Abstract: This invention presents a mobile application for real-time diagnosis and optimization of traffic signal control systems. It utilizes innovative techniques described in this document to develop virtual signal controllers into a mobile device and takes advantage of the GPS technology equipped with the mobile device to develop vehicle trajectories. The signal timing data and the vehicle movement data are then integrated into a dynamic time-space diagram for performance analysis and signal optimization. The invention is unique in that all of the functions are developed into mobile devices and thus very user friendly and easy to carry. In addition, it provides various means and functions for traffic engineers to manage, examine and fine tune traffic signals in real-time. Compared to the traditional methods that mostly rely on the engineering judgment, the present invention facilitates traffic engineers to identify the problems and make improvements in a lot more convenient and efficient way.