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This paper presents an extensive investigation regarding the impacts of green signal countdown devices (GSCD) on the intersection safety and efficiency, based on field observation of critical driver and vehicle related parameters at two similar intersections (one with GSCD and the other without GSCD) in Shanghai. Statistical analysis results have revealed that the installation of GSCD can (1) encourage drivers to pass the stop-line during the amber time with higher speeds and thus result in better utilization of the amber time and increased capacity of the intersection approach; (2) smooth the driver's response to the phase transition and effectively prevent the sudden change of speeds; (3) effectively eliminate the intersection dilemma zones by allowing the drivers to envision the phase transition and make decisions in advance; and (4) significantly reduce the number of red-light violations. Despite its effectiveness in facilitating the driver decision-making process, the installation of GSCD may adversely increase the possibility of collisions with unexpected crossing vehicles or pedestrians due to the significantly increased speeds of vehicles approaching the amber time. The analysis results from this study will offer the basis for traffic practitioners, researchers, and authorities in China to assess the impact of GSCD with respect to its operational and safety performance, and propose guidelines for its proper field installation. [ABSTRACT FROM AUTHOR]