1. An apparatus for generating supplementary information for a digital map, the supplementary information representing suitability of road use according to vehicle category, the apparatus comprising: a receiver, the receiver being operative to receive trip information representing sessions of logged journeys from a plurality of navigation devices, each session of a logged journey including vehicle category information in respect of a vehicle making the logged journey, a memory; and a processor, the processor being operative to: analyse the vehicle category information to identify at least one session corresponding to a vehicle category of interest, the vehicle category of interest including at least one selected from: a goods-vehicle; a bus; a car+caravan combination; a vehicle exceeding a predetermined weight threshold; a vehicle exceeding a predetermined width threshold; a vehicle exceeding a predetermined height threshold; a vehicle exceeding a predetermined length threshold; and a vehicle carrying predetermined hazardous material; analyse the trip information corresponding to at least one vehicle category of interest, to identify a route or route portion used by the vehicle on its journey; analyse plural routes or route portions from a plurality of sessions to identify statistically at least one trend of road use by vehicles of the vehicle category of interest, wherein the trend is a trend of vehicles of the category of interest having deviated from an expected route; and generate said supplementary information based on the at least one trend of road use by vehicles of the vehicle category of interest, the supplementary information representing suitability of road use according to vehicle category, wherein the generated supplementary information comprises information indicative of at least one road preferred by the users of the vehicles of the vehicle category of interest.