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5S E C T I O N 1 Background To address the problem of single-vehicle run-off-road (SVROR) crashes, many transportation agencies use shoulder rumble strips to alert inattentive or drowsy motorists that their vehicles have drifted out of the travel lane. As motor vehicle tires pass over the rumble strips, the drifting motorists receive auditory and tactile warnings to correct their path of steering. Due to the expected safety benefits of shoulder rum- ble strips and their relatively low installation cost, transporta- tion agencies are applying shoulder rumble strips on a widespread basis. Originally, rumble strips were installed pri- marily on rural freeways, but now transportation agencies are installing shoulder rumble strips along divided and undivided highways in both rural and urban areas, including along rural and urban two-lane roads. The expected safety benefit of shoulder rumble strips has prompted transportation agencies to expand their applica- tion of rumble strips to include installations along the cen- terlines of undivided highways. The primary purpose of centerline rumble strips is to reduce head-on crashes, oppo- site-direction sideswipe crashes, and to some degree SVROR- to-the-left crashes; however, installing rumble strips either along the shoulder or on centerline, without considering the impacts on other highway users, may lead to unintended consequences. In Section 5103 of the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU), enacted in August 2005, Congress recognized that Federally sponsored surface transportation research indicates that rumble strips improve safety. As such, Section 1401 of SAFETEA-LU lists the installation of rumble strips as a type of safety project that may be carried out under the provisions of a highway safety improvement program. A significant amount of research has been conducted on shoulder rumble strips and a lesser amount on centerline rum- ble strips. However, there remain a number of key unresolved issues related to the design, placement, and unintended effects of shoulder and centerline rumble strips. This report presents results from the entire research effort to address key un- resolved issues associated with shoulder and centerline rum- ble strips. Research Objective and Scope The primary objective of this research is to develop fur- ther guidance for the design and application of shoulder and centerline rumble strips as an effective motor vehicle crash reduction measure, while minimizing adverse effects for motorcyclists, bicyclists, and nearby residents. Guid- ance on the appropriate application of rumble strips on undivided and divided highways in both urban and rural areas is provided. This research focuses on addressing sev- eral key unresolved issues associated with shoulder and centerline rumble strips. This research was conducted in three phases. The scope of Phase I was to develop a list of key unresolved issues associated with shoulder and centerline rumble strips and select the high- est priority issues to be investigated as part of this research. Phase I was accomplished by summarizing completed and ongoing research, conducting a survey of transportation agen- cies to identify existing rumble strip policies and guidelines, identifying the gaps in research and practices, and selecting the highest priority issues for further investigation. The scope of Phase II was to conduct research on key un- resolved issues primarily associated with shoulder rumble strips, but several of the issues also relate to centerline rumble strip applications. The key issues investigated during Phase II included the following: ⢠Safety effectiveness of shoulder rumble strips on different types of roads, Introduction
⢠Optimal placement of shoulder rumble strips with respect to the edgeline, ⢠Optimum dimensions of shoulder rumble strips necessary for effective vehicular warning with least potential for adverse effects, and ⢠Minimum level of stimuli (i.e., sound or vibration) neces- sary to alert a drowsy or inattentive driver. The scope of Phase III was to conduct research on key unresolved issues primarily associated with centerline rumble strips. The key issues investigated during Phase III included the following: ⢠Safety effectiveness of centerline rumble strips on different types of roads, ⢠Safety effectiveness of centerline rumble strips along varying roadway geometry, ⢠Safety effectiveness of centerline rumble strips installed in combination with shoulder rumble strips, and ⢠Difference in safety effectiveness of shoulder rumble strips installed along the right (outside) shoulder vs. the left (median) shoulder. This research does not address transverse rumble strips. Transverse rumble strips are installed in travel lanes to warn motorists of approaching intersections, toll plazas, horizontal curves, traffic control devices, etc. In addition, this research does not specifically investigate the application of rumble strips within work zones. Several concepts and issues addressed in this report are potentially applicable to such installations, but this report does not go into detail on either the application of transverse rumble strips in travel lanes, nor installation of rumble strips in work zones. Organization of This Report This final report documents the entire research effort. The remainder of this report is organized as follows. Section 2 describes the magnitude and nature of highway safety con- cerns that could be addressed through the implementation of shoulder and centerline rumble strips. Section 3 presents the purpose, types, and dimensions of rumble strips. Section 4 summarizes the results of shoulder and centerline rumble strip research completed prior to, and during, the course of this research. Section 5 summarizes existing policies/guidelines concerning the design and application of shoulder and center- line rumble strips on rural and urban highways and, specifi- cally, presents the results of the survey conducted as part of this research. Section 6 presents the results of a safety evaluation of shoulder rumble strips completed during this research. Sec- tion 7 presents the results of a safety evaluation of centerline rumble strips completed during this research. Section 8 pre- sents details on minimum stimuli levels for effective rumble strips. Section 9 presents the results of the vehicle dynamics modeling and noise study. Section 10 provides design and applications guidance for rumble strips based on the results of the research findings. Section 11 presents the conclusions and recommendations of the research, including future research needs. Section 12 presents the references cited in the report. Appendix A presents a review of the literature in greater detail than Section 4. Appendix B presents the survey that was dis- tributed as part of the research, and Appendix C presents a detailed summary of the survey results. Appendices D through H provide supplemental information related to the safety eval- uations conducted as part of this research. For practitioners who wish to focus on the findings of this research rather than the details of the research methodology, Sections 10 and 11 will be of particular interest. 6
