In this paper, we exploit the concept of sharing GPS data for estimate traffic information. Explicitly, we implemented CarWeb, a platform to collect real-time GPS data from cars. Once receiving a sufficient amount of GPS data from cars, we are able to estimate traffic information (i.e., speed information of roads). As such, users are able to obtain the update-to-date traffic information in CarWeb platform. In addition, we propose two algorithms to estimate traffic information with GPS data given. A prototype of CarWeb is implemented and functionalities of each component in CarWeb are demonstrated in this paper.