Node Offsets relative to the previous node. Each node contains delta offsets: $(\Delta X, \Delta Y, \Delta Z)$ specified in centimeters. This vector string forms the precise 3D centerline and curvature of each travel lane.
• Stop Line Bar Coordinates: The final node of an ingress lane vector defines the physical location of the pavement stop bar.
• ConnectsTo & Maneuvers: This is the critical functional link between MAP and SPaT. The ConnectsTo data block defines:
  • Which egress lane can be reached from this ingress lane.
  • The permitted maneuver (maneuverStraightAllowed, maneuverLeftAllowed, maneuverRightAllowed, maneuverUTurnAllowed).
  • The specific SignalGroupID governing that turning path.