Vehicle-to-infrastructure — the mode that pays back first.
A roadside unit is not a radio repeater. It is an authenticated participant in the PKI, bolted to a pole, reading state out of a traffic signal controller and broadcasting a signed description of what that signal is about to do. Everything downstream — the green-wave advisory, the red-light violation warning, the bus priority request — depends on the receiver being able to prove the message came from a device with permission to speak for that intersection.
Geometry, phase, and everything the road authority wants a driver to know.
| Message | SAE name / ID | ETSI name | Contents |
|---|---|---|---|
| Signal phase and timing | SPAT (19) | SPATEM | Current signal state per movement, with time-to-change |
| Intersection geometry | MapData (18) | MAPEM | Lane-level geometry, approach and egress lanes, connections |
| Traveller information | TIM (22) | IVIM | Variable speed limits, roadworks, digital signage, advisories |
| Roadside alert | RSA (44) | — | Localised hazard broadcast |
| Signal request / status | SRM (24) / SSM (25) | SREM / SSEM | Priority or preemption request, and its status |
| GNSS corrections | RTCMcorrections (28) | RTCMEM | Differential corrections for lane-level positioning |
| Road weather | RWM (23) | — | Roadside weather and surface condition |
SPaT is typically broadcast at around 10 Hz; MAP, which describes geometry that does not change, at around 1 Hz. In Europe these all sit under ETSI TS 103 301, currently at V2.3.1 (April 2026). The message sets in full →
Where the measured benefits actually are.
US deployments have produced the field evidence. Each figure comes from a USDOT-published pilot evaluation with its own methodology and study period.
- Indiana DOT queue warning: roughly 80% reduction in hard-braking events
- Connecticut DOT worker-presence warning: collision risk with roadside workers down up to 90%
- Cleveland GCRTA transit: bus driver reaction time to pedestrian conflicts improved 19%
- Tampa THEA pilot: forward-collision conflicts down 9%, travel time down 30%
- Columbus school zones: speed compliance up from 18% to 56%
The efficiency side of the same picture — GLOSA, signal priority, the field-versus-simulation distinction — is set out with its sources on what V2X is for.
Linear returns beat quadratic ones when you are starting from zero.
Equip one intersection and every equipped vehicle that passes it benefits, from the first day. That is why the V2X services actually shipping in production vehicles are V2I services delivered over cellular — Audi's Traffic Light Information, launched in 2016 and reaching more than 22,000 connected intersections across 60-plus agencies in the US as of July 2021, and the traffic-light push features in Chinese C-V2X models. That 22,000 figure is dated to July 2021; current coverage could not be confirmed.
It is also why India's spectrum policy treats RSUs and OBUs differently. OBUs are licence-exempt under G.S.R. 466(E). RSUs require authorisation, with eligibility restricted in the MoRTH Task Force position to central and state governments and their authorised agencies. An RSU is a trusted insider in a public-safety PKI, and a compromised one can inject signed messages into every vehicle in range. That is a licence with a security bar, not a procurement footnote. India's V2X framework →
An RSU only matters if it is connected to something.
The interesting engineering is not the radio; it is the eight places an RSU touches the road authority's existing systems.
| Surface | What flows |
|---|---|
| Signal controller | RSU reads controller state and broadcasts SPaT plus intersection geometry |
| Adaptive control | Aggregated, de-identified vehicle counts and queue data feed the timing engine |
| Command centre | RSU health, tamper alerts, event streams into the city ICCC or highway control room |
| Incident and work zone | Control-room-authored hazard notifications, signed by the RSU under its own permissions |
| Priority and preemption | Emergency-vehicle and bus priority — a permission that must be cryptographically restricted, not configured |
| Back-office and data platform | Aggregated traffic, incident and asset-health data into the operator's own systems |
| Edge compute | Local fusion at the pole where backhaul is thin; the RSU as an authenticated edge node, not a dumb relay |
| PKI back end | Certificate issuance, renewal, revocation, CRL and trust-list distribution over the backhaul |
Open questions worth stating openly, because nobody has settled them: who authors a roadside message — the control room, the RSU operator, or both under separate permissions? What is the minimum interface set to standardise so that any RSU works with any traffic controller already in the field? Who is accountable when a signed roadside message is wrong but validly issued?
These are integration questions before they are procurement questions, and they are where most of the engineering effort in a real deployment actually goes. If you are scoping an RSU programme and working through them, talk to us — it is the conversation we are set up to have. The hardware side is AmbiRSU.
Where this fits.
V2V
The quadratic case, and why it has not bootstrapped.
What V2X is for
The efficiency evidence these deployments produce, field and simulated.
Message sets
SPaT, MAP, TIM and the SREM/SSEM priority exchange.
India regulation
Why RSU authorisation is a separate question from OBU licensing.
AmbiRSU
Roadside hardware treated as infrastructure — tamper detection, secure boot, hardware-held credentials.
V2X PKI
How an RSU's permission to speak for an intersection is enforced cryptographically.
Questions this page answers.
What is a roadside unit?
An RSU is a V2X transceiver mounted at a fixed location — an intersection, a gantry, a toll plaza, a work-zone trailer — that broadcasts infrastructure messages and, in most architectures, relays or aggregates messages from passing vehicles. It holds its own PKI credentials, signs everything it sends, and typically has a backhaul connection to a traffic management centre.
What is the difference between V2I and V2N?
V2I is direct radio between a vehicle and a nearby roadside unit, over PC5 sidelink or ITS-G5, at millisecond latency and with no network dependency. V2N goes over the cellular network to a server, at latencies measured in hundreds of milliseconds to seconds. V2I supports safety-critical applications; V2N supports informational ones.
How many roadside units are deployed?
There is no reliable global figure and you should be suspicious of any single number. Regionally: C-Roads reports more than 6,000 RSUs across Europe covering 30% of the TEN-T road network as of February 2026; China has more than 11,000 by one 2025 analysis; the US has no authoritative national total, with an ITS America survey finding around 18 agencies reporting deployments ranging from 2 to over 1,000 units each. India had effectively zero as of the TRAI consultation in April 2026.
Who is allowed to operate an RSU in India?
Not settled. The TRAI consultation, following MoRTH correspondence, indicates RSU authorisation restricted to “Central or State Governments or any other agencies authorized by them,” with private entities considered only for non-safety applications. This differs sharply from most jurisdictions, where RSUs are class-licensed or licence-exempt. TRAI's recommendations had not been issued as of mid-August 2026.