A method of verifying and/or improving a message indicative of an event affecting traffic flow on at least a portion of one or more navigable segments is described. The method involves obtaining positional data relating to the movement of a plurality of devices along a navigable stretch including a navigable stretch identified by the message as being affected, and using the positional data to verify and/or improve the message. The positional data is live data and is used to obtain data relating to the speed of travel of the devices along the navigable stretch including the stretch identified by the message. The speed data can be used to determine a spatial extent of the affected stretch and/or an expected speed of travel along the stretch.