Rail height effects on safety performance of Midwest Guardrail System

<b>Objective</b>: Guardrail heights play a crucial role in the way that errant vehicles interact with roadside barriers. Low rail heights increase the propensity of vehicle rollover and override, whereas excessively tall rails promote underride. Further, rail mounting heights and post embedment depths may be altered by variations in roadside terrain. An increased guardrail height may be desirable to accommodate construction tolerances, soil erosion, frost heave, and future roadway overlays. This study aimed to investigate and identify a maximum safe installation height for the Midwest Guardrail System that would be robust and remain crashworthy before and after pavement overlays. <b>Methods</b>: A research investigation was performed to evaluate the safety performance of increased mounting heights for the standard 787-mm (31-in.)-tall Midwest Guardrail System (MGS) through crash testing and computer simulation. Two full-scale crash tests with small passenger cars were performed on the MGS with top-rail mounting heights of 864 and 914 mm (34 and 36 in.). Test results were then used to calibrate computer simulation models. <b>Results</b>: In the first test, a small car impacted the MGS with 864-mm (34-in.) rail height at 102 km/h (63.6 mph) and 25.0° and was successfully redirected. In the second test, another small car impacted the MGS with a 914-mm (36-in.) rail height at 103 km/h (64.1 mph) and 25.6° and was successful. Both system heights satisfied the <i>Manual for Assessing Safety Hardware (MASH)</i> Test Level 3 (TL-3) evaluation criteria. Test results were then used to calibrate computer simulation models. A mounting height of 36 in. was determined to be the maximum guardrail height that would safely contain and redirect small car vehicles. Simulations confirmed that taller guardrail heights (i.e., 37 in.) would likely result in small car underride. In addition, simulation results indicated that passenger vehicle models were successfully contained by the 34- and 36-in.-tall MGS installed on approach slopes as steep as 6:1. <b>Conclusions</b>: A mounting height of 914 mm (36 in.) was determined to be the maximum guardrail height that would safely contain and redirect 1100C vehicles and not allow underride or excessive vehicle snag on support posts. Recommendations were also provided regarding the safety performance of the MGS with increased height.

**研究目的**：护栏高度对失控车辆与路侧护栏的交互行为起着决定性作用。过低的护栏高度会增加车辆翻车和骑跨护栏的概率，而过高的护栏则易引发车辆下穿护栏的风险。此外，路侧地形的变化会改变护栏安装高度与立柱埋置深度。为适配施工容许偏差、土壤侵蚀、冻胀以及未来路面加铺需求，提升护栏高度往往是必要的。本研究旨在探究并确定中西部护栏系统（Midwest Guardrail System, MGS）的最大安全安装高度，确保该系统在路面加铺前后均具备稳定可靠的防撞性能。
**研究方法**：本研究通过全尺寸实车碰撞试验与计算机仿真模拟，对标准高度为787毫米（31英寸）的中西部护栏系统（Midwest Guardrail System, MGS）在提升安装高度后的安全性能展开评估。针对顶部护栏安装高度分别为864毫米（34英寸）与914毫米（36英寸）的MGS，开展了两次小型乘用车全尺寸碰撞试验。随后以试验结果对计算机仿真模型进行校准。
**研究结果**：首次试验中，一辆小型乘用车以102 km/h（63.6英里/小时）的速度、25.0°的碰撞角撞击了顶部护栏高度为864毫米（34英寸）的MGS，试验顺利完成，车辆被成功引导改向。第二次试验中，另一辆小型乘用车以103 km/h（64.1英里/小时）的速度、25.6°的碰撞角撞击了顶部护栏高度为914毫米（36英寸）的MGS，试验同样取得成功。两种护栏高度均满足《公路护栏安全性能评估手册》（Manual for Assessing Safety Hardware, MASH）碰撞等级3（TL-3）的评估准则。研究团队再次以试验结果对计算机仿真模型进行校准。最终确定36英寸（914毫米）的安装高度为可安全约束并引导小型乘用车的最大护栏高度。仿真结果证实，更高的护栏高度（如37英寸）大概率会引发小型车辆下穿护栏的风险。此外，仿真结果显示，当34英寸与36英寸高度的MGS安装在坡度最大为6:1的引道坡上时，可成功约束所有测试的乘用车辆。
**研究结论**：914毫米（36英寸）的安装高度被确定为可安全约束并引导1100C型小型乘用车的最大护栏高度，且不会引发车辆下穿护栏或过度剐蹭支撑立柱的问题。本研究同时针对提升安装高度后的MGS安全性能提出了相关建议。