RSM, the Road Safety Message of SAE J2945/4: structured work zones and incidents

Updated 2026-10-09

What it is and who sends it

The Road Safety Message is a roadside message defined in SAE J2945/4 that describes a road condition in a structured way: a work zone with its reduced speed, its lane closures and the detour path; an incident; a sharp curve with an advisory speed. Where a TIM says "road construction, reduced to one lane" as a list of phrases and leaves the vehicle to interpret it, an RSM says which lanes are closed, from where to where, what the speed limit inside the zone is, when the zone starts and ends, and whether workers are present. Automated driving systems and advanced driver assistance can act on that; a human-readable TIM they can only display. The ASN.1 type is RoadSafetyMessage, DSRCmsgID 33, carried in the same MessageFrame as every other J2735 message and broadcast by an RSU from an agency's work-zone or incident data.

When it is broadcast

Property	Value
Rate	1 Hz is typical; J2945/4 allows slower for long-lived zones
PSID	0x8003 (p-encoded `8003`), the same as TIM; receivers tell them apart by `messageId`
Channel	Same C-V2X channel as everything else
Signing	Signed with the RSU's 0x8003 application certificate
Source	Agency work-zone system (often from a WZDx feed) → RSU store-and-repeat table; update when the zone changes, with `eventUpdate` incremented
Size	100 to 600 bytes; several containers can share one message

Structure, section by section

CommonContainer

Field	Meaning	Units and range	Typical value
`eventInfo.eventID`	Identifier of the event, stable across updates	octet string
`eventInfo.eventUpdate`	Increment on each revision of the event	MsgCount 0 … 127	1
`eventInfo.eventCancellation`	Whether the event is being cancelled	boolean-like Activity	false
`eventInfo.startDateTime`, `endDateTime`	Validity as `DDateTime` (year, month, day, hour, minute, second in ms)		2026-10-08 06:00 → 18:00
`eventInfo.eventRecurrence`	Daily or weekly recurrence of a scheduled zone	optional
`eventInfo.causeCode`, `subCauseCode`	ITIS group and code of the cause (road work, accident, weather…)	ITIS	1025 road construction
`eventInfo.affectedVehicles`	Vehicle groups the event applies to	ITIS 9217 … (all-vehicles, trucks, buses…)
`regionInfo[].referencePoint`	Anchor position of the region	1/10 microdegree, 0.1 m
`regionInfo[].referencePointType`	`startOfEvent` or `arbitrary`	enum	startOfEvent
`regionInfo[].approachRegion`	Where the message applies: a `broadRegion` or a list of `paths`, each a node-offset chain like a MAP lane
`regionInfo[].scale`	Offset scale factor	0 … 15	0
`crossLinking`	References to related MAP intersections or other RSMs	optional

Content containers (1 … 4 per message)

Container	What it carries	Key fields
`rszContainer` (reduced speed zone)	The zone's speed limit and the geometry it applies to	`speedLimit` (type and speed in 0.02 m/s), `rszRegion` paths, `speedLimitRegionList`
`laneClosureContainer`	Which lanes are open or closed, per lane, along a path	`laneStatus` list (lane number and open/closed flags), `laneClosureRegion`
`curveContainer`	A curve's geometry and advisory speed	`curveRegion`, `advisorySpeed`
`dynamicInfoContainer`	Workers present, moving work zone, flagger, dynamic speed	`workersPresent`, `movingWorkZone`, `dynamicSpeed`, `flaggerPresent`
`incidentsContainer`	Incident type, location, response status	`incidentType`, `incidentRegion`, `responseEquip`
`situationalContainer`	Weather, visibility and surface conditions	`weatherContainer`, `visibility`, `surfaceCondition`

The portal's template is a westbound work zone on Leesburg Pike with a reduced speed zone container (35 mph) and a lane closure container (right lane closed). Load it in the creator and open the structure explorer to see the exact member names of each container; the ? help gives each one's units.

TIM or RSM: when to use which

Question	TIM	RSM
Audience	Drivers (display, speech)	Automated and assisted driving systems, and drivers through the vehicle's own interpretation
Vehicle support today	Every OBU and most production vehicles with V2X	Growing; required by newer USDOT and state work-zone pilots
Expresses lane-level closures	Only as phrases	Yes, per lane along a path
Expresses a speed limit	As an ITIS sequence	As a typed value with a region
Expresses workers present, moving zones	Only as phrases	Yes, with dynamic flags
Authoring effort	Low	Higher; needs lane-level geometry
Source feed	Anything	Fits WZDx cleanly

Most agencies broadcast both for a work zone: a TIM so every vehicle shows something, and an RSM so capable vehicles get the structure. Both go on PSID 0x8003 in the same RSU store with separate rows.

How to create one in this portal

Open the creator and load Road Safety Message – Work Zone.
Set eventInfo.eventID to your own identifier, startDateTime and endDateTime to the zone's hours, causeCode to the ITIS cause and affectedVehicles if the zone applies only to trucks or all vehicles.
Move regionInfo[0].referencePoint to the start of the zone and edit the approachRegion.paths nodes (centimeter offsets, like a MAP lane, first node relative to the reference point).
In the containers, set the reduced speed (0.02 m/s units: 35 mph = 782) and the lane status list. Remove containers you do not need with the structure explorer; add dynamicInfoContainer when workers are present.
Encode / Validate. The hex begins 0021 (messageId 33). The map draws the reference point and the paths.
Push to my RSU on PSID 0x8003 with a delivery window equal to the event's validity. Increment eventUpdate each time you change the zone and push again; the RSU row replaces the old bytes.

How to read a decoded one

messageId 33 on PSID 0x8003 is an RSM; 31 is a TIM. A decoder that assumes TIM for everything on 0x8003 fails here.
Check the validity window in eventInfo and the eventUpdate counter against the previous message for the same eventID.
The region paths should lie on the road on the map; the first node is at the reference point.
speedLimit.speed is in 0.02 m/s, not mph.
A message with no containers is valid but useless; one with four containers that disagree about the region is a sign of hand editing.

SAE J2945/4, Road Safety Message; SAE J2735_202409 for the common types it imports; FHWA Work Zone Data Exchange (WZDx) specification as the usual upstream feed; USDOT work-zone data initiative pilots.
TIM, MAP for node offsets, units cheat sheet, TMC and ATMS integration for automating zones from a feed.