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 in a vehicle, the process 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, wherein at least one of the plurality of conditions is from a group consisting of time of day and day of week, 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 speed 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 speed in a conditional variant model associated with the condition.