Rail height effects on safety performance of Midwest Guardrail System

Objective: 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. Methods: 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. Results: 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 Manual for Assessing Safety Hardware (MASH) 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. Conclusions: 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.

研究目标：护栏高度在失控车辆与路边防护栏的交互行为中起到至关重要的作用。较低的护栏高度会增加车辆侧翻与骑越（override）的概率，而过高的护栏则会引发钻越（underride）问题。此外，路边地形的变化可能会改变护栏安装高度与立柱埋置深度。为适配施工公差、土壤侵蚀、冻胀（frost heave）以及未来路面加铺层的需求，提升护栏高度有时是必要的。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可靠的防护性能与防撞能力。

研究方法：针对标准787毫米（31英寸）高的中西部护栏系统，通过实车碰撞试验与计算机仿真模拟，评估其提升安装高度后的安全性能。针对顶梁安装高度分别为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，并以试验结果校准计算机仿真模型。

研究结果：首次试验中，一辆小型乘用车以102千米/小时（63.6英里/小时）的速度、25.0°的角度撞击864毫米（34英寸）高的MGS，护栏成功将车辆导向正确路径。第二次试验中，另一辆小型乘用车以103千米/小时（64.1英里/小时）的速度、25.6°的角度撞击914毫米（36英寸）高的MGS，试验同样取得成功。两种高度的护栏系统均满足《道路安全硬件评估手册》（Manual for Assessing Safety Hardware, MASH）的测试等级3（Test Level 3, TL-3）评价标准。随后以试验结果校准计算机仿真模型，结果显示36英寸（914毫米）的安装高度为可安全容纳并引导小型乘用车的最大护栏高度。仿真验证表明，更高的护栏高度（如37英寸）大概率会导致小型车辆钻越护栏。此外，仿真结果显示，当安装于坡度陡至6:1的引坡上时，34英寸与36英寸高的MGS仍可成功容纳乘用车模型。

研究结论：914毫米（36英寸）的安装高度被确定为可安全容纳并引导1100C型车辆的最大护栏高度，且不会出现车辆钻越或过度剐蹭支撑立柱的问题。本研究同时针对提升高度后的MGS安全性能提出了相关建议。