Pavement markings provide important information to drivers. Unlike traffic signs and signals, pavement markings are continuous and provide drivers with valuable information that helps them stay on the road and in the proper lane. What would it be like to drive down a highway without pavement markings? Sounds crazy, doesn’t it? How would you or the oncoming traffic manage? Especially at night when the value of pavement markings increases significantly. What is it that makes pavement markings visible at night? What can be done to improve nighttime visibility of pavement markings?
And what if you started feeling sleepy or became distracted while driving? Flat pavement markings would not give you any warning that you were drifting off the highway. Consider that run-off-road (ROR) crashes account for almost one-third of the deaths and serious injuries each year on the nation’s highways. Inattentive driving and driving while distracted, drowsy or fatigued have been identified as significant causal factors in ROR crashes. Statistics show that over half of Americans drive when they are too tired to get behind the wheel. Researchers at the Texas Transportation Institute (TTI) are currently investigating ways to improve highway safety through more effective pavement marking applications.
Pavement markings are different from signs in that they are typically manufactured at the location where they are applied instead of being manufactured in a factory and sent to the installation site. As a result, there is the opportunity for greater variability in the performance characteristics of pavement markings. The variability in marking performance can reduce the driving value of pavement markings, especially at night. To be effective at night, pavement markings must be retroreflective, which means that they reflect light from the vehicle’s headlamps back to the vehicle so that the driver can see the markings.
The challenge for transportation agencies is to select pavement marking materials that are appropriate for a roadway’s characteristics, such as pavement surface and traffic volumes. There is a great deal of information about pavement markings scattered among many documents, but little that is formatted to provide agency personnel with information that helps them figure out what marking material to use on a given roadway.
Since one of the key characteristics of a pavement marking is its reflective performance, or how well a driver can see it at night, it is important that anyone involved with pavement markings has the right information for a successful installation. The Texas Department of Transportation (TxDOT) recently sponsored a research project at TTI that evaluated key aspects of pavement marking effectiveness including: the retroreflectivity of markings on flexible pavements, selection of marking materials for rigid pavements, inspection issues and methods of implementing the research findings.
The Pavement Marking Handbook provided the primary means of implementing the research findings. It contains guidance, procedures and recommendations addressing many different aspects of pavement marking selection and installation. The purpose of the handbook, developed by TTI researchers with significant input from a panel of TxDOT staff and the contracting community, is to be a single source of information for anyone involved with pavement markings in Texas.
The handbook, which is currently available in draft form at http://tcd.tamu.edu under the “TxDOT Handbooks” link, is divided into two main chapters: pavement marking material selection and pavement marking installation and inspection. Each chapter serves as a stand-alone document. The material selection chapter describes the most common pavement-marking materials that are available and includes recommendations on what materials are most appropriate to use in Texas on various pavement surfaces as a function of the traffic volume. The handbook indicates that thermoplastic is the most appropriate marking material for flexible pavements. There is a range of recommendations for concrete pavements, depending upon the traffic volume and the desired service life of the marking. In general, however, epoxy, concrete-formulated thermoplastic and performed tape are the most appropriate materials.
There are many different factors that can impact the quality of a pavement marking and many of those factors are related to the manufacturing of the marking in the field. The other main chapter of the handbook provides field inspectors with information that will help them to improve the quality of the marking installation. The chapter contains information about measuring thickness, inspecting retroreflectivity and includes several “tips and trips” tables to help inspectors identify application problems associated with specific marking materials.
Ready for rumble
Researchers at TTI have been investigating a new type of pavement marking called rumble stripes—or the application of pavement markings applied over traditional rumble strips. The concept started when conventional shoulder-applied rumble strips (which have proven to be very successful in reducing ROR crashes with crash reductions of 20 to 80% being reported by various state DOTs) were applied to the centerlines of undivided highways to reduce frontal and opposite-direction sideswipe crashes. Since the centerline rumble strip (CLRS) application is relatively new, no long-term safety studies have been completed yet but short-term studies report promising results (for instance, 25% reductions in frontal and opposite-direction sideswipe crashes).
The CLRSs are different from shoulder-applied rumble strips in that the yellow centerline is applied over the rumble strip whereas the edgeline of a highway is typically applied 4 to 12 in. from the nearest edge of the shoulder-applied rumble strips. Besides adding vibration and sound for motorists drifting into the opposing lane, the CLRSs also show an additional advantage over conventional centerlines—increased wet-night visibility. In other words, during rainy nighttime conditions when traditional flat pavement markings seem to disappear the CLRSs maintain their visibility.
The added wet-night visibility of the CLRS was one of the key reasons that led Michigan, Mississippi and Pennsylvania to begin experimenting with edgeline pavement markings applied over rumble strips, also known as rumble stripes. Under this new premise, the edgeline has added wet-night visibility and, because the rumbles are milled closer to the travel lane, the drifting motorist is provided more recovery time when a shoulder exists. However, because of their design, edgeline rumble stripes provide transportation agencies a new tool for combating ROR crashes on highways with narrow or no shoulders (for instance, a prime candidate in Texas is the 38,731 miles [over 97%] of two-lane farm-to-market highways which have a pavement width of 24 ft or less). Although it is still too early to definitively state, there also is significant promise of increased edgeline life as motorists are less likely to drive on the pavement markings for extended periods because of the vibration and sound associated with the edgeline.
In Texas, studies of CLRS and edgeline rumble stripes have just recently been initiated. Because Texas highways are relatively flat and straight, many opportunities exist to pass. Therefore, one focus area of the Texas research is investigating whether CLRSs installed along sections of highway marked for passing confuse or surprise motorists as they begin their passing maneuvers. In addition, the findings also will be compared to AASHTO’s passing sight distance criteria to determine if modifications are warranted.
Texas also is conducting research on a new combination and pavement marking and rumble strip technology. This application is intended for lane lines on multilane highways. Narrow rumble strips (only 4 in. wide) are milled into lane lines and then the lane lines are restriped. This combination provides no noise or vibration but increases the wet-night visibility of the lane line. The intent of the research is to determine whether there is enough wet-night visibility to eliminate the need to use raised retroreflective pavement markers (RRPMs) for added lane line delineation. This would be particularly beneficial in Texas when the occasional snow fall accumulates and plows shear off practically all of the lane line RRPMs.