Department of Civil and Construction Engineering, Western Michigan University, Kalamazoo, Michigan.
Traffic Inj Prev. 2020;21(6):401-406. doi: 10.1080/15389588.2020.1773450. Epub 2020 Jun 4.
The main objective of this study was to assess the change in crash patterns associated with speed limit changes from 55 mph to 70 mph that occurred on some of Michigan freeway segments between year 2005 and year 2010. Many of the statistical methods used in the past to evaluate the safety impacts of raising the speed limit on freeways lack the ability to address one or more critical issues inherent in count data, such as omitted-sample bias, over-dispersion and regression-to-the-mean bias. This study used multilevel mixed-effects negative binomial regression to address these limitations, with an additional advantage of controlling for intra-cluster correlation of crashes on each freeway corridor and segments nested in the same corridor. Changes in the crash patterns between the year 2000 and year 2015 were investigated on test sites that had a change of speed limit from 55 mph to 70 mph, relative to control sites where the speed limit was maintained at 55 mph. The inclusion of random effects improved the model's ability in explaining observed crash variations on the selected freeway segments, as indicated by test statistics such as the log-likelihood ratio test, the Akaike information criterion and Bayesian information criterion (BIC)values. Further, random effects improved the significant speed limit change fixed effects during model calibration. The final mixed-effects model indicated a significant increase in fatal and injury crashes (FI), total crashes (KABCO) and road departure crashes by 11.9 percent, 21.0 percent and 23.3 percent, respectively, on freeway segments where the speed limit was raised from 55 mph to 70 mph. The increase in road departure crashes was more pronounced on curved freeway segments with the raised speed limit compared to straight segments with no speed limit changes. The 15 mph increase in the speed limit on Michigan freeways had a significant association with the increase in fatal and injury crashes, total crashes and road departure crashes. The elevated crash risks associated with the speed limit increase suggest that further studies are needed to understand changes in drivers' behaviors following a speed limit increase.
本研究的主要目的是评估 2005 年至 2010 年间密歇根州部分高速公路限速从 55 英里/小时提高到 70 英里/小时后与碰撞模式相关的变化。过去用于评估提高高速公路限速对安全性影响的许多统计方法都缺乏解决计数数据中固有一个或多个关键问题的能力，例如抽样偏差、过度分散和均值回归偏差。本研究使用多层次混合效应负二项回归来解决这些限制，并且具有控制每个高速公路走廊和嵌套在同一走廊中的部分的碰撞的内部分层相关性的额外优势。在限速从 55 英里/小时提高到 70 英里/小时的测试点上，调查了 2000 年至 2015 年之间的碰撞模式变化，相对于限速保持在 55 英里/小时的对照点。随机效应的纳入提高了模型在解释所选高速公路段观测到的碰撞变化方面的能力，这一点从对数似然比检验、Akaike 信息准则和贝叶斯信息准则（BIC）值等检验统计数据中可以看出。此外，随机效应在模型校准过程中提高了显著的限速变化固定效应。最终的混合效应模型表明，限速从 55 英里/小时提高到 70 英里/小时的高速公路段上，致命和受伤碰撞（FI）、总碰撞（KABCO）和道路偏离碰撞分别显著增加了 11.9%、21.0%和 23.3%。在提高限速的弯道高速公路段上，道路偏离碰撞的增加更为明显，而没有限速变化的直道段则没有。密歇根州高速公路限速提高 15 英里/小时与致命和受伤碰撞、总碰撞和道路偏离碰撞的增加有显著关联。限速提高导致的碰撞风险增加表明，需要进一步研究以了解限速提高后驾驶员行为的变化。
