41. A method of generating attribute models for use in a navigation system in a vehicle, the method comprising: for each of a plurality of driving sessions, during which the user travelled over a plurality of road segments in the vehicle: collecting sensor data of the vehicle for each of the plurality road segments travelled during a driving session; and associating the sensor data with at least one of a plurality of conditions, each condition descriptive of the driving session at the time the user travelled on the road segment; for each of the plurality of road segments, determining a value of a road attribute of the road segment based upon the sensor data collected for that road segment, and associated with at least one of the conditions; and for each of the plurality of conditions, storing the values of the road attribute in a conditional variant model associated with the condition, wherein storing the values of the road attribute in a conditional variant model associated with the condition comprises: partitioning values of the road attribute into separate data buckets by explicit conditions; and forming a conditional variant model for each respective explicit condition based on the values of the road attribute in that bucket.
42. The method of claim 41 , wherein the road attribute is road speed.