Movement & Phase State

In the J2735 and J2945 MAP_SPaT work a “Phase” refers to a movement phase state of a signal system.  In other words the red-yellow-green signal colors that appear at the signal head.

Although we go to very great lengths to avoid using “colors” to describe this.  And from the ISO TC 204 coordination work,  the J2735 work also supports a number of other states that are not found in the US but are common elsewhere in the world.  An example of this would be a green-flash phase proceeding the normal green phase which is used in various European countries (formally call the pre-movment in this work).

Aside: See also this article on the allowed Lane Maneuvers that are supported.  Those maneuvers that require a signal (not all do) are matched to these phase states and the behaviors that they describe.  The matching from a lane and its maneuvers is covered in this article on the use of SignalGroups.

The DE_MovementPhaseState

This is where the current phase of each movement is defined (US terminology).  A movement consists of a sequence phases, the actual order in which these states occur will vary with the deployment region.  Not every location, or every light, proceeds from red to green to yellow and back with various “arrow or ball” variations.

More generally, this is called an admissive or protected light in the US.  The terms not-protected and protected are used in J2735 and its related ISO work.  Note the terminology used below. A dark state is also provided for non-operational needs.

See the actual standard for full details, and also for a very nice State Transition Diagram (developed by Ed Folk, US DOT, as part of the world-wide coordination that took place).

The actual standard (in a terse reduced format) consists of ten ENUMERATED defined values.

MovementPhaseState ::= ENUMERATED {
   -- Note that based on the regions and the operating mode not every
   -- phase will be used in all transportation modes and that not
   -- every phase will be used in all transportation modes
   unavailable (0),
   dark        (1), 
   -- Reds
   stop-Then-Proceed (2),     Note: See this right on red article as well 
   stop-And-Remain   (3),  
   -- Greens
   pre-Movement                (4),
   permissive-Movement-Allowed (5),
   protected-Movement-Allowed  (6),  
   -- Yellows / Ambers
   -- The vehicle is not allowed to cross the stop bar if it is possible 
   -- to stop without danger. 
   permissive-clearance        (7),
   protected-clearance         (8),
   caution-Conflicting-Traffic (9)
   }
   -- The above number assignments are not used with UPER encoding
   -- and are only to be used with DER or implicit encoding

Some additional comment on the correct use of these in the US is provided below.  The most common states used in the US market for a signal controller would be :

stop-And-Remain (3),
permissive-Movement-Allowed (5),    protected-Movement-Allowed (6),
permissive-clearance (7),                      protected-clearance (8),
caution-Conflicting-Traffic (9)

Note: that there is no concept of a directional arrow image or its orientation in the above.  The arrow concept is conveyed with the term protected and the direction is inferred by the other lane to which it connects (typically to the right or to the left but therefore not limited to simple 90 degree angles).

The lack of an arrow generally means an admissive signal.  Or at the least, it implies no protection from conflicting movements by other parties.  So in the DSRC MAP-SPaT message set work, a common MUTCD sign such as R10-13 (“Left Turn Yield on Green”) is essentially implied by the use of the “permissive-Movement-Allowed” state rather than the “protected-Movement-Allowed” state.   Note: signage information can be conveyed by other DSRC message such as TIM (now renamed RSM).

Note: that there is no concept of a red clearance interval either. To be more precise, the concept of course exists and is widely implemented in most signal controllers, but it is never reflected in the SPaT data.  The fact that all the “lights are red” only appears in the SPaT by the time marks when the next phase begins.  Keep in mind the SPaT exists to serve the informational needs of the traveler (end using OBU device), not those of the signal engineer.

Considering these States further…

The unavailable Movement Phase State

   Used when the signal state is not known.  The standard states:  This state is used for unknown or error

The dark Movement Phase State

   The signal controller is off-line and all signal heads are dark. This is not the same as a having all signal heads flashing reds (a state covered next).  Note also:  The SPaT message has some status bits (see IntersectionStatusObject) that are to be used when the signal controller is off-line, being serviced or in a failureFlash operating mode.

The standard states:  The signal head is dark (unlit)

Note further:  Just as the MAP message may deal with more than one intersection in the same message, the SPaT message may do the same for multiple signal controller devices. Each Intersection State has its own (single) IntersectionStatusObject and one or more MovementList entries.

The stop-Then-Proceed Movement Phase State (a Red)

   The standard states the use case very clearly:

Often called 'flashing red' in US\
Driver Action: 
   Stop vehicle at stop line.
   Do not proceed unless it is safe.
Note that the right to proceed either right or left when
it is safe may be contained in the lane description to
handle what is called a 'right on red'

Note: This can be used for un-signalized stop signs as well.

Some signals (often found in more rural areas) turn every motor vehicle lane to all flashing red operations late at night or during off hours.  In the SPaT message, all SignalGroupIDs are issued with the non-expiring TimeMark value (minEndTime = 36001) and this phase state.

The stop-And-Remain Movement Phase State (a Red)

   The standard states the use case very clearly:

      e.g. called [a] 'red light' in US
      Driver Action:                    
        Stop vehicle at stop line.
        Do not proceed.                    
      Note that the right to proceed either right or left when
      it is safe may be contained in the lane description to
      handle what is called a 'right on red'

         The use of the concept of protected varies somewhat when directional reds are used.

To create a “red arrow” this movement phase is combined with a (protected) lane maneuver and the correct  SignalGroupID in a ConnectsTo. Note that the stop and remain state is always presumed in the SPAT for every SignalGroupID when no other information is present, so this need not be sent.

The pre-Movement Movement Phase State (a Green)

This state is not used in the US market at this time.

    pre-Movement (4),
      Not used in the US, red+yellow partly in EU
      Driver Action:                    
        Stop vehicle.
        Prepare to proceed (pending green)
        (Prepare for transition to green/go)

The permissive-Movement-Allowed Movement Phase State (a Green)

   The common permissive green ball case

      Often called 'permissive green' in US
      Driver Action:                    
        Proceed with caution,
        must yield to all conflicting traffic
      Conflicting traffic may be present in the intersection conflict area

The protected-Movement-Allowed Movement Phase State (a Green)

         The protected green ball case, with the “arrow” implied by the lane.

      Often called 'protected green' in US
      Driver Action:                    
        Proceed, tossing caution to the wind,
        in indicated (allowed) direction.

The permissive-clearance Movement Phase State (a Yellow)

   The common permissive yellow ball case

      Often called 'permissive yellow' in US
      Driver Action:                    
        Prepare to stop.
        Proceed if unable to stop,
        Clear Intersection.
      Conflicting traffic may be present in the intersection conflict area

The protected-clearance Movement Phase State (a Yellow)

         The protected green ball case, with the “arrow” implied by the lane.

      Often called 'protected yellow' in US
      Driver Action:
        Prepare to stop.
        Proceed if unable to stop,
        in indicated direction (to connected lane)
        Clear Intersection.

Note that in a turning lane a protected green arrow state may then move to a protected yellow arrow, followed by an admissive green ball which then applies to the same lane as well as other lanes (typically the through traffic.  A vehicle which is turning late in the yellow may then be presented with the green ball while in the intersection no-mans land.  Alternatively the yellow may result in a red for this lane, indicating a driver behavior of reckless control (RC).  Today local familiarity with the signalling system is the only way to “predict” the likely outcome.  The time marks in the SPaT message for pending phases can be used to inform the driver (or OBU device) which sequence will take place, removing this dilemma issue.

The caution-Conflicting-Traffic Movement Phase State (a Yellow)

   The common flashing yellow ball case

      Often called 'flashing yellow' in US
      Often used for extended periods of time
      Driver Action:
        Proceed with caution,
      Conflicting traffic may be present in the intersection conflict area

Some signals (often found in more rural areas) turn every motor vehicle lane to all flashing yellow operations late at night or during off hours.  In the SPaT message, all SignalGroupIDs are issued with the non-expiring TimeMark value (minEndTime = 36001) and this phase state.

Further comments

We concludes with a few common worked examples of common signal head encodings.

Encode up some common variations here so folks can just find the bit pattern that they need, perhaps use the Kelley cheat sheet table here

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