A mapping program for execution by at least one processing unit of a device is described. The mapping program selects a traffic segment defined in a first data structure describing congestion in an area of a map. The mapping program identifies a portion of a road in the map area that corresponds to the traffic segment by using a second data structure to correlate the traffic segment in the first data structure to the road that is defined in a third data structure. The mapping program defines a traffic congestion representation to render for the identified road portion, based on data about the corresponding road in the third data structure.