What is C-ITS? Use cases, standards & deployment basics

C-ITS, Cooperative Intelligent Transport Systems, let vehicles, roadside infrastructure, and other road users exchange data with each other in real time. That single capability is the basis for everything from a roadworks warning to full vehicle platooning.

Over 2,000 C-ITS roadside stations are already operational across the EU, backed by roughly €124 million in CEF Transport funding spread across more than 20 projects, according to CINEA. At the same time, the radio standard carrying all of that traffic, ITS-G5 or C-V2X, is still not settled the same way in every market.

This guide covers what C-ITS actually means, the three tiers of use cases behind it, and the ITS-G5 versus C-V2X standards conflict underneath them, and what each tier actually requires to build.

What C-ITS means, and why it’s not the same as ITS

Cooperative Intelligent Transport Systems (C-ITS) is the real-time exchange of data between vehicles, road infrastructure, and other road users over V2X communication. That covers vehicle-to-vehicle, vehicle-to-infrastructure, vehicle-to-pedestrian, and vehicle-to-network links, aimed at better road safety, more efficient traffic flow, and lower emissions.

C-ITS is a subset of the broader ITS (Intelligent Transportation System) category, not a synonym for it like most people thought. This matters since older ITS technology broadcasts information to every road user indiscriminately. Examples include variable message signs, inductive loop detectors, and fixed-timing signals.

C-ITS is defined by two-way, vehicle-aware communication instead. For a broader look at ITS technology, see Eastgate’s ITS applications guide.

A vehicle running C-ITS doesn’t just see a sign. It receives a message addressed to its situation, and depending on the use case, it can respond. Three message types recur across every C-ITS use case:

• DENM messages carry hazard notifications, such as a stopped vehicle or a road hazard.
• CAM messages carry basic awareness data, a vehicle’s position and heading broadcast continuously.
• CPM messages carry collective perception data, what a sensor or vehicle detects about its surroundings and shares with others.

The clearest example of C-ITS in practice is a roadworks warning. A roadside unit broadcasts a DENM message describing the closure ahead. A vehicle within range receives it and displays the warning in-vehicle before the driver could otherwise see the work zone. Roadworks and hazard warnings like this are the most common across the EU today.

One quick heads-up: “C-ITS” means something different depending on where you’re searching. Korea runs its own separate national C-ITS master plan, and it doesn’t coordinate with the EU/CAR 2 CAR framework.

C-ITS use cases: The three tiers

In C-ITS, the Day 1, Day 2, and Day 3+ labels track how much vehicles have to trust and coordinate with each other, from a one-way broadcast to a full negotiation.

Day 1: Broadcast awareness for hazard warnings

Day 1 is one-way information delivery. Infrastructure or a vehicle sends a fixed warning, and the receiving vehicle displays it. No negotiation between vehicles happens at this tier, and none is required.

C-Roads is the EU platform coordinating harmonized C-ITS deployment across member states:

• Hazardous location notification
• Road works warning
• Traffic jam warning
• Weather condition alerts
• Slow or stationary vehicle warning
• In-vehicle speed limit information

This Day 1 category also carries different labels. Some examples are Intersection Collision Warning, Emergency Vehicle Warning, Dangerous Situation Warning, Stationary Vehicle Warning, Traffic Jam Warning, and Pre-/Postcrash Warning.

Day 1 is also what’s running today, and its deployment has outpaced the other tiers precisely because it needs the least new infrastructure.

Day 2: Shared perception for vulnerable road users

Day 2 asks vehicles and infrastructure to share what their own sensors detect. This is a higher bar on data volume and positioning accuracy than Day 1 requires.

GLOSA, or Green Light Optimal Speed Advisory, is the flagship Day 2 use case. A signalized intersection communicates its signal timing. An approaching vehicle receives a speed recommendation that lets it clear the intersection on a green light, rather than braking and re-accelerating.

Vulnerable road user (VRU) warnings belong at this tier, too. Protecting a pedestrian or cyclist needs real-time positioning precision that a static broadcast message can’t provide.

GLOSA and VRU warnings don’t share a mechanism. GLOSA runs on SPaT (Signal Phase and Timing) and MAP messages from the signal controller. These carry the intersection’s timing plan and layout. VRU warnings run on CPM, the collective perception message, which carries what a sensor detected about a nearby pedestrian or cyclist.

That’s why Day 2 systems need both a live signal-timing feed and real sensor fusion on the backend, rather than a simple message relay.

Day 3+: Coordinated vehicle maneuvers

Day 3+ is where vehicles start negotiating actions directly with each other rather than just sharing awareness of the environment. Platooning, cooperative lane changes, and cooperative merging all sit here.

This tier assumes a higher level of vehicle automation, tighter latency, and more reliable requirements than Day 1 or Day 2. Moreover, Cooperative Adaptive Cruise Control sits at the boundary between Day 2 and Day 3+.

These tiers explain what C-ITS can do at each stage of maturity. The next question is what standard carries the data that makes any of it work.

ITS-G5 vs C-V2X: Why C-ITS still runs on two standards

Every C-ITS message, at every tier, has to travel over a physical radio standard, and the industry has not settled on a single one.

Ford has pushed cellular-based C-V2X rollouts abroad, while the EU’s C-Roads platform leans toward ITS-G5 in practice.

In fact, 2019 EU policy vote considered mandating a single standard and instead left the choice to manufacturers and operators, according to Volvo Trucks’ C-ITS guide.

Neither standard has won globally. In practice, the EU’s Day 1 rollouts lean toward ITS-G5, consistent with the C-Roads deployments already running in Germany and Austria. On the other hand, manufacturer-driven deployments elsewhere lean toward C-V2X. What you end up building depends heavily on which regulatory environment you’re deploying into.

The practical consequence for a deployment plan doesn’t depend on resolving that debate first. If you’re targeting the EU’s C-Roads-aligned markets, default to ITS-G5 unless there’s a specific reason to deviate.

If manufacturers in your market are already committing to cellular V2X, plan around C-V2X from day one instead of staying standard-agnostic. Building for both simultaneously is possible, but it adds real integration cost since the two standards don’t share a physical layer.

What each C-ITS tier requires to deploy

A use case’s tier maps to a specific, escalating set of hardware, security, and backend requirements. Knowing which tier you’re targeting tells you what to budget and build for.

Deploying Day 1: Roadside units and PKI certificates

Day 1 needs the least new infrastructure of any tier. It requires a roadside unit broadcasting standard DENM and CAM messages, plus a functioning public key infrastructure (PKI) layer to sign them.

Every source that touches on C-ITS security names PKI as non-negotiable, and PKI sits inside the broader IEC 62443-4-1 compliance standard for EU traffic infrastructure. Here is a brief underlying operational work:

• Issuing and rotating certificates for every roadside unit and vehicle on the network.
• Managing trust anchors across national borders, since C-Roads deployments span multiple member states.
• Keeping revocation current so a compromised certificate stops being trusted quickly.

No backend negotiation logic is required at this tier. That’s a large part of why Day 1 dominates real-world EU deployment today.

PKI operations like this are where mission-critical delivery work actually lives. Our mission-critical systems practice treats certificate issuance and revocation with the same zero-failure-tolerance standard we apply to traffic signal control.

Deploying Day 2: Sensor fusion and signal integration

Collecting and distributing collective perception messages means the backend has to fuse sensor data from multiple sources in near real time. That’s a fundamentally different job from relaying a fixed string the way Day 1 does.

Positioning accuracy has to improve on both the infrastructure and the vehicle side. GLOSA and VRU warnings only work if the position data behind them is precise enough to act on. The integration surface also expands to include traffic signal controllers specifically, sometimes running as a virtual traffic light controller rather than dedicated hardware.

GLOSA depends on live signal-timing data that a Day 1 deployment never had to touch, the kind of signal performance monitoring that is built only after Day 1 is already running.

This is also where procurement decisions start to compound. A Day 1 roadside unit and a Day 2 roadside unit can look identical on a spec sheet. Your backend contract for a Day 2 deployment still needs to specify sensor-fusion throughput and signal-controller integration explicitly.

Otherwise, the hardware you’ve bought ends up broadcasting messages no more capable than Day 1’s, regardless of what it’s technically capable of.

Deploying Day 3+: Negotiation logic and latency limits

This stage requires negotiation logic between vehicles and infrastructure. That brings it closer to a real-time coordination platform than a messaging layer. Maneuver Coordination Messages (MCMs) carry the negotiation itself. Vehicles propose and agree to a merge, a lane change, or a platoon formation instead of just broadcasting awareness of what’s around them.

Latency and reliability requirements at this tier are tight enough that they directly constrain which physical standard, ITS-G5 or C-V2X, is viable for a given deployment.

Treating this tier as operational infrastructure today would be inaccurate. It belongs in a deployment roadmap, not a current-state description.

Engineers on a deployment project most likely receive the CAR 2 CAR Communication Consortium‘s own technical use-case catalog directly. It doesn’t organize use cases by Day tier at all. It splits them into five categories instead:

• Vehicles Coordination
• Intersection Crossing Assist
• Partial and High Automation
• Advanced Warning and Information
• Agriculture-specific, for farm equipment

Where actual EU deployment stands today shapes what to plan next.

C-ITS deployment and regulation in the EU today

Directive (EU) 2023/2661 updates the original 2010/40/EU ITS framework directive. It took effect on December 20, 2023, and gave EU member states until December 21, 2025, to write it into their own national law, according to the European Parliament’s Legislative Train Schedule. So far, there are no updates on compliance data yet to say who’s actually caught up.

The European Commission fills in technical detail through later delegated and implementing acts. That’s the same mechanism it used for eCall and real-time traffic information requirements under the original 2010/40/EU directive, according to the European Commission’s ITS Directive and Action Plan.

Deployment isn’t even across the EU. Take Germany, for example. Autobahn GmbH des Bundes already runs an active C-ITS deployment today. Austria’s ASFINAG is doing something similar, running a comparable C-Roads-aligned deployment.

The Cooperative ITS Corridor, running from Rotterdam through Frankfurt to Vienna, is a separate, well-known example of cross-border cooperation. Austria’s own C-ITS strategy names the corridor as an example of that cooperation, not as the basis for its national deployment decisions.

At Eastgate, we also built a motorway traffic prediction platform for German motorway operators that runs on that same kind of real-time sensor pipeline these deployments rely on.

The bottom line

C-ITS is a tier choice from Day 1 and a standard choice between ITS-G5 or C-V2X. Treating either one in isolation is how a project ends up with hardware that can’t run the use case it was bought for.

You can plan Day 1 with real confidence today. The hardware patterns, the PKI approach, and the funding routes through programs like CEF Transport are all proven at scale. Day 2 and Day 3+ are where the standard choice starts to matter more. This is also where an early backend mistake gets expensive to unwind once a rollout is underway.

If your team is scoping a C-ITS rollout, our ITS engineering practice builds traffic signal systems and V2X platforms for transportation agencies. We’ve made these same tier and standards calls on real projects.