Park Juneyoung, Abdel-Aty Mohamed
Department of Civil, Environmental and Construction Engineering, University of Central Florida, Orlando, FL 32816-2450, United States.
Accid Anal Prev. 2015 Feb;75:310-9. doi: 10.1016/j.aap.2014.12.012. Epub 2014 Dec 25.
Numerous studies have attempted to evaluate the safety effectiveness of specific single treatment on roadways by estimating crash modification factors (CMFs). However, there is a need to also assess safety effects of multiple treatments since multiple treatments are usually simultaneously applied to roadways. Due to the lack of sufficient CMFs of multiple treatments, the Highway Safety Manual (HSM) provides combining method for multiple CMFs. However, it is cautioned in the HSM and related sources that combined safety effect of multiple CMFs may be over or under estimated. Moreover, the literature did not evaluate the accuracy of the combining method using CMFs obtained from the same study area. Thus, the main objectives of this research are: (1) to estimate CMFs and crash modification functions (CM Functions) for two single treatments (shoulder rumble strips, widening (1-9ft) shoulder width) and combination (installing shoulder rumble strips+widening shoulder width) using the observational before-after with empirical Bayes (EB) method and (2) to develop adjustment factors and functions to assess combined safety effects of multiple treatments based on the accuracy of the combined CMFs for multiple treatments estimated by the existing combining method. Data was collected for rural two-lane roadways in Florida and Florida-specific safety performance functions (SPFs) were estimated for different crash types and severities. The CM Functions and adjustment functions were developed using linear and nonlinear regression models. The results of before-after with EB method show that the two single treatments and combination are effective in reducing total and SVROR (single vehicle run-off roadway) crashes. The results indicate that the treatments were more safety effective for the roadway segments with narrower original shoulder width in the before period. It was found that although the CMFs for multiple treatments (i.e., combination of two single treatments) were generally lower than CMFs for single treatments, they were getting similar to the roadway segments with wider shoulder width. The findings indicate that the combined safety effects of multiple treatments using HSM combining method are mostly over-estimated and the accuracy of HSM combining method vary based on crash types and severity levels. Therefore, it is recommended to develop and apply the adjustment factors and functions to predict the safety effects of multiple treatments when the HSM combining method is used.
众多研究试图通过估计碰撞修正因子（CMF）来评估道路上特定单一处理措施的安全有效性。然而，由于多种处理措施通常会同时应用于道路，因此也有必要评估多种处理措施的安全效果。由于缺乏足够的多种处理措施的CMF，《公路安全手册》（HSM）提供了多种CMF的组合方法。然而，HSM及相关资料中提醒，多种CMF的综合安全效果可能被高估或低估。此外，文献中并未评估使用来自同一研究区域的CMF的组合方法的准确性。因此，本研究的主要目标是：（1）使用带有经验贝叶斯（EB）方法的前后观测法，估计两种单一处理措施（路肩振动带、拓宽（1 - 9英尺）路肩宽度）及其组合（安装路肩振动带 + 拓宽路肩宽度）的CMF和碰撞修正函数（CM函数）；（2）基于现有组合方法估计的多种处理措施的组合CMF的准确性，开发调整因子和函数以评估多种处理措施的综合安全效果。收集了佛罗里达州农村双车道道路的数据，并针对不同的碰撞类型和严重程度估计了佛罗里达州特定的安全性能函数（SPF）。使用线性和非线性回归模型开发了CM函数和调整函数。带有EB方法的前后观测结果表明，两种单一处理措施及其组合在减少总体碰撞和单车驶出路外（SVROR）碰撞方面是有效的。结果表明，这些处理措施在前期对于原始路肩宽度较窄的路段更具安全有效性。研究发现，虽然多种处理措施（即两种单一处理措施的组合）的CMF通常低于单一处理措施的CMF，但对于路肩宽度较宽的路段，它们变得相似。研究结果表明，使用HSM组合方法的多种处理措施的综合安全效果大多被高估，并且HSM组合方法的准确性因碰撞类型和严重程度级别而异。因此，建议在使用HSM组合方法时开发并应用调整因子和函数来预测多种处理措施的安全效果。
