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

Typical triggers:

1. roadworks (including mobile roadworks / safety trailers)
2. stationary or broken-down vehicle
3. traffic jam ahead
4. emergency vehicle approaching
5. adverse weather or slippery road

A DENM contains, among other things:

1. the event type (a standardized “cause code”)
2. the position and the area affected by the event
3. detection and validity times

On motorways, DENMs are typically sent by RSUs and roadworks safety trailers; vehicles generate them e.g. for emergency braking or breakdown warnings.