V2X Explained / Traffic signals
SPATEM + MAPEM · TS 103 301
How a vehicle reads a traffic light.
A connected traffic light sends two messages, and it needs both. One carries the signal timing, the other the map that says which lane the timing belongs to. On their own, each is useless; together they let a car understand the junction ahead.
The pair
A map that holds still, and timing that moves.
MAPEM
The map
The fixed geometry of the intersection: every lane, every approach, and how they connect. It changes rarely, so it is sent about once a second as a stable reference.
SPATEM
The timing
The live state of each signal group: which is green now, and when it will change. It updates constantly, because it is the part that actually moves.
Why they only work together
A timing message says "signal group 4 turns green in six seconds." That means nothing until you know which physical lane signal group 4 controls, and whether it is the lane the vehicle is actually in. The map supplies that link, tying each signal group to a lane and each lane to a direction of travel.
The two are joined by a shared intersection identifier. A receiver waits until it holds both the map and the timing for the same junction before it acts. This is where real integrations tend to break, and why both messages are defined together in ETSI TS 103 301.
What it enables
Four things a car can do once it can read the light.
Time to green
A dashboard shows the driver how long until the light changes, so nobody creeps or floors it on a guess.
Speed advice (GLOSA)
The vehicle works out the speed that catches the next green, smoothing traffic and cutting stop-start fuel burn.
Red-light warning
A car that is not going to stop in time gets warned, and so does cross traffic about to pull away.
Priority
A bus or an ambulance asks the junction for green and hears back whether it was granted.