A method of processing data relating to an electronic map, the map comprising a plurality of segments representing navigable segments in the area covered by the map, the method comprising using a processing apparatus to generate segment data and comprising the steps of: collecting historic travel data for the segment; defining a jam condition for the segment, such that where the jam condition is satisfied the segment is classified as jammed and otherwise as not jammed; generating a jam probability for the segment according to the historic travel data and the jam condition definition; generating a jam speed for the segment using the historic travel data, the jam speed being indicative of the speed on the segment when the segment is considered jammed; and associating the jam probability and the jam speed with the segment in the electronic map.