Phase I: Developing the NDOR Winter Severity Index to Improve Safety and Mobility

Although the annual direct cost of winter highway maintenance in the United States is approximately $2.3 billion (Shi, 2009), quantitatively evaluating the quality of such maintenance remains challenging. Previous studies that have measured the impact of winter weather on safety and mobility have found that in winter weather—compared to normal weather—crash rates increase by 84% and mobility losses result in a decreased average speed of as much as 24 mph, depending on storm severity and road type. Nonetheless, the success of winter maintenance activities is typically measured subjectively in many states, including Nebraska. Development of a quantitative evaluation index method requires consideration of weather conditions, the scope of the road system, the maintenance efforts undertaken for a given storm, the resulting road conditions, and the interactions among these factors. The main challenge in implementing such a method is that weather is inherently uncontrollable, and its variability complicates assessments of the relative efficiency and effectiveness of different road-maintenance activities (i.e., meeting levels of service standards, salt reduction, and budget targets). Therefore, in pursuit of a quantitative metric for evaluating highway weatherization, a vital first step is to assess the severity of individual storms through a winter index. Nebraska does not currently have a winter index to accurately measure its winter maintenance operations’ performance. Several studies have been conducted in other states to develop such winter-severity and performance indices. These studies have indicated that snow events and poor visibility contribute to reduced speeds and increased speed variability (Brown and Baass, 1997; Liang et al., 1998). For this reason, many agencies incorporate speed and volume indices as stand-ins for mobility in their performance-measurement process. Concurrently, to determine an agency’s level of performance in mitigating the impact of a winter storm on highway mobility, the storm’s severity must be quantified to normalize performance based on the severity of the storm. This winter severity index can be used to measure the degree of difficulty storms create in maintaining the targeted mobility. Before Nebraska can invest in a performance metric for winter maintenance, the state must develop an accurate winter index. To increase the efficiency of this research, the proposed work will analyze the winter indexing procedures of other states. Brief summaries of other states’ studies are provided below. By building this proposed work on the methodologies of previous studies, this research will hone the procedures to enhance efficiency and efficacy. One should note that this is a preliminary list and a more comprehensive literature review will be completed as part of this project.