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4 CHAPTER 2 REVIEW OF PRPM-RELATED LITERATURE AND JURISDICTIONAL PRACTICES This chapter summarizes (1) the findings of a literature reflective. In addition, there are two subcategories of retro- review of studies related to PRPMs and (2) the state of the reflective PRPMs: conventional raised nonsnowplowable practice according to state departments of transportation pavement markers and snowplowable pavement markers. (DOTs) that responded to the iTRANS survey. Three sections Snowplowable markers can be either raised or recessed. make up this chapter. First, an overview of current PRPM guidelines and practices is provided. This overview is followed by a section that critically reviews the knowledge about the 2.1.1 Nonretroreflective PRPMs safety effect of PRPMs. The section also assesses method- ological problems and issues arising from past research. This Nonretroreflective raised pavement markers, such as con- assessment sets the stage for the design of the current evalua- vex buttons, are made of plastic, ceramic, or aluminum. Non- tion study. The third section reviews the human factors issues retroreflective raised pavement markers are often used, in studied in past research efforts that may become relevant to conjunction with retroreflective PRPMs, as an alternative to the interpretation of the study results and formulation of painted markings. The "Manual on Uniform Traffic Control guidelines for PRPM implementation. Devices" (MUTCD) (3) provides guidelines on the spacing requirements of nonretroreflective PRPMs when used as a replacement to painted markings. The "Roadway Delineation 2.1 OVERVIEW OF PRPM Practices Handbook" (2) indicates that the nonretroreflective CURRENT PRACTICES PRPMs will provide daytime visibility and the retroreflective PRPMs the nighttime visibility. PRPMs were developed to provide delineation over a wider range of environmental conditions than could be achieved with standard pavement marking materials. Retroreflective PRPMs 2.1.2 Retroreflective PRPMs provide a clear, definitive outline of pavement markings even under adverse visibility conditions such as rain, fog, and dark- There are two types of retroreflectors: prismatic (also called ness. Standard paint lines are ineffective during rainy condi- cube-corner) and spherical lens (4). The marker that is most tions because rain often accumulates on the painted markings, commonly installed today is the wedge-shaped, cube-corner thus reducing the retroreflectivity of the paint, whereas the retroreflector. Prismatic retroreflectors are manufactured with raised pavement markers stand above the pooled water (2). different face designs, and spherical lens retroreflectors are Nonsnowplowable PRPMs are also effective in providing a manufactured with different glass bead types. Changing either "wake up call" for the driver who wanders out of the travel the face design or the glass bead type in retroreflectors can lane. The wake up call is created by the vehicle vibration and be expected to give different results for daytime and night- audible tone when crossing over the PRPMs. Currently, there time delineation. References such as ASTM F923-00 (4), are many types and models of pavement markers on the mar- the "Roadway Delineation Practices Handbook" (2), and ket, including both retroreflective and nonretroreflective types. the "Guidelines for the Use of Raised Pavement Markers" (5) PRPMs are developed with a variety of configurations and provide guidance in the selection of appropriate markers. characteristics. Some markers are wedge shaped, and some are The American Society for Testing and Materials (ASTM) round or oval. Markers are available with and without replace- has produced the following standard specifications: able retroreflective inserts, and the variations are numerous. Commercially available markers vary in all aspects, such as ASTM D 4383, "Standard Specification for Plowable, size, shape, composition, and capabilities. Snowplowable Raised Retroreflective Pavement Markers" (6), and markers have also been developed to reduce the vulnerabil- ASTM D 4280, "Standard Specification for Extended ity of the marker to snowplow activity. In general, there are Life Type, Nonplowable, Prismatic, Raised Retroreflec- two main types of PRPMs: retroreflective and nonretro- tive Pavement Markers" (7).