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====== What is C-ITS? ======

**C-ITS** stands for **Cooperative Intelligent Transport Systems**: vehicles and road infrastructure exchange information directly with each other over radio, without going through any network operator or manufacturer cloud.

This page explains the technology itself. Which vehicles and which infrastructure actually transmit is covered on [[en_alt:senders|Senders]]; where it is deployed is on [[en_alt:coverage|Coverage]].

===== How it works =====

C-ITS messages are broadcast on **5.9 GHz** using **ITS-G5** (also called WLANp or IEEE 802.11p) — a variant of Wi-Fi adapted for fast-moving stations and short handshakes. There is no association, no access point and no pairing: a station simply transmits, and everyone in range can hear it.

Range is a few hundred metres in built-up areas and up to a few kilometres with a clear line of sight. That is why the map only shows live data where a receiver is nearby.

The band is divided into several channels. Almost everything interesting happens on the **control channel, G5CC at 5900 MHz**, which is what every receiver in this wiki listens to by default:

^ Channel ^ Frequency ^
| G5CC | 5900 MHz |
| G5SC2 | 5890 MHz |
| G5SC1 | 5880 MHz |
| G5SC3 | 5870 MHz |
| G5SC4 | 5860 MHz |

If you want to look at a service channel instead, the receiver firmwares let you change the channel — but expect to see far less traffic there.

**The messages are not encrypted.** Anyone with suitable hardware can receive and read them — which is what makes this project possible in the first place. Messages can, however, be cryptographically **signed**, so a receiver can tell whether a message genuinely comes from an authorised sender. To protect privacy, vehicles do not use a fixed identifier but transmit under **changing pseudonyms**.

Participants fall into two groups:

  * **RSUs (Road Side Units)** — stationary senders: traffic lights, roadworks signs, toll gantries.
  * **OBUs (On-Board Units)** — mobile senders: trams, buses, cars, emergency vehicles.

===== Message types =====

Several standardised message types travel over C-ITS. These are the ones that matter in practice.

^ Type ^ Full name ^ In one sentence ^
| [[en_alt:c-its#cam|CAM]] | Cooperative Awareness Message | Periodic status broadcast from a vehicle |
| [[en_alt:c-its#denm|DENM]] | Decentralized Environmental Notification Message | Warning about an exceptional situation |
| [[en_alt:c-its#mapem|MAPEM]] | MAP Extended Message | The geometry of an intersection |
| [[en_alt:c-its#spatem|SPATEM]] | Signal Phase and Timing Extended Message | The current state of a traffic light |
| [[en_alt:c-its#ivim|IVIM]] | Infrastructure to Vehicle Information Message | Road sign and infrastructure information |

==== CAM ====

Compatible vehicles broadcast CAM messages at 1–10 Hz, typically four times per second. A CAM contains, among other things:

  - GPS position
  - Speed
  - Heading
  - Longitudinal and lateral acceleration
  - Vehicle length and width
  - Status of the lights, e.g. indicators, low beam, rear fog light

Since 2019 more and more cars have been equipped with C-ITS and transmit this data, as long as Car2X is enabled in the vehicle's menu. This is the data we use to draw vehicles on the map:

{{:cam2.png?0x420|Vehicles on the live map, drawn from CAM messages}}

==== DENM ====

DENMs are **event-driven** warning messages (ETSI EN 302 637-3). While CAMs are broadcast continuously, a DENM is only sent when something exceptional happens, and it is repeated for as long as the event is valid.

Typical triggers:

  - Roadworks, including mobile roadworks and safety trailers
  - A stationary or broken-down vehicle
  - Traffic jam ahead
  - An emergency vehicle approaching
  - Adverse weather or a slippery road

A DENM contains the event type as a standardised //cause code//, the position and the area affected, and the detection and validity times.

On motorways, DENMs typically come from RSUs and from roadworks safety trailers. Vehicles generate them for events such as emergency braking or a breakdown.

==== MAPEM ====

MAPEMs describe the **topology of an intersection** (ETSI TS 103 301): the geometry of its lanes, which ingress and egress lanes exist, and which manoeuvres connect them.

A MAPEM contains the reference position of the intersection, lane geometries as offset points, lane attributes (vehicle lane, bike lane, crosswalk) and the permitted connections between lanes, referenced by signal group.

On their own MAPEMs are mostly static. The dynamic part — the actual signal states — is transmitted separately as SPATEM. Together the two enable applications such as GLOSA (Green Light Optimal Speed Advisory).

==== SPATEM ====

SPATEMs transmit the **current state of a traffic light** (ETSI TS 103 301): which signal groups are red, amber or green right now, and how that state is expected to change.

A SPATEM contains the current phase per signal group, predicted change times (for example "likely green in 12 s") and references to the intersection topology from the matching MAPEM.

C-ITS traffic lights send SPATEMs several times per second. Together with MAPEM they are what lets some vehicles display a countdown to green.

==== IVIM ====

IVIMs carry **road sign and infrastructure information** (ETSI TS 103 301 / ISO 19321) — electronic road signage, essentially.

Typical contents:

  - Variable speed limits from overhead gantries
  - Lane closures and lane management
  - Warnings shown on variable message signs
  - Sign information for a stretch of road

IVIMs are sent by RSUs, for example along the ASFINAG motorway network. Unlike a DENM, an IVIM applies to a defined section of road rather than to a single point.

===== Further reading =====

Background on the technology and on the policy behind it. Sources for individual deployments are listed with the regions themselves, on [[en_alt:coverage|Coverage]].

  * [[https://de.wikipedia.org/wiki/C-ITS|Wikipedia: C-ITS]] (German)
  * [[https://www.bmimi.gv.at/themen/mobilitaet/alternative_verkehrskonzepte/telematik_ivs/publikationen/citsstrategie.html|Federal Ministry: C-ITS strategy for Austria]]
  * [[https://c-its-deployment-group.eu/activities/c-its-deployments/|C-ITS Deployment Group: C-ITS is now ready for use in Germany]] (04/2021)
  * [[https://www.its-viennaregion.at/|ITS Vienna Region]] — the ITS competence centre for Vienna, Lower Austria and Burgenland
  * [[https://www.youtube.com/watch?v=G2VGzrJVO2Q|Video: How to Improve Public Transport with C-ITS — Vienna]] (05/2026)