A navigation apparatus 200 comprises a processing resource 260 configured to: determine a route to a selected destination; obtain weather-related data; and modify speed data for a plurality of route segments in dependence on the weather-related data. For each route there is a respective classifier that classifies the road type for the route segment, and the processing resource is configured to modify the speed data for each route segment based on the weather-related data and the road type represented by the classifier for that route segment.