A method of generating map data indicating a deviation from expected jam conditions on a segment of a plurality of segments in an area covered by an electronic map, the segment having associated therewith a jam probability indicating the likelihood of a jam on that segment. The method comprises establishing an expected jam condition for the segment based on the jam probability of that segment, and then obtaining live data indicating the jam conditions on at least one of the plurality of other segments in the area. A revised jam condition can then be established for the segment using the obtained live data.