SRM and SSM: the request and the answer in signal priority

Updated 2026-10-09

What they are and who sends them

The Signal Request Message (SRM, DSRCmsgID 29) is sent by a vehicle that wants the traffic signal to favor it: a transit bus running late, a snow plow on a route, a freight truck that should not have to stop, an ambulance on a call. It names the intersection, the lane it will arrive on and leave by, when it expects to be there, and who it is. The Signal Status Message (SSM, DSRCmsgID 30) is the roadside's reply: for each active request at the intersection it echoes the requester and says whether the request is granted, rejected, processing or something else. The messages are a conversation; the rest of the system, from the priority request server to the controller's priority logic, is described in signal priority. This article is about the bytes.

When they are broadcast

Property	SRM	SSM
Sender	Vehicle OBU	RSU, on behalf of the priority request server or controller
Rate	Once per second while approaching, or on change; J2945/B gives the rules	Once per second while any request is active, or on change
PSID	0xE0000016 (p-encoded `E0000016`)	0xE0000015 (p-encoded `E0000015`)
Signing	Signed with a certificate whose PSID 0xE0000016 and service-specific permissions (SSP) authorize the vehicle's role; an RSU checks the SSP before honoring a request	Signed with the RSU's 0xE0000015 application certificate
Size	50 to 120 bytes	30 to 80 bytes

SRM structure

Field	Meaning	Units and range	Typical value	Where it comes from
`timeStamp`	Minute of the year	optional	403680	OBU clock
`second`	Milliseconds in the minute when the SRM was built	0 … 59999	45000	OBU clock
`sequenceNumber`	Increments per SRM from this vehicle	0 … 127	1	OBU
`requests[].request.id`	Intersection `region` and `id`; must match the MAP and SPaT		region 0, id 1201	from the MAP the vehicle heard
`requests[].request.requestID`	Chosen by the vehicle; echoed in the SSM	0 … 255	7	OBU
`requests[].request.requestType`	`priorityRequest` (new), `priorityRequestUpdate` (changed arrival time), `priorityCancellation`	enum	priorityRequest	OBU logic
`requests[].request.inBoundLane`	CHOICE `lane` (MAP laneID), `approach` (ApproachID) or `connection` (LaneConnectionID)		lane 3	matched from the MAP
`requests[].request.outBoundLane`	Same choice for the exit	optional	lane 4	matched from the MAP
`requests[].minute`, `second`	Expected time of arrival at the stop line	minute of year + ms	403680, 45000	OBU estimate
`requests[].duration`	How long service is wanted	ms	20000	OBU estimate
`requestor.id`	CHOICE `entityID` (4-byte TemporaryID) or `stationID`		entityID `BEEF0001`	OBU
`requestor.type.role`	BasicVehicleRole: `transit`, `truck`, `emergency`, `police`, `fire`, `ambulance`, `roadWork`, `dot`, `safetyCar`…	enum	transit	vehicle configuration
`requestor.type.subrole`	Agency-defined refinement, `requestSubRole1` … `14`	enum	requestSubRole1	agency policy
`requestor.type.request`	Importance, `requestImportanceLevel1` … `14`	enum	requestImportanceLevel3	agency policy
`requestor.type.hpmsType`	VehicleType: `bus`, `car`, `axleCnt5Trailer`…	enum	bus	vehicle configuration
`requestor.position`	`position` (lat, long, elevation), `heading` (1.5° units), `speed` (`transmisson` and `speed` in 0.02 m/s)			GNSS
`requestor.name`, `routeName`	Vehicle and route labels	up to 63 chars	"Metrobus 28A", "28A"	transit CAD
`requestor.transitStatus`	8-bit: loading, anADAuse, aBikeLoad, doorOpen, charging, atStopLine	bit string	`00000000`	vehicle
`requestor.transitOccupancy`	`occupancyEmpty` … `occupancyFull`	enum	occupancyMed	passenger counter
`requestor.transitSchedule`	Seconds ahead (+) or behind (−) schedule in 10 s units	−122 … 121	−3 (30 s late)	transit CAD

The spelling transmisson inside TransmissionAndSpeed is the standard's, not a typo in the portal.

SSM structure

Field	Meaning	Units and range	Typical value
`timeStamp`, `second`, `sequenceNumber`	As in the SRM, for the SSM itself		403680, 45500, 1
`status[].sequenceNumber`	Per-intersection counter	0 … 127	1
`status[].id`	The intersection	region, id	0, 1201
`status[].sigStatus[]`	One package per active request	1 … 32
`sigStatus[].requester.id`, `request`, `sequenceNumber`	Echo of the SRM's `requestor.id`, `requestID` and `sequenceNumber` so the vehicle can match the reply		`BEEF0001`, 7, 1
`sigStatus[].requester.role`, `typeData`	Echo of the requester's role and type	optional	transit
`sigStatus[].inboundOn`, `outboundOn`	Echo of the lanes		lane 3, lane 4
`sigStatus[].minute`, `second`, `duration`	Echo of the requested service time	optional
`sigStatus[].status`	PrioritizationResponseStatus: `unknown`, `requested`, `processing`, `watchOtherTraffic`, `granted`, `rejected`, `maxPresence`, `reserviceLocked`	enum	granted

maxPresence means the vehicle has been asking too long and the controller will not extend further; reserviceLocked means the same vehicle was just served and must wait; watchOtherTraffic is a grant with a caution that conflicting priority is also being served.

How to create a pair in this portal

Open the creator and load SRM. The template is a late Metrobus on route 28A approaching intersection 1201 northbound on lane 3, leaving by lane 4, asking for 20 seconds of service.
Set request.id.id to your intersection and inBoundLane/outBoundLane to lane IDs from your MAP (load the MAP template to see them). Set minute and second to the expected arrival. Set the role and importance that your agency's policy assigns; the ? help lists the enumerations.
Encode / Validate. The hex begins 001D (messageId 29). The map shows the requestor's position and the intersection.
Load SSM. Copy the SRM's requestor.id.entityID, requestID and sequenceNumber into sigStatus[0].requester, mirror the lanes and times, and set status. Encode; the hex begins 001E (messageId 30).
Decode both in the decoder and confirm the summary links the SSM to the SRM. In the live view, a vehicle with an outstanding request shows its state (requested, granted, rejected) on the intersection marker.
For a bench test, push the SSM on PSID 0xE0000015 from the RSU to see an OBU display the grant. SRMs are vehicle messages; push one on 0xE0000016 only to exercise a priority request server with a synthetic vehicle.

How to read a decoded one

The SRM's request.id must match the intersection the vehicle is actually approaching; a wrong ID is silently ignored by the server.
inBoundLane must be a lane that exists in the current MAP with a connectsTo; a request on an egress lane or a crosswalk lane is rejected.
minute and second should be a few seconds to a minute in the future; an arrival in the past is a stale or replayed request.
In the SSM, requester.id, request and sequenceNumber must equal the SRM's; if the server regenerates requestID the vehicle cannot match the answer.
A status that stays requested or processing for many seconds usually means the controller side (NTCIP 1211 or the vendor priority server) never received the call; see the troubleshooting table in signal priority.
Check the SSM's second against the SRM's: a reply more than a second or two later points at a slow server path.

SAE J2735_202409, SignalRequestMessage and SignalStatusMessage modules; SAE J2945/B, requirements for signal priority and preemption applications; NTCIP 1211 for the controller-side request and status model.
Signal priority, MAP, SPaT, MAP to SPaT, SCMS for the certificate permissions a requester needs.