Module 62 - CV273

CV273: Introduction to SPaT / MAP Messages

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CV273: Introduction to SPaT / MAP Messages

Table of Contents

Introduction/Purpose - 2

Samples/Examples - 2

Reference to Other Standards - 6

Case Studies - 6

Glossary - 6

References - 8

Study Questions - 9

Module Description

This module is an introduction to the Signal Phase and Timing (SPaT) and MAP Data messages that may be broadcasted at signalized intersections to assist the deployment of applications related to signalized intersections in a connected vehicle environment. These two messages are defined by SAE J2735_201603, Dedicated Short Range Communications (DSRC) Message Set Dictionary. I101: Using ITS Standards - An Overview, and CV261: Vehicle-to-Infrastructure (V2I) ITS Standards for Project Managers, are recommended prerequisites for participants.

1. Introduction/Purpose

Connected vehicles (CV), a component of the Cooperative Intelligent Transportation System (C-ITS) environment, have the potential to significantly reduce vehicular crashes, provide operators of surface transportation systems with more timely and accurate system performance data to better manage their systems, and provide travelers with access to specific traveler information. To maximize these benefits, agencies should deploy systems that conform to established standards. Proper deployment of standards-conformant equipment and systems will support interoperability, minimize future integration costs, make procurements easier, and facilitate regional and national integration.

The purpose of this module is to introduce transportation managers and specification writers on the purpose and contents of the SPaT and MAP messages, which are key messages exchanged between CV applications related to signalized intersections. The module will focus on two aspects for using the messages—what information can be provided by each standardized message, and implementation considerations related to each message. The implementation considerations will discuss what information needs to be provided to satisfy an operational need of the agency, security considerations, and existing tools to support implementation and efforts to standardize implementation of these two messages to facilitate interoperability, as well as regional and national integration.

2. Samples/Examples

Figure 1 is an example of the timing values for a basic signalized intersection. The timestamp and other times are in tenths of a second in the current hour, while eventState is an enumerated value of the current state of the movement, defined in SAE J2735_201603 (DE_MovementPhaseState).

Please see Extended Text Description below.

(Extended Text Description: Author's relevant description for this figure from Slide #39 of the presentation: This slide entitled "What are the optional elements of the SPaT message?" fully consists of a chart that shows the values for the states data frame for the SPaT message for specific time points along the x-axis within a full signal cycle. At the top are two color bars (red, green, yellow) showing the signal indication for main street green and the cross street as time progresses through the cycle length. Below is text indicating timestamps at which data was recorded. Data was recorded at:

Below the time points is a chart with data values for each timestamp. The first row contains the timeStamp (in tenths of a second within the current hour) as it would appear in the SPaT message. Below this row is data values for each signal group—signalGroup 2 for main street and signalGroup 4 for the cross street. The following data is recorded:


Figure 1. Example Timing Chart (Source: NTCIP 1202 v03, Figure 13)

Figure 2 and Figure 3 presents parts of an example of a use case provided in ISO TS 19091.

Table A.12 – SA2: Red light violation warning

Use Case Name Red Light Violation Warning
Category Safety
Infrastructure Role Data provider
Short Description This use case describes provision of signal timing information to approaching vehicles to help prevent red light violations
Goal Roadside equipment sends MAP and SPaT in real-time to approaching vehicles, which utilize the information to notify driver of need to stop to avoid potential red light violation

RSE transmit performance adequate for approaching vehicles' timely DSRC reception

Security Management System in place to allow OBE to check RSE messages

RSE messages meet minimum performance requirements

Intersection approach roadway segments mapped to sufficient accuracy and differentiates lanes governed by each signal phase

Positioning performance adequate to match vehicle with lane-specific signal phases, if applicable

OBE driver interface/algorithm with appropriate timing/inputs established

Geographic Scope Local signalized intersection & approaching roadway segments
Actors OBE-equipped vehicles with red light violation warning application RSE connected to local traffic signal controller
Illustration (example) The figure shows an illustration of a Red Light Violation Warning. There is an aerial view of a four-way intersection with one vehicle approaching. There are traffic signals and crosswalks on each approach to the intersection. The vehicle is OBE equipped and there are yellow rings around it representing wireless communication. On the right at the corner of the intersection is a traffic signal controller adjacent to Roadside Equipment. There is a black arrow labeled "SPaT" indicating SPaT messages flowing from the Traffic Signal Controller to the Roadside Equipment. There is an arrow representing information flowing from the Roadside Equipment to the vehicle. There is a label with the number 1 on the road to the left of the car representing step 1 of the Main flow when the vehicle enters DSRC range. There is a label with the number 2 next to the Roadside Equipment representing step 2 of the main flow when MAP and SPaT information is transmitted from the Roadside Equipment. There are labels with the numbers 3, 4, and 5 on the vehicle representing steps 3, 4, 5 of the main flow occur within the vehicle. There is a driver shown with the vehicle with a label with the number 6, representing step 6 of the main flow when the driver receives information to stop in time to prevent a violation.
Preconditions MAP message reflects current intersection geometry

Figure 2 ISO TS 19091 Example Use Case (Part 1)

Main flow (example) 1. OBE-equipped vehicle enters DSRC range (i.e. for OBE receive/RSE transmit) of RSE (note that if another medium is used, the same assumption applies).
2. RSE transmits MAP and SPaT information
3. OBE verifies that RSE messages are acceptable (authentic, valid, meet MPR)
4. OBE matches vehicle location to intersection geometry/lane and associated signal phase
5. OBE determines if vehicle is expected to violate red indication based on vehicle trajectory and other information
6. If violation is expected, OBE provides information to driver to stop at appropriate time (in time to stop)
Alternate flow(s) 4a. OBE also utilizes turn signal information and/or other vehicle parameters to match with signal phase
5a. OBE also considers vehicle stopping parameters (e.g. size/weight)
6a. OBE utilizes other information (e.g. image processing of traffic light, etc.) as a backup to determine whether information to stop should be provided to driver
6b. OBE initiates action directly with vehicle if violation is expected
Post-conditions Vehicle crosses stop bar before red onset or stops on red before entering intersection
Information Requirements

Current Maneuver permitted, remaining time for maneuver, yellow clearance time, red clearance time, next maneuver to be serviced

MAP: Intersection Geometry, Permitted maneuvers


Determining turn lane prior to approach

Violation based on current signal phase vs. future

MPR acceptability of current RSE messages

Source docs/references CEN#1 p. 3; USDOT J2735SE Candidate Use Case #1, p. 4

Figure 3 ISO TS 19091 Example Use Case (Part 2)

3. Reference to Other Standards






4. Case Studies

5. Glossary

To include additional descriptions/acronyms used primarily in the module. List out in alphabetical order.

Term Definition
Data Elements Smallest named item of data that conveys meaningful information and has a defined set of attributes
Data Frames Collections of data elements
Intelligent Transportation Systems (ITS) Systems that apply data processing and data communications to surface transportation, to increase safety and efficiency. ITS systems will often integrate components and users from many domains, both public and private.
Interoperability The ability of two or more systems or components to exchange information and to use the information that has been exchanged. The dependence of the CV Environment on successful exchange of data between independent components results in a requirement that all V2I deployments.
MAP A message containing roadway geometric information. See SAE J2735.
Message A structured string of data elements used to convey information.
On-Board Unit (OBU) This term refers to the complement of equipment located in the vehicle for the purpose of supporting the vehicle side of the applications. It is likely to include the DSRC radios, other radio equipment, message processing, driver interface, and other applications to support the use cases described herein.
Revocable lane A lane definition that can be enabled or disabled. Each revocable lane represents a possible regulatory state for a given physical lane. Whether the lane is enabled or disabled is broadcasted in the SPaT message.
Roadside Unit (RSU)

Devices that serve as the demarcation component between vehicles and other mobile devices and existing traffic equipment.

Note: From DSRC Roadside Unit (RSU) Specification Document v4.1.

Signal Phase and Timing (SPaT) A message type that describes the current state of a signal system and its phases and relates this to the specific lanes (and therefore to maneuvers and approaches) in the intersection. See SAEJ2735.

BSM - Basic Safety Message

C-ITS - Cooperative Intelligent Transportation System

CV - Connected Vehicle

GNSS - Global Navigation Satellite System

GPS - Global Positioning System

IEEE - Formerly Institute of Electrical and Electronics Engineers

ISO - International Standards Organization

ITS - Intelligent Transportation Systems

NMEA - National Marine Electronics Association Message

OBU - On-Board Unit

PRL - Protocol Requirements List

RSU - Roadside Unit

RTCM - Radio Technical Commission for Maritime Services Message

SAE - Formerly Society of Automotive Engineers International

SPaT - Signal Phase and Timing

SRM - Signal Request Message

SSM - Signal Status Message

TS - Technical Specification

USDOT - United States Department of Transportation

V2I - Vehicle-to-Infrastructure

V2V - Vehicle-to-Vehicle

6. References

Connected Vehicle Basics

Deployment (General)

7. Study Questions

1. Which of the following user needs for a signalized intersection is not addressed with SPaT data?

  1. Receive currently allowed vehicle movements
  2. Receive lane location descriptions
  3. Receive suggested vehicle speeds
  4. Receive estimated times when signal indications will change

2. Signal timing information for how many intersections can be included in a single SPaT message?

  1. Only one signalized intersection
  2. Only one signalized and one non-signalized intersection
  3. Up to two signalized intersections along an arterial
  4. Up to 32 signalized intersections

3. Which of the following attributes for a lane is included in a MAP message?

  1. The centerline locations of a lane
  2. The permitted direction of travel of the lane
  3. The permitted vehicle types that may use the lane
  4. All of the above

4. When broadcasting SPaT and MAP messages, which of the following issues must be considered?

  1. Only one intersection is described in each SPaT and MAP message
  2. All MAP messages must be accompanied by a SPaT message
  3. Other standards may limit the number of bytes in a message
  4. SPaT and MAP messages must use the same broadcast rate

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