The effectiveness of in-vehicle navigation systems in alleviating non-recurrent congestions is examined. Computer simulation modeling is used to assess the effectiveness of four different types of navigation systems consisting of static map, dynamic map, route guidance and advanced route guidance systems. The study is conducted for the Irvine network, Orange County, California. The results of the study shows that the performance of descriptive in-vehicle navigation systems (static map and dynamic map) is dependent on the initial network flow conditions and could lead to severe worsening of traffic condition. The prescriptive systems (route guidance and advanced route guidance) could substantially improve the traffic situation; the performance of these systems is dependent on the level of market penetration, i.e. the fraction of drivers equipped with the navigation system. The appropriate level for the Irvine network is shown to be 30% or more.