An improved Moran's I method is proposed to describe the spatio temporal features of road traffic flow based on the traditional Moran's I. The spatio temporal distribution classification of road traffic status including homogenous congested and uncongested traffic, heterogeneous congested and uncongested traffic is then given through Moran plotting graph. To verify the applicability of the proposed method, the traffic flow data along the second ring-shape urban expressway in Beijing is introduced for analysing the spatio temporal distribution and evolution of road traffic status from global and local autocorrelation through using the proposed method.