Other topics that are of possible interest but that are not included in this report are:

• Optics. Optics include glass beads and other types of retroreflective optics. NCHRP Report 743 describes a test method that can be used to assess the initial quality of Type I beads, but it is not a field test. Field tests are needed. There is also a need to look at some of the specified characteristics of markings–for instance, bead roundness is a specification within M247 , but it seems the specification was written around the distribution of roundness that is typically produced from the process of using recycled glass. Data exist that demonstrate the rounder the glass, the better the retroreflectivity. Other characteristics of M247 should also be reconsidered. In addition, there are characteristics of beads that are not addressed in M247 or elsewhere that are known to impact retroreflective performance, such as inclusions and crazing.
• Coatings. Various coatings are used on glass beads to prevent clogging, encourage wicking, and change buoyancy for embedment. There is very limited knowledge in the public domain concerning the effectiveness of these coatings. Field tests would be very useful for inspectors.
• Binder. There is a need to determine what types of innovative binders are available to promote durability and bead retention.
• Texturing. A variety of texturing techniques are used for markings for various reasons. The effectiveness of the texturing has not been studied.
• Contrast markings. There are a variety of ways to add contrast to markings. There is no uniformity across the United States, and there has been little study of the effectiveness of contrast markings.
• Color. It has been over a decade since NCHRP Report 484 documented the feasibility of having the United States go to an all-white pavement marking system. The environmental reasons driving the effort have since been resolved, and there seems to be little interest in moving toward an all-white system. However, white markings typically have higher initial and maintained retroreflectivity levels.
• Eye tracking. Recent research shows that markings are the focus of unfamiliar drivers as they negotiate horizontal curves, even when there are other in-curve devices present such as chevrons; however, the research was not able to quantify the value of markings due to limited resources. There appears to be promise in using eye trackers to help better understand how drivers use markings to maintain their lane position and prepare for upcoming roadway features such as curves.
• Heavy metals in glass beads. While this was an issue a couple of years ago, legislation has passed to place limits on heavy metals. The issue seems to have faded.
• Measurements of retroreflectivity. Measuring pavement marking retroreflectivity on profiled markings, rumble stripes, and open-graded and large-aggregate surfaces continues to be a challenge. Wet retroreflectivity measurements are also an issue since they typically require traffic control, which limits their use in specifications.
• Specifications. There is some momentum among agencies to move toward a performance-based specification rather than a method-based specification. In a performance-based specification, an agency specifies the minimum retroreflectivity it desires, initially and/or over a specified period (such as a year or more).
• Photoluminescent markings. In early 2014, a demonstration project in the Netherlands featured photoluminescent edge line markings. While the demonstration project was popular on social media, little technical information has been released about the markings, such as how the brightness is maintained throughout the night, whether photoluminescence works in wet conditions, and what colors are available besides radioactive green. Additional work is needed to determine the feasibility of photoluminescent markings.