What is C-ITS?

C-ITS stands for Cooperative Intelligent Transport Systems: vehicles and infrastructure communicate with one another.

Typical C-ITS participants are:

Stationary RSUs - Road Side Units

  1. Traffic lights (traffic signal installations)
  2. Roadworks signs
  3. Toll gantries on motorways

Vehicles, OBUs (On-Board Units)

  1. Trams
  2. Buses
  3. Cars
  4. Emergency vehicles

C-ITS packets are transmitted on 5.9 GHz. The packets are unencrypted, but they can be signed.

An overview of which vehicles and infrastructure actually send C-ITS: Who is sending C-ITS?

C-ITS Messages

Several standardised message types travel over C-ITS. The message contents are specified by ETSI. Machine readable message specifications are published as ASN.1 files (https://forge.etsi.org/rep/ITS/asn1).

All messages share a common header. This header includes the messageId, which specifies the type of message.

Message Id Type Full name In one sentence
1 DENM Decentralized Environmental Notification Message Warning about an exceptional situation
2 CAM Cooperative Awareness Message Periodic status broadcast from a vehicle
4 SPATEM Signal Phase and Timing Extended Message The current state of a traffic light
5 MAPEM MAP Extended Message The geometry of an intersection
6 IVIM Infrastructure to Vehicle Information Message Road sign and infrastructure information
9 SREM Signal Request Extended Message Request message for green light at intersection
10 SSEM Signal request Status Extended Message Reply message to SREM
14 CPM Collective Perception Message Sensor information

Other, less common, message types are:

While ETSI defines the structure of C-ITS messages and their permitted value ranges, some values remain open to interpretation—for example, a role may be represented as an integer without specifying what a value such as 42 actually means. ETSI can also be vague about when and how often messages should be transmitted, sometimes specifying only a broad range such as 1–100 Hz.

C-Roads (https://www.c-roads.eu/), an EU-funded platform for harmonising C-ITS, addresses these ambiguities. It defines concrete use cases, specifying which messages should be sent, how information should be modelled within those messages, which values to use, and under what conditions and frequency messages should be transmitted.