1. A navigation apparatus comprising: a processing resource configured to determine a route to a selected destination, to obtain weather-related data and to modify speed data for a plurality of route segments in dependence on the weather-related data, wherein for each route segment there is a respective classifier that classifies the road type for the route segment, and wherein 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.