I. Introduction
Traffic prediction is the cornerstone of intelligent transportation systems. Accurate prediction methods are critical for practical traffic applications. For instance, road traffic speed and state forecast is of great significance for dynamic management of urban traffic, improvement of quality and efficiency of the logistics industry, and route planning of public travel. Due to the irregular structure of the traffic network and the variable temporal characteristics of traffic flow, the traffic forecasting problem presents challenges that traditional time series forecasting problems do not possess.