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.

研究目标：探究现有关于数字倒计时计时器（digital countdown timer, DCT）提升信号交叉口安全性与运行效率的有效性的相关证据。研究方法：本研究遵循系统评价与荟萃分析优先报告条目（Preferred Reporting Items for Systematic Reviews and Meta-Analysis, PRISMA）指南开展系统综述。通过关键词检索电子数据库以获取相关文献。经文献筛选与方法学质量评估后，共纳入14项研究，并采用叙述性分析对纳入研究的结果进行综合梳理。研究结果：三类数字倒计时计时器对交叉口安全性与运行效率的影响存在差异。绿灯倒计时计时器（Green signal countdown timer, GSCT）可减少闯红灯行为、I型两难区域分布以及追尾碰撞发生概率，但会增加黄灯启亮后的过街行为，并对II型两难区域分布与交叉口通行能力的影响存在分歧。与之相对，红灯倒计时计时器（Red signal countdown timer, RSCT）可提升交叉口通行能力，尽管其降低闯红灯行为的效果会随时间逐渐减弱。同样，连续式倒计时计时器（Continuous countdown timer, CCT）可显著提升交叉口通行能力，但对闯红灯行为与黄灯启亮后的过街行为的影响存在不确定性。研究结论：鉴于现有关于数字倒计时计时器对交叉口安全性与运行效率影响的证据有限且结论不一致，目前尚无足够依据推荐在信号交叉口安装任意一类数字倒计时计时器以提升其安全性与运行效率。不过，红灯倒计时计时器与连续式倒计时计时器均可显著提升交叉口通行能力，尽管其对交叉口安全性的影响尚不明确。未来研究需借助更丰富的实地观测数据，进一步验证数字倒计时计时器预期的安全与运行效益。