Office of Operations 

Research and Development 

1

Office of Operations 

Research and Development 

  Adds to the

WHAT IS CARMA?

The CARMASM Program is the Federal 

Highway Administration’s initiative to 

enable development and collaboration 

for CDA research. CARMA’s purpose is 

to transform the surface transportation 

ecosystem by improving efficiency and 

safety through automated vehicles 

working together and with roadway 

infrastructure.

BACKGROUND

The SAE J3216 Standard1 defines the 

CDA cooperation classes for applications 

to enable cooperation between vehicles 

and other entities to improve safety and 

efficiency. The SAE J2735 Standard2

defines a message set for V2X 

communication using the available 5.9 

GHz bandwidths for Wireless Access in 

Vehicular Environments (WAVE) 

communications systems. WAVE 

provides an opportunity to expand the 

message set to support general V2X 

intent sharing and agreement seeking 

about future actions and to use it for 

5G and other wireless communication 

technologies. In addition, the 

technology could support general 

descriptions of traffic controls.

THE CARMA SUITE OF TOOLS

The CARMA suite of tools, including 

CARMA PlatformSM, CARMA CloudSM, 

CARMA StreetsSM, and CARMA 

Messenger, allows informed and 

dynamic cooperation among 

vehicles, infrastructure, and other 

roadway users. All these tools work 

together with the open-source 

software V2X Hub to enhance 

networked, wireless communications 

between automated vehicles, 

infrastructure, and personal devices.

A set of six new message types 

(described on page 4) that address 

needs for intent sharing, CDA 

agreement seeking, traffic control 

information, and maneuvering 

recommendations was developed 

through the CARMA Program. The 

message structures are flexible to 

support evolving research needs and 

are structured in accordance with the 

Standard J2735 Standard2

 to better 

facilitate incorporation. These new 

messages are used for:

 Intent sharing: A CARMA Platform

vehicle can receive information

about where other CARMA vehicles

intend to go and the maneuvers 

they plan to execute. This information 

allows the CARMA vehicle to plan a 

trajectory that safely avoids other 

vehicles while taking advantage of 

known gaps in traffic lanes. This 

capability is enabled by the SAE 

CDA Class B3

 communication class.

 Agreement seeking: A CARMA

Platform vehicle can communicate

with neighboring entities to seek

agreement on a mutually beneficial

set of cooperative maneuvers such

as smoothly weaving two traffic

streams. This capability is enabled by

the SAE CDA Class C

communication class.

4

 Traffic control information: CARMA

Cloud can communicate changes in

traffic controls to CARMA Platform

vehicles with a high level of detail

related to geometry and traffic

control timing.

 Maneuvering recommendations:

CARMA Streets can communicate

maneuvering recommendations,

such as cooperative passing

through a four-way stop, with

CARMA Platform vehicles.

The CARMA Ecosystem has created six new message

types to enable vehicle-to-everything (V2X)

communication between vehicles and other entities

for various CDA cooperation classes, as defined in 

SAE J3216,

1 which enables true cooperative behavior.

Cooperative Driving Automation

(CDA) Message Set to Improve

Intelligent Transportation Systems

(ITS) Communications

©oonal/ iStock/Getty Images Plus

1 SAE International. 2021. Taxonomy and Definitions for Terms Related to 

Cooperative Driving Automation for On-Road Motor Vehicles. J3216_202107. 

Warrendale, PA: SAE International. https://www.sae.org/standards/content/

j3216_202107, last accessed September 17, 2021.

2 SAE International. 2016. Dedicated Short Range Communications (DSRC) 

Message Set Dictionary. J2735_201603. Warrendale, PA: SAE International. 

https://www.sae.org/standards/content/j2735_201603/, last accessed 

September 17, 2021.

3 SAE International. 2011. Class B Data Communication Network Messages 

- Detailed Header Formats and Physical Address Assignments (STABILIZED Apr 

2011). J2178/1_201104. Warrendale, PA: SAE International. https://www.sae.

org/standards/content/j2178/1_201104/, last accessed September 17, 2021

4 SAE International. 1993. Class C Communications Protocol Proposal for 

Off-Road Vehicles. 930007. Warrendale, PA: SAE International. 

https://www.sae.org/publications/technical-papers/content/930007/, last 

accessed September 17, 2021.
2 3

OFFICE NAME TITLE OF TECHBRIEF

Figure 1 depicts several practical 

cooperative use cases where the 

new message set is needed. Each 

box represents a single message 

and shows its type (refer to the 

message type details on page 4). 

The groupings represent use case 

events that work together. The 

number is the sequence in which 

the message is broadcast within 

that use case:

 Messages may be repeated

periodically as information is

updated.

 Message semantics are the focus

of the diagram.

 Security provisions operate

behind the scenes and may

include authentication

certificates, message encryption,

or other practices.

CARMA Adds to CDA Message Set to Improve ITS CARMA Adds to CDA Message Set to Improve ITS 

 

 

 

 

 

Figure 1. Diagram. CARMA Message Sets.

Source: FHWA

CMV = commercial motor vehicle; DOT = department of transportation; ID = identification.

To learn more about CARMA, 

visit: highways.dot.gov/research/

research-programs/operations/

CARMA
TITLE OF TECHBRIEF

For more information, please contact Taylor Lochrane, CARMA Program Manager: 

Taylor.Lochrane@dot.gov

FHWA-HRT-21-104

HRDO-10/10-21(WEB)E

4

CARMA Adds to CDA Message Set to Improve ITS 

The six new message types developed by the CARMA program, which fall under either mobility or geofence messages, 

can be used to enhance capabilities for intent sharing, agreement seeking, traffic control information, and 

maneuvering recommendations.

Mobility messages: Each message has a common set of information at the beginning that includes fixed 

identifications for the sender and optional identifications for a targeted recipient (either of which could be a 

vehicle, infrastructure, pedestrian, or cyclist), as well as temporary basic safety message identifications, plan 

identifications, and timestamps.

Geofence messages: Each message is applied to part of a single traffic lane and designed to communicate 

traffic control updates to CARMA vehicles via CARMA Cloud.

Broadcasts a message to inform other vehicles of the message sender’s (primary vehicle’s) current location 

and intended path over the next several seconds.

Proposes a specific relationship with one or more neighboring entities (e.g., cooperative lane merge, 

pedestrian crossing, or commercial motor vehicle safety check) and indicates the urgency of the plan. This 

could include a proposed vehicle trajectory. Entities can mean vehicles, road infrastructure, pedestrians, 

and other road users.

Accepts or rejects another entity’s proposal.

Communicates specific relationship parameters, such as platoon operations or emergency vehicle presence 

(if on shoulder, alerts for the move-over law).

MOBILITY PATH

MOBILITY REQUEST

MOBILITY RESPONSE

MOBILITY OPERATION

Sent periodically from a vehicle to the roadside infrastructure, specifying the vehicle’s desired travel route 

and current map information, and asking for any available updates to traffic rules (e.g., speed limits and lane 

closures) along that route.

Broadcasts in response to a traffic control request and contains any changes to traffic controls (e.g., lane 

markings, speed limits, and lane use restrictions) since the requesting entity’s last update. The message is 

customized for the requesting entity.

TRAFFIC CONTROL REQUEST

TRAFFIC CONTROL MESSAGE

To learn more about CARMA, visit: highways.dot.gov/research/research-programs/operations/CARMA

Recommended citation: Federal Highway Administration, CARMA Adds to Cooperative Driving Automation (CDA) 

Message Set to Improve Intelligent Transportation Systems (ITS) Communications (Washington, DC: 2021) 

https://doi.org/10.21949/1521690