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Publication Number: FHWA-HRT-07-059
Date: October 2007
Updates to Research on Recommended Minimum Levels for Pavement Marking Retroreflectivity to Meet Driver Night Visibility Needs
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As traffic control devices, pavement markings relay a wide variety of information to drivers. They are unique in terms of traffic control devices because drivers do not have to shift their attention away from the roadway in order to receive continuous information. Properly implemented longitudinal pavement markings convey the following information: (1)
Pavement markings become one of the key methods of conveying this information to the driver at night, so their proper placement and maintenance are critical for safe driving. (2) In order for pavement markings to be seen by drivers at night, they must be retroreflective. Retroreflectivity is a measure of an object’s ability to reflect light back towards a light source along the same axis from which it strikes the object. In the case of retroreflective pavement markings, incoming light from vehicle headlamps is reflected back towards the headlamps and, more importantly, the driver’s eyes. The retroreflective property of pavement markings is what makes the pavement markings visible to nighttime drivers. Pavement markings are made retroreflective by embedding glass beads in the marking material (sometimes called the binder material). Rather than scattering light, as the pavement marking material would do without the glass beads, the beads refract the incoming light in such a way that it is returned back towards the driver’s headlamps. The most common measurement of retroreflectivity is the Coefficient of Retroreflected Luminance (RL), which can be described as “the ratio of the luminance of a projected surface of retroreflective material to the normal illuminance at the surface on a plane normal to the incident light.” (3) Retroreflective measurements can be used to assess the efficiency of pavement markings in terms of their ability to retroreflect headlamp illumination.
Retroreflective measurements are made with a standard geometry that represents what a driver in an average passenger car would see during inclement weather conditions at night. The standard geometry is based on a viewing distance of 30 m (98 ft). Handheld and mobile pavement marking retroreflectometers used in the United States must be based on the standard 30-m (98-ft) geometry. This includes the minimum retroreflectivity levels recommended in this report.
Pavement markings, like many other roadway materials, deteriorate over time. As pavement markings deteriorate, they lose their ability to retroreflect headlamp illumination. As a result, retroreflective measurements of pavement markings decrease over time. There is general agreement that this reduced performance may be a causative agent in the rate and severity of nighttime crashes, although previous research has not yet quantified the relationship. It should be pointed out that a recent study found no safety difference between high and low retroreflectivity for longitudinal nonintersection markings. (4)
Several surveys have shown that “brighter” markings provide a higher comfort level and are preferred by nighttime drivers. Drivers of all ages and from all parts of the United States feel that markings that are bright and easy to see are important to driver safety and that agencies should make marking visibility a priority. Many drivers believe that pavement marking visibility could
While the Manual on Uniform Traffic Control Devices (MUTCD) requires that pavement markings be illuminated or retroreflective, it contains no minimum maintenance retroreflective requirements. (6) In 1992, Congress mandated that such standards for signs and pavement markings be developed, and research to develop these standards has been ongoing. The research for minimum in-service retroreflective requirements for traffic signs was accelerated, leading to a Notice of Proposed Rulemaking that was posted in the Federal Register in July 2004. Currently, the Federal Highway Administration (FHWA) is conducting research to develop a standard for minimum levels of pavement marking retroreflectivity. The FHWA expects to initiate the pavement marking retroreflectivity rulemaking process once the research is concluded and the results are analyzed and considered. While previous research has been undertaken to recommend minimum pavement marking retroreflectivity levels, the need exists to update the earlier research due to changes in roadway user characteristics, vehicle preferences, headlamp performance, and available research tools.
In the early 1990s, the FHWA sponsored research in which the Ohio University’s Computer-Aided Road-Marking Visibility Evaluator (CARVE) model was used to determine driver night visibility needs for various pavement marking treatments.(1) This model was developed and calibrated using results of various pavement marking studies published in the literature. Using CARVE, a table of recommended minimum levels of pavement marking retroreflectivity was developed as a function of posted roadway speed and the presence of retroreflective raised pavement markers (RRPMs).(1) However, the CARVE model is limited because it has not been updated to reflect changes in roadway marking materials, headlamps, or types of roadway surfaces. Subsequently, the Target Visibility Predictor (TARVIP) model was developed based on the CARVE model by the University of Iowa to address these shortcomings, with additional features that allow the user to define roadway profiles, adjust headlamp configurations, and use newer roadway marking materials.
With this new modeling tool, an opportunity exists to analyze and recommend updated minimum maintained pavement marking retroreflectivity levels. These recommendations will consider what minimum retroreflectivity levels will best serve drivers operating in a broad range of visibility scenarios.
The objective of this research is to develop updated minimum maintained pavement marking retroreflectivity levels. The following goals were pursued to accomplish this objective:
The project scope was limited to the investigation of dry, dark, rural, straight roads and longitudinal pavement markings. Transverse pavement markings, horizontal and vertical curves, and wet conditions are outside the scope of this project.