Retroreflective pavement markers (RPMs) have been used throughout the world since they were patented in the United Kingdom in 1934. Their use was called into question by NCHRP Project 17-28, Pavement Marking Materials and Markers: Safety Impact and Cost Effectiveness, which was completed in 2004, and summarized in NCHRP Research Results Digest 305. The research, using the number of crashes in California by severity per unit of time and distance, produced results that were controversial among many researchers and practitioners: “no statistically significant relationship was found between safety and the retroreflectivity of pavement markings and markers.” While the research limitations have been pointed out in numerous areas, the only conclusion that can be agreed on by all is that more research is needed, particularly, on the interaction between the driver and roadway environment. One effort to address the issue was SHRP 2 Safety Project S08D, Analysis of Naturalistic Driving Study Data: Roadway Departures on Rural Two-Lane Curves (https://www.trb.org/Main/Blurbs/171254.aspx). The Naturalistic Driving Study (NDS) is the largest driving study ever undertaken and has produced unparalleled data from which to study the role of driver performance and behavior in traffic safety and how driver behavior affects the risks of crashes. The NDS data record how drivers really drive and what they are doing behind the wheel. In S08D, NDS and the Roadway Information Database (RID) were used to develop relationships between driver, roadway, and environmental characteristics and risk of a roadway departure on rural two-lane curves on paved roadways. The results of the research, based on driver performance rather than crash analysis, strongly suggest that the use of countermeasures such as RPMs that better delineate the curve may provide better advance information for drivers to perform safely. Research is needed to further analyze the performance of RPMs on driver performance at night, especially by drivers 60 years of age or older, on all roadways. The aim of this study was to develop a guide for transportation practitioners that describes and evaluates the safety benefits, as shown by driver performance or other methods, of Retroreflective Pavement Markers (RPM), both raised and recessed, for nighttime driving, particularly during wet weather. The guide addresses criteria and information for use and placement of RPMs. The final report, NCHRP Research Report 1015,  is now available. A brief practitioner's Guide and Powerpoint presentation may be found by searching the TRB webpage.