A method is provided of providing jam warning messages in relation to jams affecting navigable segments of a network of navigable segments. The method comprises, for each of one or more identified jams, obtaining a jam tail flow speed for the jam. The jam tail flow speed is preferably determined from the analysis of positional data relating to the current or relatively recent movement of a plurality of devices associated with vehicles along at least some navigable segments of a navigable network with respect to time. The jam tail flow speed is used to generate, or to determine whether to generate, a jam warning message in respect of the jam.