005_FHWA_Pavement_Marking_Synthesis.pdf

Synthesis of Pavement Marking Research www.safety.fhwa.dot.gov

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June 2015

SYNTHESIS OF PAVEMENT MARKING RESEARCH

Form DOT F 1700.7 (8-72)

1. Report No.

FHWA-SA-15-063

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle Synthesis of Pavement Marking Research

5. Report Date June 2015

6. Performing Organizations Code

7. Authors Paul J. Carlson

8. Performing Organization Report No.

9. Performing Organization Name and Address Leidos, Inc. Texas A&M Transportation Institute 11951 Freedom Drive The Texas A&M University System Reston, VA 20190 College Station, Texas 77843-3135

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

Contract DTFH61-10-D-000024

12. Sponsoring Agency Name and Address Office of Safety Federal Highway Administration 1200 New Jersey Avenue, SE Washington, DC 20590

13. Type of Report and Period Covered Final Report

14. Sponsoring Agency Code

HSA

15. Supplementary Notes Projects were performed with the cooperation and participation of the Federal Highway Administration, U.S.

Department of Transportation.

Project Title: Highway Safety Visibility Program Technial Assistance

16. Abstract This report provides a synthesis of recent research focused on longitudinal pavement markings. It offers a review of 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 operational performance, visibility, and safety, highway agencies can increase the effectiveness of their pavement marking policies. 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.

17. Key Words retroreflectivity, traffic control devices, pavement markings, operational performance, visibility, safety, durability

18. Distribution Statement No restrictions.

19. Security Classif. (of this report) Unclassified

20. Security Classif. (of this page) Unclassified

21. No of Pages

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized.

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SYNTHESIS OF PAVEMENT MARKING RESEARCH

SI* (MODERN METRIC) CONVERSION FACTORS

APPROXIMATE CONVERSIONS TO SI UNITS

SYMBOL WHEN YOU KNOW MULTIPLY BY TO FIND SYMBOL

LENGTH

in ft yd mi inches feet yards miles

25.4 0.0305 0.914 1.61 millimeters meters meters kilometers mm m m km

AREA

in2 ft2 yd2 ac mi2 square inches square feet square yard acres square miles

645.2 0.093 0.836 0.405 2.59 square millimeters square meters square meters hectacres square kilometers mm2 m2 m2 ha km2

VOLUME (NOTE: volumes greater than 1000 L shall be shown in m3) fl oz gal ft3 yd3 fluid ounces gallons cubic feet cubic yards

29.57 3.785 0.028 0.765 millileters liters cubic meters cubic meters mL L m3 m3

MASS

oz lb T ounces pounds short tons (2000 lb)

28.35 0.454 0.907 grams kilograms megagrams (or "metric ton") g kg

Mg (or "t")

TEMPERATURE (exact degrees) oF Fahrenheit 5 (F-32)/9 or (F-32)/1.8 Celsius oC

ILLUMINATION

fc fl foot-candles foot-Lamberts

10.76 3.426 lux candela/m2 lx cd/m2

FORCE and PRESSURE or STRESS lbf lbf/in2 poundforce poundforce per square inch

4.45 6.89 newtons kilopascals

N kPa

APPROXIMATE CONVERSIONS FROM SI UNITS

LENGTH

mm m m k millimeters meters meters kilometers

0.039 3.28 1.09

0.0621 inches feet yards miles in ft yd mi

AREA

mm2 m2 m2 ha km2 square millimeters square meters square meters hectacres square kilometers

0.0016 10.764

1.19 2.47

0.386 square inches square feet square yard acres square miles in2 ft2 yd2 ac mi2

VOLUME

mL L m3 m3 millileters liters cubic meters cubic meters

0.034 0.264

35.314 1.307 fluid ounces gallons cubic feet cubic yards fl oz gal ft3 yd3

MASS

g kg

Mg (or "t") grams kilograms megagrams (or "metric ton")

0.035 2.202 1.103 ounces pounds short tons (2000 lb) oz lb T

TEMPERATURE (exact degrees) oF Fahrenheit 1.8C+32 Celsius oC

ILLUMINATION

lx cd/m2 lux candela/m2

0.929 0.2919 foot-candles foot-Lamberts fc fl

FORCE and PRESSURE or STRESS

N kPa newtons kilopascals

0.225 0.145 poundforce poundforce per square inch lbf lbf/in2

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380.

(Revised March 2003) iii

SYNTHESIS OF PAVEMENT MARKING RESEARCH

Table of Contents

CHAPTER 1: Introduction

Background

Purpose of this Report

Organization of this Report

Report Summary

CHAPTER 2. MUTCD Warrants

CHAPTER 3. Pavement Marking Width

Operational Performance

Visibility

Safety

CHAPTER 4: Dry Pavement Marking Retroreflectivity

Operational Performance

Visbility

Safety

Test Methods and Specifications

CHAPTER 5: Wet Pavement Marking Retroreflectivity

Operational Performance

Visibility

Durability

Test Methods and Specifications

CHAPTER 6: Combined Impacts of Markers

CHAPTER 7. Pavement Marking Selection Tools

CHAPTER 8. Future of Pavement Markings

CHAPTER 9. Other Topics of Interest

References iv

SYNTHESIS OF PAVEMENT MARKING RESEARCH

List of Figures

Figure 1. Graph. Pavement marking detection distance as a function of retroreflectivity.(27)

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

Figure 3. Screenshot. Results screen.(49) v

SYNTHESIS OF PAVEMENT MARKING RESEARCH

List of Abbreviations and Symbols

AADT Annual average daily traffic

AASHTO American Association of State Highway and Transportation Officials

FHWA Federal Highway Administration mcd/m2/lx millicandelas per square meter per lux

MoDOT Missouri Department of Transportation

MUTCD Manual on Uniform Traffic Control Devices

NCHRP National Cooperative Highway Research Program

NCDOT North Carolina Department of Transportation

NPA Notice of Proposed Amendments

QR Quick Response

RFID Radio frequency identification

RRPM Raised retroreflective pavement marker vpd Vehicles per day vi

SYNTHESIS OF PAVEMENT MARKING RESEARCH

Chapter 1: Introduction | June 2015

SYNTHESIS OF PAVEMENT MARKING RESEARCH

CHAPTER 1: Introduction

BACKGROUND

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:

• When are markings needed?

• What types of markings should be used?

• How should the markings be maintained?

• Do narrow highways need both center line and edge line markings? If not, is one safer than the other?

• What width should pavement markings be?

• How bright do they need to be?

• How can pavement markings be optimized when used in areas with roadway lighting and/or raised pavement markers?

• What is the value of different types of glass bead coatings?

• How round do the glass beads (the optics of most markings) need to be?

More research is needed to definitively answer these questions and determine specific methods of enhancing the performance of pavement markings.

PURPOSE OF THIS REPORT

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.

June 2015 | Chapter 1: Introduction

SYNTHESIS OF PAVEMENT MARKING RESEARCH

ORGANIZATION OF THIS REPORT

The structure of this report is based on the parameters that impact pavement marking placement and performance. The report is structured as follows:

• Chapter 2 summarizes when and where longitudinal pavement marking are typically installed.

• Chapter 3 addresses research findings related to longitudinal pavement marking width.

• Chapter 4 addresses research findings related to pavement marking retroreflectivity, based on the factors within the control of highway owners/operators.

• Chapter 5 addresses wet nighttime pavement marking retroreflectivity. Although wet pavement marking retroreflectivity is a subset of retroreflectivity described in Chapter 4, this chapter is separated out due to heightened interest in this area.

• Chapter 6 addresses the impacts of raised retroreflective pavement markers (RRPMs).

• Chapter 7 describes research that provides guidance on selecting the most appropriate pavement marking materials.

• Chapter 8 includes thoughts concerning the future of pavement markings.

• Chapter 9 briefly describes related topics not specifically addressed in this report.

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.

REPORT SUMMARY

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 6-inch 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.

Chapter 1: Introduction | June 2015

SYNTHESIS OF PAVEMENT MARKING RESEARCH

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.

June 2015 | Chapter 2: MUTCD Warrants

SYNTHESIS OF PAVEMENT MARKING RESEARCH

CHAPTER 2. MUTCD Warrants

Longitudinal pavement markings (center lines, lane lines and edge lines) are a traffic control device and are included in the

MUTCD. The MUTCD defines characteristics of pavement markings such as width, pattern, and color. The MUTCD also includes the standards that define when and where pavement markings are used on roadways in the United States.

The 1993 Department of Transportation Appropriations Act directed the FHWA to include warrants for longitudinal pavement markings in the MUTCD. The Markings Technical Committee of the National Committee on Uniform Traffic Control Devices provided FHWA with warrant criteria recommendations based upon available research and engineering judgment for center line and edge line markings. FHWA concluded a series of rulemaking efforts on January 3, 2000, when the final rule modifying the

MUTCD was adopted by establishing warrants for center line and edge line longitudinal pavement markings. The warrants are based on traffic volume, functional classification, and roadway width.

Since the pavement marking warrants were added to the MUTCD, several studies have been conducted regarding the safety effect of applying more aggressive criteria than the MUTCD warrants. The most recent study was released in 2013, in which

MoDOT investigated the safety effects of striping edge lines on low-volume rural highways (i.e., roadways with annual average daily traffic [AADT] of 400 to 1,000 vehicles per day [vpd]). Prior to this study, MoDOT’s policy was to stripe routes with an AADT of 1,000 vpd or greater with both an edge line stripe and a center line, while routes with traffic volume less than 1,000 vpd only had a center line stripe. By adding edge lines to routes with 400 vpd or greater, MoDOT increased the number of miles of fully striped roadways by nearly 7,500 miles.(5)

MoDOT started the additional edge line striping in 2009 and completed it in 2012. One district completed its eligible routes in

2009—the 570 center line miles of highways in that district were used in this study. A total of 576 crashes occurred on these highways from 2006 to 2008. Of these 576 crashes, 105 involved either a fatal or disabling injury crash (35 severe crashes per year).

Of the severe crashes, 79 involved the run-off-the-road right crash type. Overall, there were a total of 340 run-off-the-road right crash types in the combined locations.

The after data included years 2010 and 2011. During this two-year span, a total of 327 total crashes occurred, 46 of which involved either a fatal or disabling injury crash (23 severe crashes per year). Of the severe crashes, 31 involved the run-off-the-road right crash type. Overall, there were a total of 173 run-off-the-road right crash types in the combined locations.

Using the empirical Bayes method for analysis, the study showed a 15.2 percent decrease in total crashes for all crash types

(significant at the 95 percent confidence level). In addition, severe crashes decreased by 19.3 percent (though this data was not able to gain statistical significance due to the small sample size).

SUMMARY: The MUTCD warranting criteria are based on traffic volume, roadway width, and functional classification.

When the MUTCD warranting criteria were drafted, no specific research was available to help set the criteria. For agencies interested in increasing safety on low volumes highways, there is now research suggesting that enhanced safety benefits can be obtained by implementing more aggressive criteria than those currently in the MUTCD. Additional research is needed to assess specific thresholds for defining the added safety.

Chapter 3: Pavement Marking Width | June 2015

SYNTHESIS OF PAVEMENT MARKING RESEARCH

CHAPTER 3. Pavement Marking Width

In the United States, most longitudinal markings are nominally 4 inches wide. The MUTCD defines the width of a normal pavement marking as 4 to 6 inches, and a wide pavement marking as at least twice the width of a normal marking (at least

8 inches). In practice, however, many agencies consider 6-inch wide longitudinal pavement markings to be wide markings.

As demonstrated in this chapter, much of the research on pavement marking width also considers 6-inch pavement markings to be wide.

OPERATIONAL PERFORMANCE

A 2013 study published by FHWA includes a literature review of the impacts of pavement marking width on observed driver speed and lateral placement; however, the findings of the review were inconclusive.(6) The same FHWA report includes a before-and-after study of vehicle speed and vehicle lateral placement approaching and throughout horizontal curves on two-lane highways. The study produced findings similar to the previous research. While some particular instances of either lateral placement and/or change in speed were found to be statistically significant, the findings were not consistent and the magnitude of the change was not deemed practical. For the conditions studied, it appears that wider edge lines had no practical impact in terms of vehicle lateral placement and speed.

SUMMARY: Based on the latest research, 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 beneficial.

VISIBILITY

A motorist operating a vehicle uses pavement markings to guide him or her along the roadway. It is generally believed that, to be effective, pavement markings must be easily seen—increasing the width of the markings is one way to increase their visibility.

Some agencies have reported that the visual appearance of wider pavement markings is enough to justify their use because the roadway looks better and even safer compared to 4-inch markings.(7) In general, research provides some evidence to suggest that drivers can see wider markings at longer distances. These improvements may improve lane keeping and positively impact safety.

Researchers have performed multiple studies to evaluate pavement marking visibility as it relates to width. The results of pavement marking width studies from the 1980s and early 1990s are inconclusive in terms of identifying visibility gains.

Some research showed increased visibility for wider lines,(8,9,10) while other research showed no consistently statistical or practical differences.(11,12) More recent studies have provided more consistent results.

A 1995 study found that increasing pavement markings from 4 inches to 8 inches provides a statistically significant increase in average detection distances for young drivers on a left curve.(8) A 2001 study found that 6-inch markings have a statistically significant improvement over 4-inch markings for detection distances among both older and younger drivers under dry conditions at night.(11) A 1996 simulator study found that 8-inch markings provide a marginal improvement in average detection distance over 4-inch markings for both older and younger drivers at low levels of marking brightness.(14)

A 2006 study found that increasing markings from 4 inches to 6 inches resulted in an increase in detection distance, but found no increase in detection distance when increasing width from 6 inches to 8 inches.(13) A 2010 study comparing various

4 inch and 6 inch markings under wet and dry conditions found that some markings can produce marginal improvements in detection distances under wet conditions.(15) The findings show that the structural design of the marking, combined with the type of retroreflective optics, is an important aspect to understand in terms of the relationship between width and visual

June 2015 | Chapter 3: Pavement Marking Width

SYNTHESIS OF PAVEMENT MARKING RESEARCH

nighttime performance under both dry and wet conditions. A 2006 empirical study showed that theoretical calculations of marking detection distance as a function of marking width are invalid, and more work is needed to develop the mathematical relationships between marking width and detection distances.(20)

A 2010 eye-tracking study suggests that increasing edge line width from 4 to 6 to 8 inches along horizontal curves provides a reduced driver workload in the driving environment by providing more time for drivers to focus their foveal (central) vision on critical driving tasks.(16) The same study also determined that brighter pavement markings did not impact driver eye-looking patterns. Combined, these findings suggest that increasing the width of pavement markings may be more valuable than increasing their retroreflective performance—at least above a particular retroreflective threshold that may be adequate.

SUMMARY: Although the research has some inconsistencies, the most recent studies indicate that wider pavement markings can increase detection distance. Research 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. Increasing pavement marking width may provide more benefits than those gained from installing and/or maintaining markings with relatively high retroreflectivity levels.

SAFETY

Being perhaps the most important performance indicator of the effect of pavement marking width, safety evaluations have been reported for nearly two decades. In general, as more data are available and more advanced study techniques are implemented, the effect of pavement marking width is better understood; however, the effect of increasing pavement marking width by 2 inches has been elusive. Some agencies have reported that the visual appearance of wider pavement markings is enough to justify their use because the roadway looks safer.(7)

In 1987, a study from Virginia evaluated wider edge lines using a before-and-after approach with run-off-the-road and opposite-direction crashes. The data were from three years prior to installing 8-inch wide edge lines and two years after installation at three test sections.(17,18) The analysis resulted in a 13.6 percent reduction in both run-off-the-road and opposite-direction crashes, which was not statistically significant when compared to the comparison sites. Another 1987 before-and-after crash study conducted in New Mexico suggested that wider lines have no safety benefit in terms of reducing crashes.(15) Both of these studies were hampered by insufficient data.

In 2012, researchers presented the most comprehensive analysis yet of wider pavement markings and their impacts on the frequency and severity of non-intersection/interchange non-winter crashes (in terms of frequency and severity).(29) The analysis excluded intersection and interchange crashes as well as those in the winter months of November through March. Researchers studied the data from two-lane highways from three different States using current statistical analysis techniques to handle the unique characteristics of the data, including differences of how, when, and the extent to which States made the transition to wider lines. Also adding to the challenge was the implementation process and timing of transitioning to wider edge lines in each

State.

This detailed approach provided a number of results and while there are minor differences among the results, they all indicate consistent and positive safety effects of wider edge lines on two-lane highways. The crash frequency analysis suggests that wider edge lines are effective in reducing the frequency of crashes on rural two-lane highways, especially with regard to relevant target crashes such as single-vehicle crashes and run-off-the-road crashes.(6)

The same study also tried to identify the safety benefits of pavement markings wider than 4 inches on multilane divided highways. Using similar approaches but with less data, the research failed to produce the same conclusive findings. At the

Chapter 3: Pavement Marking Width | June 2015

SYNTHESIS OF PAVEMENT MARKING RESEARCH

same time, the wider edge line markings were not shown to be a detriment either. One of the possible explanations is the standards to which these different types of roads are built. Since many two-lane highways have lower design standards than multilane facilities, it is not unreasonable to expect that subtle changes like adding 2 inches of pavement marking width will have a measurable impact on the lower standard highways. This is an interesting finding because agencies often prioritize the placement of wider pavement markings on multilane divided highways before their two-lane, two-way highways.

In a follow-up study, researchers developed and compared benefit-cost ratios for wide edge line markings and other low-cost safety countermeasures such as rumble strips and post-mounted delineators.(21) Using earlier research findings,(29) specifically the conclusion that wider edge lines have been shown to reduce total target crashes 15 to 30 percent and fatal plus injury target crashes 15 to 38 percent, the researchers estimated that the benefit-cost ratio for wide edge lines is $33 to $55 for each $1 spent. Coincidentally, this benefit-cost ratio is similar to the benefit-cost ratio for rumble strips alone, although there are obvious differences between the two countermeasures. While rumble strips address crashes where the driver is distracted, drowsy, or inattentive and can be effective even when obscured by snow or rain, wider edge lines seem to be most effective where the driver is looking at the roadway/striping, or where the driver’s peripheral vision is detecting the marking.(22)

SUMMARY: Wider edge line pavement markings have been shown to have a positive safety relationship on rural two-lane two-way highways. They also provide a high benefit-cost ratio on these roadways. However, on multilane divided highways, wider pavement markings have not been found to have a significant positive safety impact. The North Carolina Department of Transportation (NCDOT) is currently conducting a follow-up study to validate the safety benefits described for rural two-lane, two-way highways. The research described here provides possible justification for wider edge lines on some roads. Future research could be of interest to determine the safety benefits of wider markings on horizontal curves

June 2015 | Chapter 4: Dry Pavement Marking Retroreflectivity

SYNTHESIS OF PAVEMENT MARKING RESEARCH

CHAPTER 4: Dry Pavement Marking Retroreflectivity

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.

OPERATIONAL PERFORMANCE

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.

SUMMARY: Pavement marking retroreflectivity has not been shown to impact operational measures of performance such as vehicle speed or lateral placement. While research of this type is generally focused within horizontal curves, there is no reason to think that the findings would be different along tangent sections. If additional research is performed in this area, it should include new considerations (e.g., the benefits of retroreflectivity when dry conditions are not ideal, such as when oncoming vehicles are approaching and nighttime visibility is limited).

VISIBILITY

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.

Chapter 4: Dry Pavement Marking Retroreflectivity | June 2015

SYNTHESIS OF PAVEMENT MARKING RESEARCH

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.

SUMMARY: 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 is performed in this area, it should include objective human factors testing with an aim to derive replacement retroreflectivity levels based on nighttime driver needs.

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:

• While the study incorporated large amounts of crash data and used contemporary statistical techniques, there were limitations. For instance, the research only included crashes from California and modeled retroreflectivity (no measurements were made).

• While the study included efforts to overcome the possible limitations in modeling retroreflectivity, these efforts presupposed that markings in California reach a value where there is an adverse impact on safety.

• The pavement marking maintenance policy of California is such that higher volume highways are restriped up to three times a year with paint or every two years with thermoplastic markings. As a result, there is only the occasional roadway with retroreflectivity levels below 100 mcd/m2/lx (in other words, most markings were adequate for nighttime driving).

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.

Chapter 4: Dry Pavement Marking Retroreflectivity | June 2015

SYNTHESIS OF PAVEMENT MARKING RESEARCH

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:

• Nighttime.

• Single-vehicle nighttime.

• Fatal plus injury nighttime.

• Single-vehicle nighttime fatal plus injury.

The results were different for yellow center line and white edge lines:

• The effect of retroreflectivity of the yellow center lines on nighttime crashes and single-vehicle nighttime crashes with low retroreflectivity (≤150 mcd/m2/lx) was found to be statistically significant. The results suggest that expected crash frequency decreases as the center line retroreflectivity increases, specifically in the low range (≤150 mcd/m2/lx).

• The retroreflectivity for white edge lines was found statistically significant for nighttime crashes and single-vehicle nighttime crashes. The results suggest that the expected crash frequency decreases as the white edge line retroreflectivity increases.

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).

SYNTHESIS OF PAVEMENT MARKING RESEARCH

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 the edge line and the center line retroreflectivities.

• The difference of retroreflectivity values (i.e., white–yellow).

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.

SUMMARY: The relationship between pavement marking retroreflectivity and safety has proven elusive. As more data are becoming available, research is beginning to show consistent findings regarding the positive relation between retroreflectivity and nighttime crashes. The research findings described in this section provide consistent evidence that specifying and maintaining adequate pavement marking retroreflectivity can increase safety. However, additional research could potentially strengthen the understanding of how specific retroreflectivity levels impact nighttime safety.

TEST METHODS AND SPECIFICATIONS

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…