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SAE J2735-Draft-Rev29 [issued: 12-11-08]
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This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
for both simple and complex signalized intersections to be modeled without additional overhead in the
message unless that overhead is needed (say to relate pedestrian lanes states, or other events that may not
be present in every intersection). Consult the prior definitions for specific details, but here is a general
overview of the structures defined in the SPAT message.
The overall use of the SPAT message is to reflect the current state of all lanes in all approaches in single
intersection. Any preemption or priority then follows in a structure for the whole intersection. Lanes that
are at the same state (with the same end time) are combined. Thus the simplest SPAT message consists on
two such states, one for the then active lanes/approach, and another for all the other lanes that at that time
share the state being stopped (a red state). The stopped (red) lanes are optionally not sent at other times
(the presumption being that any lane not enumerated in the SPAT is in fact set red).
Here is a message fragment illustrating this:
SPAT Message
Msg id = 0x0c (indicates a SPAT message)
SPAT id = TBD
(indicates a unique value for this intersection)
States
State #1
Lane Set
(list of lanes this applies to)
1, 2
Movement State(signal state or pedestrian state)
SignalState = Green light
TimeToChange = 12.3 seconds
YellowSignalState =
State #2
Lane Set
(list of lanes this applies to)
3,4.5.6, etc...
Movement State(signal state or pedestrian state)
SignalState = Red light
TimeToChange = Indeterminate for this state
YellowSignalState =
Preempt = none present
The SPAT message consists of a sequence of MovementStates for each lane in the intersection.
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The
SPAT status information is associated with the lanes found in the MAP message by the use of shared lane
numbering values. The overall framework consists of the regionally unique intersectionID (required), the
collation of current lane states, any signal-wide preemption data, and some optional content (such as the
human readable name of the intersection) as follows. Some additional information regarding the internal
preemption or priority request status of the signal controller itself can be obtained in the Signal Status
Messages (SSM) message.
Up to 255 unique states can be sent, although more commonly only active states are sent at any time (the
phase of the active lane-approach, and all other lanes which are in a red-phase). Considering the
MovementState data frame further we see that it includes a great deal of timing information:
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In these messages all lanes are given a unique number regardless of what approach they may
belong to. Therefore, an “approach” in a traffic engineering sense of the word always consists of one or
more defined lanes in these messages. Lane numbering value assignment is arbitrary, but some
conventions or best practices are expected to apply.