1. A navigation apparatus comprising: a processing resource configured to: receive, for each of a plurality of road segments of a digital map, a classifier associated with the road segment, each classifier being one of a set of general classifiers of road segment properties; receive weather-related data, the weather related data comprising a plurality of speed modification factors, each speed modification factor associated with a different classifier in the set of general classifiers, and each speed modification factor computed based on a weather condition and a reaction of roads having the corresponding classifier to the weather condition; for each of the plurality of road segments, modify speed data for the road segment by: using the classifier associated with the road segment to determine a selected speed modification factor from among the plurality of speed modification factors; and modifying the speed data based on a product of an expected speed of travel for the road segment for normal conditions and the selected speed modification factor; and perform a navigation operation using the modified speed data, wherein the navigation operation comprises at least one of determining and providing a route to a destination and determining and providing an estimated time of arrival (ETA).