Updated 2026-10-09

Why these exist

SAE J2735_202409 defines more than thirty message types. A connected intersection deployment lives on six of them (BSM, SPaT, MAP, TIM, SRM and SSM) plus PSM, RSM and SDSM. The rest cover emergency vehicles, positioning corrections, probe data, cooperative maneuvers and newer applications standardized in companion documents. The portal's decoder recognizes all of them by messageId, and the creator has templates for EVA and RTCM. This page tells you what each one is so a stray message in a capture is not a mystery.

"Placeholder module" means the 2024 release reserves the message ID and ships an ASN.1 module whose body is still NULL; the content is being defined in the companion standard, so a decode shows only the frame.

Emergency Vehicle Alert (EVA, 22)

An emergency vehicle broadcasts an EVA alongside its BSM to say what it is and what it is doing: an rsaMsg (a Road Side Alert carrying an ITIS event such as ambulance-units, a heading slice, an extent and a position), responseType (emergency, nonEmergency, pursuit, stationary…), details (siren and lightbar in use, single or multi-vehicle response, privileged event flags), and the responder and equipment types. Vehicles use it to show "emergency vehicle approaching" and to decide whether to yield; a priority request server may treat it as evidence alongside an SRM. Care about it if you run EMS or fire preemption pilots; the portal template is an ambulance responding on Broad Street. PSID practice varies by deployment.

Road Side Alert (RSA, 27)

The simplest roadside warning: a typeEvent ITIS code, an optional description, a priority, a heading slice, an extent and a position. It predates TIM and is what EVA embeds. Few deployments send stand-alone RSAs today because a TIM does the same with regions and validity; care about it if you decode an EVA or support older roadside software.

RTCM corrections (28) and NMEA corrections (24)

An RSU with a surveyed GNSS reference can rebroadcast RTCM 3 differential correction frames inside a J2735 wrapper (msgs as raw RTCM octets, with an optional anchor point and header) so vehicles nearby reach lane-level accuracy; NMEA corrections carries NMEA sentences the same way. PSID 0x8000 is assigned to RTCM. Care about it for lane-level MAP matching pilots and for automated vehicle testing. The portal template holds a sample frame; replace msgs with your own correction bytes.

Common Safety Request (CSR, 21)

A vehicle asks nearby vehicles to include specific optional data in their BSMs, listed as requests of data element identifiers. Rarely implemented; care about it only if a test plan calls for it.

Intersection Collision Avoidance (ICA, 23)

A roadside or vehicle message that warns of a predicted collision at a named intersectionID and laneNumber, with the offending vehicle's core data, path and eventFlag. Superseded in practice by applications that compute the same from BSMs and SPaT, and by SDSM for unequipped vehicles; care about it in historical pilot data.

Probe Vehicle Data (PVD, 26), Probe Data Management (PDM, 25), Probe Data Config and Report (35, 36)

The probe family collects anonymized vehicle snapshots (position, speed, environment sensors) for traffic monitoring. The RSU broadcasts a PDM telling vehicles how often to sample and when to report; vehicles upload PVD with up to 32 snapshots. J2945/C replaces this in 2024 with the Probe Data Config and Report messages. Care about it for travel-time and road-weather data programs; the portal decodes PVD and labels the snapshots.

Traffic Light Status Message (TLSM, 50)

New in 2024, a compact per-signal status with a cycle counter and cycle time, intended for regions whose signal systems are organized by traffic light rather than by movement. In the United States SPaT remains the message vehicles consume; care about TLSM if you exchange data with deployments that use it.

Road Weather Message (RWM, 34)

Defined in SAE J2945/3, a CHOICE of an upload from vehicles (wiper, traction and temperature observations) and a download from the roadside (road weather advisories). Care about it for winter operations pilots alongside snow-plow priority (see signal priority).

Maneuver Sharing and Coordinating Message (MSCM, 42)

Defined in SAE J3186, a vehicle proposes or responds to a maneuver (lane change, merge, intersection crossing) with a maneuverID, the intended path, a responseFlag and a reason code. Care about it for automated driving and cooperative merging research; infrastructure may relay but does not originate it.

Cooperative Control Message (CCM, 40)

Defined in SAE J2945/6, the platooning message: control inputs and state shared between following vehicles. Research and freight corridor pilots only.

Road Geometry and Attributes (RGA, 43)

Reserved in 2024 for SAE J2945/A, a successor to MAP with richer road geometry for automated vehicles. The module is a placeholder; care about it only to recognize the ID.

SCPR and SCPS (46, 47)

Signal Control and Prioritization Request and Status, reserved in 2024 as the next generation of SRM and SSM aligned with NTCIP 1211's model. Modules are placeholders; today's priority deployments use SRM and SSM.

Toll and road-user charging messages (37 … 39, 48, 49)

Defined in SAE J3217, these advertise toll points, report usage and acknowledge it, plus configure and report road-user charging. Care about them for tolling interoperability work; they are not part of a safety deployment.

Test messages (240 … 255)

Sixteen message IDs reserved for vendors' and labs' private test content. If one shows up in a capture, somebody is testing; the portal shows the frame and the raw bytes.

See also the standards overview, the units cheat sheet and capturing messages.