Browsing by Author "Alsady, Abdullah S"

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EVALUATION OF THE SAFETY IMPACTS OF SPEED LIMIT INCREASES ALONG HORIZONTAL CURVES ON RURAL FREEWAYS IN MICHIGAN
(Michigan State University, 2025) Alsady, Abdullah S; Savolainen, Peter T
In May 2017, the State of Michigan enacted legislation that increased the maximum posted speed limit from 70 mph to 75 mph on approximately 614 miles of rural limited-access freeways. While previous studies have evaluated the broader operational and safety impacts of this change, limited research has focused on its effects on roadway segments with geometric constraints—particularly horizontal curves. These curves are recognized as high-risk locations due to reduced sight distance, elevated lateral acceleration demands, and speed compliance challenges. This study addresses that gap by assessing the safety impacts of the speed limit increase specifically on horizontal curves. The analysis used crash data spanning five years before (2012–2016) and five years after (2018–2022) the speed limit change. A total of 116 horizontal curve segments were selected as treatment sites, while 81 comparable segments served as controls. Two analytical approach was employed: a cross-sectional regression analysis to identify key predictors of crash frequency and severity, and a before–after evaluation using the Empirical Bayes (EB) method to adjust for regression-to-the-mean and other confounding factors. Site-specific Safety Performance Functions (SPFs) were developed using data from control sites to predict expected crashes across total crashes, fatal and incapacitating injury crashes (KA). The EB analysis revealed a 46.4% statistically significant increase in total crashes across all treatment sites after the speed limit increase. KA crashes rose by 36.5%, though this increase was not statistically significant. In summary, this study provides strong empirical evidence that speed limit increases on horizontal curves can lead to higher crash frequencies. The findings emphasize the importance of incorporating geometric assessments and targeted safety countermeasures—such as advisory speed signs—when modifying speed policies. These insights are especially relevant for transportation agencies seeking to enhance safety while maintaining efficient travel on rural freeway networks.
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