Generally, trees are naturally occurring fixed objects that are found along many roadways and that potentially pose safety risks to errant motorists. Unfortunately, trees have been responsible for numerous fatal and serious injury crashes during run-off-road events. This study included an incremental benefit-to-cost (B-C) analysis that used the Roadside Safety Analysis Program to investigate the efficacy of safety treatment alternatives for trees on roadways with volumes of less than 500 vehicles per day (vpd) and speed limits of 55 mph (88.5 km/h) or greater. The study was based on a parametric analysis of site characteristics from a field survey in Kansas. It used four tree groupings, three tree diameters, and four lateral offsets from the roadway to configure 120 scenarios. Three safety treatment methods were considered: (a) a do-nothing option representing the baseline condition; (b) tree removal, with cost estimates coming from reliable sources; and (c) a crashworthy guardrail system. For various reasons, the guardrail system was no more cost-effective than the do-nothing or tree removal options. B-C ratios were used to recommend tree removal on the basis of several pertinent variables. In all cases, B-C ratios for tree removal were never less than 1.0, which indicated limited justification for keeping the trees. Tree removal was considered the safest and primary alternative when trees were far from other fixed obstacles. Because these guidelines are based solely on B-C analyses, the road designer or engineer is encouraged to use them as a foundation for making safety improvements but also to consider site-specific investigation and analysis.
Guardrail terminals have evolved to the point where they absorb energy while utilizing tension in the rail to countermand the compression. However, non-gating terminals have yet to be developed. In the present study, the possibility of a non-gating guardrail terminal was investigated. Specifically, the combination of lateral and longitudinal forces that produce non-gating performance were determined from computer simulation. Next, a prototype terminal was crash tested at the research team’s laboratory. A terminal head was designed to deform the guardrail, and its internal structure was adjustable to control the longitudinal force. Posts were designed to control lateral forces by modifying their section modulus. This controlled the force at which the posts buckled in response to a collision. A prototype was subjected to two 15° crash tests using an SUV and a small car. In both tests, the kinetic energy of the test vehicle was fully absorbed and the Manual for Assessing Safety Hardware (MASH) criteria would have been met. Neither vehicle passed beyond the terminal head, making these test results the first of their kind.
Crash cushions vary in geometry and cost. In this study, crash cushions were categorized in three different categories: redirecting with repair costs greater than $1,000 (RGM), redirecting with repair costs less than $1,000 (RLM), and nonredirecting sacrificial (NRS). Typically, RGM systems are less expensive initially, but life-cycle costs are high. RLM systems typically reciprocate this trend. NRS crash cushions (e.g., sand barrels) are generally less expensive but require total replacement after a crash has occurred, which may be impractical at hightraffic volume locations. Due to limited funding, there is often a need to identify the most cost-effective crash cushion category for highway scenarios with different roadway, traffic, and roadside characteristics. This study was commissioned to determine benefit-cost ratios for each crash cushion category in a wide range of roadway and roadside characteristics using the probability-based encroachment tool, Roadside Safety Analysis Program. Only RGM and RLM systems were cost effective for freeways and divided rural arterials, but all three categories competed against the unprotected condition on undivided rural arterials and local roads.
The objective of the present study is two-fold. First, the elucidation of the biomechanics of penetrating trauma as a result of guardrail intruding into the occupant compartment. Second, the evaluation of the biomechanical efficacy of hybrid tension-compression guardrails to better protect occupants. The nine fatally guardrail penetrating crashes occurred between 2016 and 2019 were analyzed to study the mechanism of injuries. Four car-to-guardrail crash tests were conducted using a hybrid guardrail that integrated the commonly used W-beam with a new design of tension-based end terminal. The test included the impact of a bogey-type platform, small sedan vehicles, and a pick-up truck at highway speeds onto the guardrail. The impact orientation was varied to simulate the frontal and oblique corner crashes with a speed ranging from 90 to 111 kph. The real-world studies showed that the fatal injuries were due to impaling guardrail regardless of vehicular speed and size. The occupants not in the trajectory of the guardrail in the same vehicle sustained minor injuries despite experiencing a similar energy level. In these cases, the crash severity was survivable without the guardrail penetration. The mean pre-impact speed, change in speed, and vehicular acceleration was 117 kph, 20 kph, and 97 m/sec2, respectively. The hybrid guardrail system deflected the vehicle without any penetration into the occupant compartment. The mean peak accelerations in crash tests were below injurious threshold levels. The present research shows that the hybrid guardrail system not only eliminated the intrusion into occupant survival space but also deflected the vehicle.
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