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9  Introduction 1.1 Project Background Pavement markings are perhaps the most common traffic control device (TCD) used to com- municate roadway information to drivers. They serve to transmit information on lane designa- tion and curvature as well as reinforce the messages of other TCDs. To be effective, pavement markings must convey information in all lighting and weather conditions. As a result, pavement markings on public roads contain retroreflective elements, such as glass beads, so that light from vehicle headlights is returned to the eye of the driver at night. While most pavement markings provide adequate retroreflectivity under dry nighttime condi- tions, wet nighttime conditions dramatically reduce the visibility of most markings. When rain begins to fall, the retroreflective performance of almost all pavement marking materials is sig- nificantly degraded. The degraded performance generally continues until the markings are dry or daylight arrives. In order to provide adequate nighttime visibility in wet and rainy conditions, many agencies choose to supplement their pavement markings with raised pavement markers. Raised pavement markers are TCDs as defined in the Manual on Uniform Traffic Control Devices (MUTCD) (Federal Highway Administration 2012). Formally, they are defined by the MUTCD as a device, mounted on or in a road surface, that has a height generally not exceeding approximately 1 inch above the road surface for a permanent marker or not exceeding approxi- mately 2 inches above the road surface for a temporary flexible marker that is intended to be used as a positioning guide and/or to supplement or substitute for pavement markings. Raised pavement markers can be nonretroreflective (such as ceramic traffic buttons) or retro- reflective. Retroreflective pavement markers (RPMs) have a retroreflective surface to provide delineation in nighttime wet and rainy conditions. Types of RPMs include raised RPMs, snow- plowable RPMs, and recessed RPMs (Figure 1). The three types of RPMs all serve the same purpose but differ in how they are installed and maintained: ⢠Raised RPMs are plastic devices adhered to the roadway surface and are typically used in areas without significant snowplow activity. ⢠Snowplowable RPMs are installed in a casting that is embedded in the road surface. The snowplowable RPMs are used in areas with substantial winter weather where snow plowing is needed. ⢠When RPMs are used in snowy regions, they are installed into grooves cut into the pavement and referred to as recessed RPMs. As mentioned above, RPMs are primarily used to provide traffic lane delineation for night- time drivers, especially in wet and rainy conditions. While not required by the MUTCD, their perceived value is evident since many state and local agencies choose to install and maintain them because pavement markings traditionally lack the ability to meet nighttime driver needs C H A P T E R 1
10 Performance Criteria for Retroreflective Pavement Markers in wet and rainy conditions. The RPM language in the MUTCD is in Part 3 and describes how RPMs are to be used once a decision is made to use them. There are no generally accepted national policies or guidelines describing when to use RPMs, and more so, there are practically no scientific guidelines for describing RPM performance levels needed for visibility, safety, or other possible metrics. The brief introduction here demonstrates that while RPMs are not required, many agencies use them. Their use is not uniform from agency to agency, and their effectiveness is not well established. The need for this research is quite clear. 1.2 Project Objectives The lack of uniformity in the application of RPMs stems from the absence of guidelines for their use based on scientific research. While the effects of RPMs have been previously studied, the findings have been mixed; different approaches have led to different conclusions. The variations in conclusions drawn regarding RPM efficacy in the existing literature are largely attributable to difficulty in isolating the effect of the RPMs themselves. When RPMs are used, it is common for them to be installed as a part of a larger construction project. Consequently, there are a variety of other roadway factors that change when RPMs are deployed (e.g., new pavement markings and new roadway surface), which confounds their specific, observable impact, particularly in terms of crash rate. The challenges associated with dealing with these confounding factors are described in greater detail in the body of this document. The objective of this research was to isolate and identify the effects of RPMs from a cohesive, three-pronged investigation of driver visibility, behavior, and safety. This was accomplished by developing a series of closed-course experiments, reviewing naturalistic driving data, and con- ducting an extensive review of the existing safety literature (after deeming a safety analysis not feasible due to lack of quality data). Such a multifaceted approach provides the best opportunity to craft guidelines that account for different ways RPMs affect the driving experience. Specific items the research covered included the following: ⢠Use of RPMs on two-lane rural, multilane urban, limited access, and other roads ⢠Use of RPMs on tangents and horizontal curves ⢠Use of RPMs in conjunction with other types of roadway delineation devices or treatments, including lighting ⢠Visibility needs of older drivers ⢠Different types of RPMs ⢠Performance measures for RPMs Raised RPM Snowplowable RPM Recessed RPM Figure 1. Types of RPMs.
Introduction 11  1.3 Report Organization This report is presented in several chapters that cover the major research activities of the proj- ect. Chapter 2 covers an in-depth literature review of the safety and operation impacts of RPMs as well as RPM visibility-related studies. Chapter 3 presents the results of a survey of state agencies and a summary of state agency practices. Chapter 4 covers the human factors study that evaluated the visibility and driver behavior impacts of RPMs on a closed-course test facility. Chapter 5 presents the results of a driver performance evaluation using the second Strategic Highway Research Program (SHRP 2) naturalistic driving study (NDS) data. Chapter 6 consists of a discussion of several smaller efforts of the project. These efforts included a before-and-after field speed study, RPM retroreflectivity measurement, and an RPM safety analysis that lacked sufficient data to complete. Chapter 7 summarizes the findings from the study and suggests additional research. A valuable resource, âPractitionerâs Guide for the Use of Retroreflective Pavement Markers,â is found in Appendix F.
