U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000
Pavement markings are a relatively new concept in the overall history of paved roads. Prior to the mid-1900s, the majority of our highways were "naked"–highways without white and yellow pavement markings that exist on all major highways in the United States today. The research on adding markings to naked highways goes back over half a century when most highways did not have markings. While the findings from these studies are based on naïve statistical analysis methods, the studies all clearly indicate that pavement markings provide crash reductions.(1-4) A 2004 synthesis on pavement marking research includes a brief description of the value of installing pavement markings on naked highways.(1)
Pavement markings delineate the travel lanes and other features of the roadway, providing benefits that are inherently linked to safety. Pavement markings provide lane position assistance in the near field and roadway alignment guidance in the far field. They have been designed to be visible to drivers in both day and night conditions. Pavement marking research has covered topics such as durability, visibility, vehicle operational impacts using speed and lane position, and safety impacts using crash types such as run-off-the-road and head-on crashes. Less research has been devoted toward how to cost-effectively optimize the performance of pavement markings–i.e., their durability, best practices on size and brightness, and interactions with other treatments such as roadway lighting or raised pavement markers. Some other questions that may need more research are:
More research is needed to definitively answer these questions and determine specific methods of enhancing the performance of pavement markings.
This report provides a synthesis of recent research focused on longitudinal pavement markings. It offers a review of the United States standards for when and where longitudinal pavement markings are used. It also covers two of the most important parameters in terms of increasing the effectiveness of pavement markings: width and retroreflectivity. These items are within the control of owners and operators. By better understanding how width and retroreflectivity impact performance, highway agencies would be able to increase the effectiveness of their pavement marking policies and improve safety.
This synthesis is intended to provide a reference about what is known on the subject and what questions still remain. The research recommendations are based on the synthesized material. Ongoing or pending research that is known is noted where appropriate.
The structure of this report is based on the parameters that impact pavement marking placement and performance. The report is structured as follows:
Chapters 3 through 5 are divided into three key performance metrics: operational impacts, visibility, and safety. Where appropriate, standards and specifications are summarized. Each section of this report includes a brief summary and recommendations for future research. Remarks are also provided where ongoing work is underway or pending.
Specifically excluded from this synthesis are object markers and pavement marking design issues that are the purview of the Manual on Uniform Traffic Control Devices (MUTCD). Additionally, this report does not include pavement marking symbols, crosswalk markings, and marking types other than traditional longitudinal markings. This is a synthesis of research rather than a synthesis of practice.
The MUTCD describes when and where longitudinal pavement markings shall and should be installed, using criteria based on traffic volume, roadway width, and functional classification. When the MUTCD criteria were drafted, specific research was not available to help set the criteria. Research results (such as those from the Missouri Department of Transportation [MoDOT] described in Chapter 2) have become available and suggest that there may be enhanced safety benefits to implementing criteria more aggressive than those currently described in the MUTCD warrants. However, additional research may be needed to assess what criteria would be appropriate.
The default pavement marking width is generally 4 inches in the United States, although 6inch wide pavement markings are also used. The width of pavement markings up to 6 inches has negligible impacts on operating speed and lateral placement on two-lane highways. Additional research regarding the operational performance of pavement markings wider than 6 inches or on highways other than two-lane, two-way facilities may be of interest. Regarding visibility of wider lines, there are some inconsistencies in the research, but the most recent studies indicate that wider pavement markings can increase pavement marking visibility. Research also has shown that wider pavement markings may provide a way to reduce driver workload along horizontal curves.
More research is needed to understand the trade-offs between using wider and brighter pavement markings. Many agencies strive for higher retroreflectivity levels without considering wider markings. Increasing pavement marking width may offer more benefits than installing and/or maintaining markings with relatively high retroreflectivity levels. Wider pavement markings have been shown to have a positive safety relationship on rural two-lane, two-way highways; however, on multilane divided highways, wider pavement markings have not been found to have a significant positive safety impact.
The research on the effects of dry pavement marking retroreflectivity has demonstrated little to no impact on operational measures of performance such as vehicle speed or lateral placement. While research of this type is generally focused within horizontal curves, the findings are likely to be similar along tangent sections. If additional research is performed in this area, it should include new considerations, such as studying the benefits of retroreflectivity when dry conditions are not ideal (e.g., when on-coming vehicles are approaching and nighttime visibility is limited).
Research shows that pavement marking detection distance provides an objective way to measure pavement marking visibility, although it may not be directly tied to a specific driving task. Subjective studies and visibility modeling have been used to determine visibility-derived recommendations for minimum maintained retroreflectivity levels. If additional research in performed in this area, it should include objective human factors testing with an aim to derive replacement retroreflectivity levels.
As more data become available, research on the relationship between retroreflectivity and safety is starting to show consistent evidence that specifying and maintaining adequate pavement marking retroreflectivity can increase safety. Additional research is needed to strengthen the understanding of how specific retroreflectivity levels impact nighttime safety.
Currently, a national model for a performance-based specification for pavement marking retroreflectivity does not exist. While a test method and specification for handheld pavement marking retroreflectivity measurements exist, similar test methods and specifications for mobile equipment do not. There is also no national certification program for mobile retroreflectivity measurements, which are similar to other mobile measurements such as pavement friction.
Wet-retroreflectivity is one of the newest topics of interest for pavement marking performance. The research on its effects show, like dry pavement marking retroreflectivity, that wet pavement marking retroreflectivity performance does not impact operational measures of performance such as vehicle speed or lateral placement. Additional research in this area could include new considerations, such as determining the benefits of retroreflectivity during wet conditions and when driving can be particularly challenging (e.g., driving on undivided highways with opposing vehicle headlamp glare).
Research has shown that dry retroreflectivity is almost always higher than wet retroreflectivity, but wet performance cannot be predicted based on dry retroreflectivity measures. The wet retroreflectivity performance is very dependent on the pavement marking optics. More work is needed to validate appropriate research recommendations for minimum wet retroreflectivity. Additional research is also needed to understand how wet retroreflective pavement marking visibility affects nighttime crashes.
Markings that provide long-lasting wet retroreflectivity performance are not yet widely available. Initial research results with a new, more controlled ASTM test method for measuring the performance of pavement markings under a continuous condition of wetting indicate placing wet-night markings in a groove may be necessary (at least in climates with regular snow plow activities) to help maintain an adequate performance level. The ASTM test method for continuous wetting has recently been approved; however, there are still some concerns about the repeatability of the test method.
Other items in this report include a discussion of the combined impacts of raised pavement markers, pavement marking selection tools, ideas about the future of pavement markings, and a list and brief description of other important aspects of pavement markings.
