I. Introduction
Traffic congestion has become an ubiquitous problem for many big cities in both developed and developing countries, which can result in various issues including decreasing the road traffic capacity, slowing down the vehicle speed and increasing the air pollution. Currently, Intelligent Transportation System (ITS) has been widely adopted to improve traffic conditions. As one important goal in ITS, accurately predicting traffic conditions in real time can help the drivers better plan their routes, shorten travel time and improve traffic conditions, and thus effectively alleviate the issues of air pollution, energy consumption and traffic accidents. Therefore, the study of traffic flow prediction is of great importance in ITS and has attracted increasing research interest recently.