In this study, we investigate the method to increase the accuracy of traffic estimation based on multiple GPS data from GPS-equipped vehicles in Bangkok area, Thailand. We propose an algorithm to predict an average speed of a road segment by combining hourly speed profiles of the road with real-time GPS velocity data. The algorithm utilizes incremental weighted update of average travel velocity from both sources. The velocity of data from GPS is used for update every time the data comes in, whereas the speed profiles are used for update when there is no data for a period of five minutes. The outputs are compared with a speed obtained with the position-based method. In this algorithm provides the low average speed error results around 8 km/h approximately.
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