1. A machine-readable medium encoded with instructions, that when executed by one or more processors, cause the processor to carry out a process for generating attribute models for use in a navigation system, the process comprising: computing a value for each desired attribute along one or more road segments covered by a driving session for a user, wherein the desired attribute comprises at least one selected from a group consisting of road speed, road familiarity, road safety, and disfavored roads; computing a default value for unseen road segments not yet traveled; and forming one or more attribute models for each attribute based on the corresponding attribute values.