A method of generating expected average speeds of travel across one or more segments in the area covered by an electronic map, each segment representing a navigable segment in the area covered by the map and having associated therewith: an average speed of travel across that segment; segment data comprising at least one jam probability, the jam probability indicating the likelihood of a jam on that segment; and a jam speed indicative of the speed of travel on that segment when it is considered jammed. The method comprises using the segment data to predict whether a jam condition exists on at least one or each of the segments; and outputting the expected average speed of travel across these segments as: a) a first speed, where a jam condition is predicted to exist, the first speed being the jam speed for the relevant segment or a speed based thereon, or b) a second speed, where a jam condition is not predicted, the second speed being the average speed for the relevant segment or a speed based thereon.