Effects of Digital Countdown Timer on Intersection Safety and Efficiency: A Systematic Review

<b>Objective</b>: To investigate the available evidence referring to the effectiveness of digital countdown timer (DCT) to improve the safety and operational efficiency of signalized intersection.<b>Methods</b>: A systematic review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement guidelines. Relevant literature was searched from electronic databases using key terms. Based on study selection and methodological quality assessment, fourteen studies were included in the review. Findings of the included studies were synthesized in a narrative analysis. <b>Results</b>: Three types of DCT had different effects on intersection safety and operational efficiency. Green signal countdown timer (GSCT) reduced red light violation, type I dilemma zone distribution, and rear-end collision likelihood, but increased crossing after yellow onset and had mixed impacts on type II dilemma zone distribution and intersection capacity. In contrast, red signal countdown timer (RSCT) increased intersection capacity, despite its effectiveness of reducing red light violation dissipated over time. Likewise, continuous countdown timer (CCT) significantly enhanced intersection capacity, but had mixed influences on red light violation and crossing after yellow onset. <b>Conclusions</b>: Due to the limited and inconsistent evidence regarding DCT's effects on intersection safety and efficiency, it is not sufficient to recommend any type of DCT to be installed at signalized intersections for improving the safety level and operational efficiency. Nevertheless, it is apparent that both RSCT and CCT enhance intersection capacity, though their impacts on intersection safety are unclear. Future studies need to further verify those anticipated safe and operational benefits of DCT with enriched field observation data.