Enhancing School Zone and School Bus Safety

Safety issues in school zone areas have been one of the most important topics in the traffic safety field. This research project assesses the safety effects of different roadway countermeasures in school zone areas. Although many studies have evaluated the effectiveness of various traffic control devices (e.g., sign, flashing beacon, speed monitoring display), there is a lack of studies exploring different roadway countermeasures that might have significant impacts on the school zone safety. In this research project, the most crash-prone school zone was identified in Orange and Seminole Counties, Florida, based on crash rate, which is defined as crash per thousand daily vehicle miles traveled. The results showed that Westridge Middle and Sadler Elementary schools were the top two crash-prone school zones. Afterward, a microsimulation network was built in VISSIM to test different roadway countermeasures in the school zones. Before applying different countermeasures, the network was calibrated and validated by traffic volume and travel time in order to replicate the real field. Three different countermeasures—two-step speed reduction, decreasing the number of driveways, and converting the two-way left-turn lane (TWLTL) to a raised median—were implemented in microsimulation and compared with the field condition. For each countermeasure, we also ran different sub-scenarios. In two-step speed reduction, we analyzed three sub-scenarios that were defined by the maximum speed limit on the main roadway. The number of driveways was reduced by 25%, 50%, 75%, and 100%, so four sub-scenarios were used to analyze in this countermeasure. We replaced TWLTL with a raised median, so all the left-turning vehicles made left turns either at the intersection or median. Therefore, two sub-scenarios, intersection U-turn and median U-turn, were analyzed. Surrogate safety measures are widely used as indicators to evaluate crash risk in the microsimulation software as it cannot directly measure the traffic crashes. In this research project, three surrogate safety measures were used; two of them were developed from time-to-collision (TTC) notations. Three surrogate safety measures—time-exposed time to collision (TET), time-integrated time to collision (TIT), and (3) time-exposed rear-end crash risk index (TERCRI) —were utilized in this research project as indicators for safety evaluation. The higher value of surrogate safety measures indicates higher crash risk. The results showed that all the sub-scenarios in two-step speed reduction and decreasing driveway access reduced TET, TIT, and TERCRI values significantly compared to the base condition. Moreover, the combination of two-step speed reduction and decreasing driveway access countermeasures outperformed their individual effects as well as the base condition. The one-way ANOVA analysis showed that all the sub-scenarios were significantly different from each other. Sensitivity analysis was also conducted to capture the impact of different sub-scenarios for different values of TTC threshold. The results show that all the sub-scenarios in two-step speed reduction and decreasing the number of driveway access reduced TET, TIT, and TERCRI values significantly for different values of TTC threshold, which ranged from 1 to 3 s. Conversely, for converting the TWLTL to the raised median, the crash risk was higher than the base condition because the value of TET, TIT, and TERCRI was much higher than the base condition. Therefore, the results of this research project provide useful insights for transportation and safety planners.

学校区域的交通安全问题一直是交通安全领域的核心议题之一。本研究项目旨在评估学校区域内不同道路安全对策的安全效益。尽管已有诸多研究评估了各类交通管控设施（如交通标志、闪光警示灯、速度监测显示屏）的有效性，但针对可能对学校区域交通安全产生显著影响的多样化道路安全对策的探索性研究仍较为匮乏。本研究基于事故率（即每千车日行驶里程对应的事故数），在佛罗里达州奥兰治县与塞米诺尔县筛选出了事故多发的学校区域。研究结果显示，韦斯特里奇中学（Westridge Middle）与萨德勒小学（Sadler Elementary）为该区域内事故发生率最高的两所学校区域。随后，研究团队借助VISSIM搭建了微观仿真路网，用于测试学校区域内的各类道路安全对策。在实施各类对策前，研究团队通过交通流量与行程时间对路网进行了校准与验证，以确保仿真结果能够复现真实道路场景。本次研究共设置三类道路安全对策：两步式减速措施、减少车道出入口数量，以及将双向左转车道（two-way left-turn lane, TWLTL）改造为凸起式中央分隔带，将其在仿真路网中实施并与真实场景进行对比。针对每一类对策，研究团队均设置了多组子场景开展仿真测试。在两步式减速措施的子场景中，研究基于主路最高限速设置了三组子场景进行分析。针对减少车道出入口的对策，研究分别设置了出入口减少25%、50%、75%与100%的四组子场景开展分析。将双向左转车道改造为凸起式中央分隔带后，所有左转车辆仅可在交叉口或中央分隔带处完成左转，因此研究设置了交叉口掉头与中央分隔带掉头两组子场景进行分析。由于微观仿真软件无法直接量化交通事故，替代安全指标（surrogate safety measures）被广泛用于评估交通碰撞风险。本研究共采用三类替代安全指标，其中两类基于碰撞时间（time-to-collision, TTC）方法推导而来。本次研究选用的三类替代安全指标分别为：暴露型碰撞时间（time-exposed time to collision, TET）、积分型碰撞时间（time-integrated time to collision, TIT），以及暴露型追尾碰撞风险指数（time-exposed rear-end crash risk index, TERCRI），将其作为安全评估的核心指标。替代安全指标的数值越高，则代表碰撞风险越高。研究结果显示，相较于基准场景，两步式减速措施与减少车道出入口的所有子场景均能显著降低TET、TIT与TERCRI的数值。此外，两步式减速措施与减少车道出入口对策的组合方案，其安全效益优于单独实施任一对策的效果，同时也优于基准场景。单因素方差分析（one-way ANOVA）结果显示，所有子场景的仿真结果均存在显著差异。研究还开展了敏感性分析，以探究不同TTC阈值（取值范围为1至3秒）下各子场景的影响变化。结果表明，在TTC阈值1至3秒的所有取值下，两步式减速措施与减少车道出入口的所有子场景均能显著降低TET、TIT与TERCRI的数值。与之相反，将双向左转车道改造为凸起式中央分隔带的方案，其碰撞风险高于基准场景，因为该方案下TET、TIT与TERCRI的数值远高于基准场景。综上，本研究的结果可为交通规划与安全规划人员提供极具参考价值的决策依据。