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In addition to the width of pavement markings, agencies can also control the retroreflectivity level of markings. Retroreflectivity is a nighttime characteristic of the marking, but since crashes are overrepresented during nighttime conditions, the effect of retroreflectivity is a focus of many research projects. Generally speaking, agencies can specify the retroreflectivity of newly installed markings, which is common. Agencies can also specify the performance level of pavement markings as they wear. While this is not as common as specifying retroreflectivity levels for newly installed markings, the concept is being used more frequently.
The vast majority of specified pavement marking retroreflectivity levels are for dry markings, not wet markings. This section summarizes work related specifically to dry pavement marking retroreflectivity. Wet pavement marking retroreflectivity is the focus of Chapter 5.
A 2007 FHWA study reported on the effectiveness of pavement marking delineation on curves to induce consistency in vehicle speed and lateral position based on a nighttime driving experiment.(23) This was a well-controlled field study where conditions of a specific two-lane, two-way rural highway were changed from night to night and participants drove the section in an instrumented vehicle. The research showed that the use of brighter pavement markings (during dry conditions) does not improve speed consistency between an approach tangent and the midpoint of a horizontal curve. The same study showed that the use of brighter pavement markings does not affect the driver lane position differential between an approach tangent and the midpoint of a horizontal curve.
Pavement marking visibility studies typically use detection distance as the measure of effectiveness. Detection distance has been measured in different ways. In a static setup, the driver counts the number of skip lines visible. In a dynamic setup, where the research participant is driving the vehicle, the driver is tasked with detecting either the beginning or end of a long line, an isolated skip line,(24) or a discontinuity such as a taper.(25) The results are reported in maximum nighttime detection distances.
These studies have repeatedly shown that pavement marking detection distances increase as retroreflectivity increases, but the relationship is not linear.(26,27) In other words, if the retroreflectivity level is doubled, the detection distance will increase, but it will not be doubled. Research findings from a human factors experiment are shown in Figure 1. The research included pavement markings with retroreflectivity levels of 100, 300, and 800 mcd/m2/lx. For the 100 mcd/m2/lx marking, the average detection distance was about 300 feet. Increasing the retroreflectivity to 300 mcd/m2/lx produced about another 100 feet of detection distance, and then increasing the retroreflectivity to 800 produced about another 100 feet of detection distance.
Figure 1. Graph. Pavement marking detection distance as a function of retroreflectivity.(27)
While several studies have been conducted to determine minimum maintained retroreflectivity levels, they have been conducted in different ways. Subjective studies have been performed that use rating of various pavement markings in situ. In these studies, nighttime drives are conducted while the research participant rates the markings. Using measured retroreflectivity levels for the same markings, researchers assess the participants' ratings of the same markings. Most of these types of subjective studies show that markings with retroreflectivity levels of 100 ± 30 mcd/m2/lx are preferred minimum levels for nighttime driving.(1) Because there is not a definitive minimum retroreflectivity level required for nighttime driving, this value of 100 mcd/m2/lx has been used several times throughout this report as a reference point to make comparisons and call out important aspects of other research efforts.
As mentioned previously, another way to determine minimum retroreflectivity levels is to use a visibility metric such as the end-detection distance. FHWA's ongoing efforts to revise the MUTCD to add minimum retroreflectivity levels for pavement markings are based on a combination of approaches. The proposed minimum retroreflectivity levels were modeled using a combination of physics associated with retroreflectivity and results from related human factors studies.(28) The FHWA-proposed minimum maintained retroreflectivity levels are visibility derived-in other words, they specify the retroreflectivity that is needed under specific situations to maintain a specified amount of preview time. There is a belief that visibility performance is a reasonable surrogate for safety.
Similar to pavement marking width, pavement marking retroreflectivity and its effect on nighttime crashes has been studied for years. There is a general belief that a positive relationship exists, but researchers have not been able to fully describe it. The amount and quality of retroreflectivity data have been increasing, however, because mobile retroreflectometers have become more prevalent. Some agencies have been collecting system data periodically for nearly a decade, providing researchers more opportunity to discover the impacts of retroreflectivity on safety.
In 2006, researchers in New Zealand studied the safety impacts of brighter pavement markings and reported that there was not a conclusive improvement in safety.(30) This study took advantage of a 1997 policy change in New Zealand that required a minimum maintained retroreflectivity level of 70 mcd/m2/lx. Using a before-and-after approach, the authors compared crash rates before the change in policy. One assumption of the study was that markings were brighter during the after period. These results may not be directly applicable to the United States, since in New Zealand roadways are delineated as a function of traffic volume. As volumes increase, they progressively apply the following treatments: delineators, center lines, edge lines, and then raised retroreflective pavement markers (RRPMs). Roadways with center lines also have delineators. As a side note, research has shown that supplemental delineation treatments, such as delineators or RRPMs, overpower the potential effect of pavement markings.(31)
Also in 2006, the results of a National Cooperative Highway Research Program (NCHRP) study concluded: "…the difference in safety between new markings and old markings during non-daylight conditions on non-intersection locations is approximately zero."(32) This finding has been discussed and debated for years, and continues to fuel debate in the highway safety space. A close evaluation of the study reveals several points that are worth noting:
In addition to the concerns regarding the modeled retroreflectivity levels, the researchers binned the retroreflectivity levels in bins divided in a linear fashion. The lowest bins for the edge lines included retroreflectivity levels from 21 to 183 mcd/m2/lx, thus including markings ranging from inadequate to adequate in the same bin (according to a synthesis of perception studies reported elsewhere(35)). Eight additional bins included retroreflectivity levels up to 413 mcd/m2/lx. It might have been more productive to form the bins in a logarithmic function since the performance of retroreflectivity has been repeatedly shown to be best modeled logarithmically rather than linearly.(33,34)
In 2007, researchers reported results from an effort using North Carolina that aimed to develop a statistical association between measured pavement marking retroreflectivity and traffic crash frequency.(36) The results suggest that increased levels of the average pavement marking retroreflectivity on multilane highways may be associated with lower expected target crash frequencies.
However, the association was small in magnitude and not statistically significant. On two-lane highways, the association between pavement marking retroreflectivity and crash frequency was larger in magnitude and marginally significant. The retroreflectivity levels used in this study were above 100 mcd/m2/lx with an overall average of 240 mcd/m2/lx.
In 2008, a similar effort in Iowa included three years of measured retroreflectivity.(37) These data were analyzed along with crash records from the same year. The distributions and models of the entire database, and a subset including only two-lane highways, did not show that pavement marking retroreflectivity correlated to crash probability. Truncating the data to only records with retroreflectivity values less than 200 mcd/m2/lx revealed a statistically significant relationship.
In 2010, researchers in Iowa expanded their effort to study the statistical relationship between crashes and longitudinal pavement marking retroreflectivity.(38) While this effort included five years of retroreflectivity data, it was only on certain segments–the crash data set included 1,343 records, which constitutes approximately 1.6 percent of all records statewide. This small sample size was noted as a challenge for the statistical analyses, since the occurrence is a rare event within the whole data set. Retroreflectivity was found to be a significant parameter in the probability of crash occurrence when only data from the interstate roads were analyzed and when the data were divided into three subsets by line type (white edge lines, yellow edge lines, and yellow center lines). In the final set of analyses for white edge lines and yellow center lines, crash occurrence probability was found to increase when longitudinal pavement marking retroreflectivity decreased. While the extent of the study and the data set are not sufficient to identify a definitive relationship, the researchers noted that the results represent a potential relationship that supports additional analyses to link pavement marking retroreflectivity and highway safety through crashes.
Another study on the relationship between retroreflectivity and safety was published in 2013.(39) Using data from Michigan, the researchers evaluated relationships between crashes and longitudinal pavement marking retroreflectivity. The retroreflectivity data consisted of pavement marking measurements representing white edge lines, white lane lines, yellow edge lines, and yellow center lines. The data included crashes and retroreflectivity measurements from 2002 to 2008.
This study only considered nighttime crashes that occurred under dry conditions at non-intersection and non-interchange segments from April through October. The following specific types of crashes were initially identified as target crashes for this study:
The results were different for yellow center line and white edge lines:
The findings lend support to the positive safety effects of maintaining the retroreflectivity of pavement markings. There was no attempt to develop or validate thresholds of retroreflectivity; however, the findings do provide compelling evidence demonstrating that maintenance of pavement marking retroreflectivity can have a positive effect on safety.
A follow-up analysis of the Michigan data was presented in 2014 with a focus on how the combined associations of edge line and center line retroreflectivity levels relate to nighttime crashes on two-lane rural highways.(40) This study included only two-lane highways (no freeways) and only actual retroreflectivity readings (no imputed retroreflectivity records).
The results of this follow-up work continue to provide evidence that the retroreflectivity of pavement markings relates to safety (this time for two-lane highways in Michigan). In particular, this analysis found that the retroreflectivities of both white edge and yellow center lines jointly contribute to the relationship between nighttime crashes. The relationship was captured by two parameters:
The sum of retroreflectivities was found to relate to nighttime crashes in inverse proportion. Conversely, the difference of retroreflectivities was found to relate to night crash frequency in direct proportion. In other words, sites with brighter pavement markings tend to have fewer nighttime crashes.
A follow-up paper published in 2015 builds from the retroreflectivity-safety analysis.(41) Data from North Carolina were obtained and used to continue to evaluate how pavement marking retroreflectivity relates to nighttime safety on rural two-lane highways. The North Carolina data were used to test the robustness of the statistical models derived from the Michigan data. Additional analyses were also explored and described. Using the results from this paper, previous research, and the state of the practice, recommendations and their implications were developed for safety-derived minimum retroreflectivity levels for pavement markings.
The current test method for measuring retroreflectivity under dry conditions is ASTM E1710. This test method states that equipment used to measure pavement marking retroreflectivity has to meet certain criteria, such as having a measurement geometry of 30 meters. ASTM E1710 is written for handheld measurement equipment and includes a precision and bias statement to provide users an indication of the errors associated with such measurements.
While various types of mobile measurement equipment are being used around the United States, there is no national specification for mobile equipment. The Texas Department of Transportation currently requires that all users of mobile retroreflectivity equipment have their equipment and operators certified on an annual basis.
The most common specifications for pavement markings include American Association of State Highway and Transportation Officials (AASHTO) M247, which describes most beads used for pavement markings. In addition, ASTM D7585 includes sampling procedures for inspection of pavement marking retroreflectivity.
A performance-based specification for thermoplastic markings is under development within ASTM D04.38. Discussions have been held to initiate a more comprehensive performance-based specification within ASTM, but the work has not yet started. There is no national model for a performance-based pavement marking specification.