Impact of Shoulder Width and Median Width on Safety (2009)

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7Problem Statement Every roadway design starts with the intention to provide a safe facility that addresses mobility concerns, accommodates the physical and social environment, and is financially feasi- ble. To achieve such designs, engineers rely on guidelines and policies, which aim to address these goals. Sometimes, how- ever, it may not be practical to conform to the highest possible values of all these guidelines. For example, adherence to a certain geometric specification may create environmental concerns, affect historical structures, be economically unfea- sible, or otherwise affect a community in an undesirable way. To address such issues, the roadway design may need to deviate from the prevailing optimum value expressed in guidelines and policies. An understanding of the impacts of such alternative designs on both the safety and the operational character of the roadway is essential to making an informed choice among possible designs. The AASHTO publication A Policy on Geometric Design of Highways and Streets (commonly referred to as the Green Book) provides guidance to the designer by referencing a recommended range of values for critical dimensions of the design of new roadway alignments and those undergoing major reconstruction (1). The Green Book provides guidelines—not standards—that permit sufficient flexibility to encourage distinctive independent and appropriate designs for specific situations. However, such flexibility can introduce uncertainty since there may be insufficient data to quantify potential trade-offs when evaluating design variations. Even though the Green Book indicates that the referenced guidelines provide for a safe, comfortable, and aesthetically pleasing roadway, designers may have little information regarding the safety and operational consequences that can result if they deviate from the recommended range of values or select one value from over another within the range. The Green Book provides control values for the design of new alignments or those undergoing major reconstruction that allow flexibility. For most control values, the Green Book indi- cates that the recommended ranges provide a safe, comfortable, and aesthetically pleasing roadway; however, there are cases where additional flexibility is necessary and, therefore, the design exception process is available. This process allows the designer to consider other design values for a specific element in order to better fit the design to the situation. In many cases, there is little research that quantifies such decisions and their impact on safety (2, 3). The concept of guidelines was emphasized even more in Flexibility in Highway Design (4), a recent publication by U.S. DOT, and is further stressed in AASHTO’s Guide for Achieving Flexibility in Highway Design (5). The previously used approach typically deemphasized the design’s impact on human and natural environments, increasing the possibility of creating wide swaths of pavement cutting through communities and natural resources. This approach was typically justified by arguing that it results in designs with increased safety, but this result was not always achieved. A critical review of design guidelines by Hauer (6) stated that several design guidelines are based on empirical data from decades ago, with some not validated through research. Also, research demonstrated that other design values lower than those suggested in the Green Book work well to achieve flexibility in design while balancing the concerns of safety and capacity. While safety must always be considered when selecting design values, the ramifications of cost restrictions and environmental concerns might warrant consideration of a reduced value for a design element. Proper designs should assess competing constraints and create a solution that meets mobility and safety objectives. Research Objectives and Approach This research brought together existing research knowledge and project experience to provide a reference guide for planners and designers. This research effort will provide the highway design community with information resources and decision C H A P T E R 1 Introduction and Research Approach

tools for designing roadways where design flexibility may be appropriate to the roadway context. The research was conducted in two phases. The first phase comprised a literature review and the development of method- ology for data collection and analysis in the second phase. In the second phase, data were collected and analyzed to develop an understanding of the safety and operational impacts from design element trade-offs. The two-phase project included seven specific tasks: • Phase I Tasks – Task 1: Review of past and ongoing work relative to rural multilane highway crash prediction models and accident modification factors (AMFs). – Task 2: Development of data acquisition plan identifying states with crash databases that could be used in the next phase as well as specific data elements. – Task 3: Development of a data analysis plan for manip- ulating the collected data and developing appropriate prediction models and AMFs. – Task 4: Development of an interim report and work plan. • Phase II Tasks – Task 5: Acquisition of the appropriate data and analysis of the data to develop the appropriate models and AMFs. – Task 6: Development of guidelines that could be used by designers in evaluating the safety consequences from design element trade-offs for multilane rural highways. – Task 7: Preparation of final report. The first phase of the project identified a plan for data col- lection analysis that would produce reasonable models and AMFs for future use. The plan was reviewed by the NCHRP project panel, and adjustments were made to the direction of the work. The major adjustment was a departure from the original objective of identifying the safety implications from several design elements and focusing only on several specific cross-section elements: lane width, shoulder width and type, median width and type, and (possibly) clear zone width. This refocused approach allowed for a better coordination with other ongoing NCHRP projects on the development of the Highway Safety Manual (HSM) (7). The HSM is envisioned to be a comprehensive document of current knowledge related to roadway safety treatments as well as to contain tools for predicting the safety effects of different roadway design alter- natives for various classes of roadways. Several NCHRP projects were initiated in support of HSM development at the same time as Project 15-27, and efforts were undertaken to coor- dinate with some of these projects to produce compatible results. The project team worked very closely with the team of NCHRP Project 17-29, “Methodology to Predict the Safety Performance of Rural Multilane Highways,” which aimed to develop predictive tools and the HSM chapter for rural multilane highways. Organization of the Report This report presents the findings and conclusions of the research to develop crash prediction models and AMFs for specific design elements of multilane rural highways. The research results are included along with recommendations for future research. The remainder of the report is organized in the following chapters: • Chapter 2: Literature Review—presents the current knowl- edge on AMFs and identifies potential needs for the work undertaken herein. • Chapter 3: Data Analysis—documents the methodology followed to analyze the collected data, includes a description of the data used, and presents the results from the analysis. • Chapter 4: Design Elements Recommendations—presents the proposed guidelines for the various design elements as a result of this research. • Chapter 5: Conclusions and Suggested Research—includes a summary of the study objectives, project findings, and recommendations for future research work. An appendix discusses the use of prediction models to determine the relative safety of design element choices. 8