In conjunction with GPS-enabled smart-phones, the method of Floating Car Data (FCD), a part of Intelligent Transportation Systems (ITS), offers faster and cheaper procedure to gather and display real-time traffic data. However, this type of phone at its current state is only able to provide data at low accuracy; thus, additional procedures are required to improve the data accuracy. In this research, an FCD-based monitoring system with virtual trip line (VTL) is designed and implemented, and some formulas are used in the proposed algorithm to minimize the data error. The developed system and algorithms are empirically evaluated using data collected by a probe vehicle traveling on Jakarta Inner Ring Road in Jakarta, Indonesia. As the results, the real-time traffic monitoring system is developed which is able to display the traffic data in less than 15~s for 90% of the cases. In addition, the research also establishes the optimal length of the VTL and the optimal weighted moving average filter to correct the geo-location and velocity errors.