A navigation guidance system is provided which provides direction information from a remote server to a mobile user unit for guiding user to a desired destination. The navigation guidance system maintains time-related models for traffic speeds expected on road segments of a road database. By maintaining such time-related models, the system can more accurately provide the user with road guidance instructions to arrive at a chosen destination by a given time. The system can also provide more accurate route guidance information to a destination over different routes, even during rush hour or at off peak times. These time related models can be changed as a result of changing traffic patterns or as a result of an incident on a road segment. In a preferred embodiment, therefore, the system continuously monitors each calculated route and advise the users of alternative routes or of updated departure or arrival times as appropriate.