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Research – Pavement Markings

Chapter 6: Combined Impacts of Markers

Pavement markings are intended to provide the road user an indication of lane position and roadway alignment. Under wet nighttime conditions, many traditional pavement markings lose much–if not all–of their retroreflective performance. As noted in the previous chapter, significant progress has been made in terms of developing pavement markings that maintain their dry retroreflectivity performance in various conditions of wetting. However, the traditional way to provide wet nighttime visibility is the use of RRPMs. Research efforts have been undertaken to understand the durability of RRPMs, but little research has been conducted to understand how the performance of RRPMs and pavement markings can be combined.

One of the most recent studies was a multi-phase study commissioned by the FHWA that focused on understanding the combined impacts of center line and edge line pavement marking retroreflectivity and pavement markers.(45, 46, 47) The initial focus of this effort was to determine how the presence of markers might impact the need for various pavement marking retroreflectivity levels.

The first part of the study, published in 2003, consisted of a simulator study designed to address the question, "How much can the retroreflectivity of roadway pavement markings be reduced if raised pavement markers of a certain retroreflectivity are installed on the road?" The researchers discussed the concept of a trading ratio, which refers to trading off the retroreflectivity of markings with the installation of markers. Curve recognition distance was used to determine the trading ratio of markers to marking condition.(45) The study concluded that the trading ratio was 0.55 for yellow center lines on non-freeway roads, which means pavement markings could degrade as much as 45 percent with the addition of pavement markers and still maintain the same overall curve detection distances.

A second simulator experiment, published in 2004, investigated the interaction between center line and edge line luminance on driver curve recognition distances.(46) Just as the addition of pavement markers increases road delineation luminance, the addition of edge lines to center lines also increases the delineation luminance. In a parallel to their previous experiment, the researchers looked for the trading ratio for the addition of edge lines that would allow for degradation in center line retroreflectivity while maintaining the same curve detection distance. The researchers calculated the trading ratio between center lines and edge lines to be 0.41, which means center line luminance may be allowed to degrade by as much as 59 percent with the addition of edge lines, while maintaining the same curve detection distances.

The limited contrasts available with their simulator in both simulator experiments were 150:1, which is "far below those likely to be experienced on a real roadway at night." This limitation in contrast emphasized the need for a field validation study. The researchers conducted the field validation of the two previous experiments.(47) The field study was conducted on a newly constructed highway, not opened to other traffic, and delineated to look like a two-lane rural highway; 12-foot wide lanes matched the lanes used in the two previous simulator studies. The road grade was a constant 0.84 percent downhill, and there was no illumination by fixed roadway lighting. The closed road did have some ambient illumination from nearby buildings and other roads. Six trials were conducted, which took about nine hours per participant over seven nights.

The field validation called into question the use of simulators for pavement marking studies. The researchers found that, because of the limited contrast ratios simulators can project, simulators are poor representations of real-world conditions. The researchers revised the previous simulator findings to say that their trading ratios between center line markers and markings are valid only for markers that are very faded or have low retroreflectivity. In the field, pavement markers were so much brighter than the pavement markings that they produced trading ratios that were practically zero (less than 0.001). In other words, the effect of pavement markers is so much more powerful than pavement markings in curve detection distances that yellow center lines could be allowed to deteriorate by more than 99 percent in retroreflectivity if new pavement markers were added. The researchers therefore questioned the concept of a trading ratio in providing useful engineering information. In general, the issue of "trading" the retroreflectance of longitudinal lines with lines that have pavement markers is not entirely resolved or agreed upon by the research community. Findings from the second simulator study were validated in that low-luminance edge lines combined with low-luminance center lines produced higher curve detection distances than low-luminance center lines alone.

Also in 2004, an NCHRP report was published focusing on the safety effectiveness of snowplowable raised pavement markers on two-lane and four-lane highways.(48) Crash prediction models were developed for various crash types: total, fatal and injury, nighttime, nighttime fatal and injury, daytime, daytime fatal and injury, wet weather, dry weather, and guidance-related. Using data from four northern States, the authors discovered that nonselective use of raised pavement markers on two-lane roadways, overall, does not significantly reduce total or nighttime crashes, nor does it significantly increase these crash types. Mixed results were discovered for selective use (e.g., poor crash history) of raised pavement markers on two-lane roadways. On four-lane freeways, the use of raised pavement markers showed neither a positive nor a negative overall safety effect on total and nighttime crashes. However, some significant reductions were recorded for wet weather crashes at those locations on four-lane freeways, and there are indications that they are only effective in reducing nighttime crashes where the AADT exceeds 20,000 veh/day.

In 2011, researchers measured luminance (instead of retroreflectivity) to assess the visibility of various delineation treatments including pavement markings and markers. To assess their performance, the visibility level was used instead of retroreflectivity. This metric allows the results to be normalized for direct comparison. It can also provide an indication of the impacts of continuous versus intermittent delineation.(49) An example of the results is shown in Figure 2.

Bar chart rates type of curve delineation by visibility.
Figure 2. Bar chart. Comparison of visibility by delineation type.(46)

In this example, the markings were applied within the week prior to the measurements using low volatile organic compound paint and Type I AASHTO M247 beads. The RRPMs were non-plowable and were installed in a 0.5-inch deep groove according to the contract specification. The guardrail delineator tabs were the butterfly variety and were seven years old. The results demonstrate the added value of RRPMs for wet nighttime conditions. They also provide some evidence that performance metrics other than retroreflectivity can lead to better understanding the impacts of various delineation treatments such as pavement markings.

SUMMARY: The use of simulators to study the combined impacts of center line and edge line pavement marking retroreflectivity and pavement markers did not result in definitive findings. Additional work from the field suggests that visibility metrics other than retroreflectivity may be more useful to better understand how various delineation treatments perform in terms of providing visibility. Overall, the use of snowplowable markers has been shown to have mostly negligible impact (positive or negative) on nighttime safety. An NCHRP research project is currently being planned that is designed to develop enhanced RRPM recommendations (NCHRP Project 05-21).

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