Abstract: With large-scale and complicated spatio-temporal characteristics, visual analytics of taxi GPS data is a challenging issue.In this paper, we present a visual analytic method for road traffic analysis based on taxi GPS data, and we adopt two encoding schemes, the discrete arrow graph and the continuous stack graph, to explore the volume, direction, speed and other information of road traffic flow on widened roads based on OpenStreetMap.Douglas-Peucker algorithm and the velocity clustering algorithm are used for data reduction and improving rendering respectively. The preprocessed taxi GPS data are stored in a cloud computing platform in a distributed manner, and MapReduce is utilized to accelerate data and query processing.We test the validities of our proposed encoding schemes on Hangzhou taxi GPS data.Experimental results show that our method can effectively and accurately reveal the status of road traffic in Hangzhou.