Quantities regarding road traffic states such as traffic volume or speed may be changed over time. It is necessary to know a short-term change within one day or within several hours when evaluating the reliability of travel time. Not only that but also it is necessary to understanding the long-term variation over several years or decades, when making the transport policy such as the one on the tolling. The aim of this study is to conduct long-term time-series modeling for monthly traffic volume data of Japanese urban/interurban highways including Hanshin Expressway and East Nippon Expressway and the daily traffic volume data of Hanshin Expressway. By employing a structural time series model, this study estimates the several components of traffic volume variability comprehensively.