The present invention combines the geographical coverage possible with fixed, pre-defined
route segment costs (e.g. the legal
speed limit) with, wherever possible, richer time dependent costs. A user of, for example, a portable navigation device, can therefore continue
route planning as before to virtually any destination in a country covered by the stored map
database, but wherever possible, can also use traffic data with time-dependent costs, so that the effect of congestion with any time
predictability can be accurately taken into account as an automatic,
background process. It leaves the user to simply carry on driving, following the guidance offered by the navigation device, without needing to be concerned about congestion that exists now, and whether it will
impact his journey.