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SAE J2735-Draft-Rev29 [issued: 12-11-08]
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285 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Annex H Map and SPAT Message Use and Operation
1.
Introduction
There are four messages currently defined to support intersection mapping needs and relating signal phase
and timing data to OBUs. These message support the intelligent intersection needs of the VII program. All
of these are the result of field experience involving several dozen intersections where similar prototype
messages have been operating. The data content used in those messages was similar, but used a proprietary
encoding, now replaced by the standard BER-DER encoding format specified here. These four messages
(listed below) are mature but supporting documentation on how they are to be used remains to be
developed. This annex serves as a short introduction to the intended general use of the messages.
The four subject messages are:
Signal Phase and Timing Message
(SPAT)
Relates the current intersection signal light phases
Map Data
(MAP)
Relates the Physical Geometry of the intersection
Signal Request Message
(SRM)
Requests preempt or priority services
Signal Status Messages
(SSM)
Relates the internal state of the signal controller
1.1
Intended Audience
This document is written primarily for application and system programmers who write compliant software;
system architects who drive the DSRC message creation, distribution and consumption processes; and
content designers and managers such as city managers and their staff.
1.2
Philosophy of SPAT and MAPs
In normal use the OBU units are expected to receive the MAP message before entering the intersection.
This map message conveys all the physical geometry for one or more intersections and well as the
regulatory information (allowed maneuvers) for the intersection and assigns specific lane numbers to both
drivable vehicle lanes and other features of each intersection. When in the range of the intersection, the
SPAT message is broadcast from an RSU with the current signal state at all times. OBU users can relate
the (dynamic) SPAT message information to specific lanes of travel in the (static) MAP message and
determine the phase state of the intersection and for how long that state will persist. Two additional
messages (SRM and SSM) are used to request and the determine priority and preemption events. These
two messages are typically used by public safety and public transport OBUs only.
At a high level, the MAP message contains all the static (unchanging) information relating to one or more
intersections in the intersections data frame. This information (consisting of both required and optional
informational content) is determined for the intersection and broadcast in such a way that a cache of local
intersections can be maintained by the OBU. Besides describing the lane geometry paths and the allowed
maneuvers for each lane, the intersection data frame can provide additional information regarding barriers,
pedestrian walkways, shared roadways and rail lines that may affect vehicle movement.
By contrast the SPAT message contains the state of the intersection and got how long this state will persist
for each approach and lane that is active. The SPAT and MAP share a common lane numbering
assignment between them to allow mapping.
2.
The overall framework of the SPAT
The Signal Phase and Timing message (SPAT) uses a simple framework to provide a basic summary of the
signal state at any given time (dynamic data). The many optional elements defined in this message allow