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Nighttime Visibility


Methods for Maintaining Pavement Marking Retroreflectivity



Improving safety throughout the transportation network is the primary goal of the Department of Transportation.(1) The intent of this proposed rule is to advance this safety strategy in regard to nighttime visibility on our Nation’s roads. In 2012, 33,561 people died in motor vehicle traffic crashes in the United States.(2) While only a quarter of travel oc. curs at night,(3) about one-half of traffic fatalities occur during nighttime hours.(4) This translates into a nighttime fatality rate that is approximately three times greater than that of daytime. There are many reasons for this disparity such as alcohol and fatigue, but no one factor can be singled out for all nighttime traffic crashes. It is, however, reasonable to expect that pavement markings be visible to drivers at night to facilitate safe nighttime driving.

Pavement markings play one of the most important safety functions on our roads. They are widely accepted as being beneficial to drivers in that they communicate the intended travel path for short-range operations and the roadway alignment for long-range delineation. To ensure consistent application of pavement markings, their characteristics and warranting criteria are described in the Manual on Uniform Traffic Control Devices (MUTCD),(5) which sets the national standard for traffic control devices.

Pavement markings have been repeatedly shown to reduce crashes. Recent crash studies as well as those more than a half-century old have conclusively shown that adding edge lines to rural two-lane highways can reduce crashes and fatalities. Findings from a recent paper demonstrate that the benefits from pavement marking edge lines can be achieved with narrow pavement widths (18 feet or less) and traffic volumes as low as 1,000 vpd.(6)

While the presence of pavement markings has been consistently shown to reduce crashes, the nighttime crash rate (as described in the opening paragraph) remains out of proportion compared to the daytime crash rate. Retroreflection (more commonly referred to as retroreflectivity) is a property of the pavement marking that can be measured and is a key indicator of the nighttime visibility of pavement markings. Maintaining pavement marking retroreflectivity is consistent with the FHWA’s goal of improving safety on the Nation’s streets and highways, and many safety and operational strategies depend on pavement marking visibility that meets the needs of drivers. Furthermore, recent research confirms the value of maintaining longitudinal pavement marking retroreflectivity.((7),(8)

After having analyzed and considered technical research results as well as input from participants of FHWA-sponsored workshops, FHWA has developed proposed minimum maintained pavement marking retroreflectivity levels for the MUTCD.(9) Improvements in pavement marking visibility will also support the FHWA’s efforts to be responsive to the needs of older drivers whose visual capabilities are declining. This is important because the number of older drivers is expected to increase significantly in the coming years. As of 2008, 32.2 million drivers in the United States were at least 65 years old. It is estimated that by 2020, there will be more than 40 million licensed drivers 65 years and older.(10)

The importance of pavement marking retroreflectivity is recognized in the current MUTCD, which includes the following standard statement regarding pavement markings:

"Markings that must be visible at night shall be retroreflective unless ambient illumination assures that the markings are adequately visible. All markings on Interstate highways shall be retroreflective." (5)

However, there are no specific performance requirements in the MUTCD regarding pavement marking retroreflectivity levels needed by nighttime drivers. To address this, the United States Congress, as part of the 1993 Department of Transportation Appropriations Act (Pub. L. 102-388; October 6, 1992), directed the Secretary of Transportation to revise the MUTCD to address minimum retroreflectivity standards for signs and pavement markings. The Congressional directive stated that:

"The Secretary of Transportation shall revise the MUTCD to include a standard for a minimum level of retroreflectivity that must be maintained for traffic signs and pavement markings which apply to all roads open to public travel."

The opening statements of the MUTCD define the purpose of traffic control devices and the principles for their use. Traffic control devices, including pavement markings, are meant to promote highway safety and efficiency by providing for the orderly movement of all road users.(5) (Note: The MUTCD is incorporated by reference in 23 CFR 655.601. It is available the FHWA’s Web site at http://mutcd.fhwa.dot.gov.) The MUTCD provisions for pavement marking visibility apply to each agency or official responsible for managing and operating roadways open to public travel.

On December 21, 2007, the FHWA published in the Federal Register a final rule amending the 2003 Edition of the MUTCD to include standards, guidance, options, and supporting information relating to maintaining minimum levels of retroreflectivity for traffic signs. The 2009 Edition of the MUTCD also includes the same sign retroreflectivity language.

It is generally believed that maintaining the daytime performance of pavement markings (i.e., placement and color) is more easily accomplished than maintaining the nighttime performance. Nighttime performance of pavement markings can be more difficult to maintain for a variety of reasons. One of the primary differences between daytime and nighttime pavement marking performance is a material property called retroreflection. Retroreflection is a special type of reflection that redirects incident light, such as that from headlights, back toward the source. Pavement markings are made out of many different materials (water-based paints, solvent-based paints, thermoplastic, and epoxy, to name a few); are placed on different pavement types (asphalt and concrete); and are different colors (primarily yellow and white). As a result, the ability of the pavement marking to redirect headlamp illumination back toward the vehicle, thereby making the marking visible for the nighttime driver, varies. The commonly accepted practice for determining pavement marking retroreflectivity is measured at a standard 30-meter geometry and expressed in units of millicandelas per square meter per lux (mcd/m2/lx). A standard protocol including sampling requirements is defined in ASTM D7585, Standard Practice for Evaluating Retroreflective Pavement Markings Using Portable Hand-Operated Instruments.(11)

The nighttime visibility of pavement markings provided through retroreflectivity is difficult to assess during daytime conditions using visual inspection methods. Furthermore, the retroreflective properties of all pavement marking materials may be initially inconsistent due to installation practices, then degrade over time, making pavement markings continuously less visible (i.e., less bright) at night. Environmental conditions, such as radiation from the sun, moisture, and pollutants, cause a substantial amount of deterioration in retroreflective performance. However, loss of retroreflectivity can also occur due to traffic, roadway debris, and snowplowing activities.

As pavement markings degrade and become less retroreflective, their effectiveness in communicating the intended travel path and roadway alignment to road users at night diminishes. If left unattended, pavement marking retroreflectivity can diminish to the point that the markings cannot be seen in time for nighttime drivers to react properly. Thus, to maintain nighttime effectiveness, pavement markings must be replaced before they reach the end of their useful retroreflective life. Until recently, little information was available about the levels of retroreflectivity necessary to meet the needs of drivers and thereby define the useful life of pavement markings. Research has led to the development of recommended minimum maintained levels of pavement marking retroreflectivity for longitudinal pavement markings considering currently available materials, vehicle fleet characteristics, and the capabilities of a significant majority of the driving population.(12)

One of the key concerns identified during the agency workshops held in the summer of 2007, when the FHWA first started developing potential MUTCD language for minimum pavement marking retroreflectivity levels, was that the rule needs to provide flexibility for agencies to adopt various pavement marking maintenance methods without having to measure the retroreflectivity of all their pavement markings.(9) In this report, the FHWA has outlined maintenance methods that agencies can implement to maintain minimum pavement marking retroreflectivity levels in conformance with the proposed MUTCD requirements. As the proposed rulemaking states, agencies will need to implement pavement marking maintenance methods that incorporate the consideration of minimum retroreflectivity levels to provide for nighttime visibility. This document describes methods for maintaining minimum pavement marking retroreflectivity levels.

Pavement Marking Materials and Standards

There is currently no nationally accepted specification or standard containing established minimum retroreflectivity levels for pavement markings, including newly installed markings or markings at the end of their service life. For many years, ASTM D6359 included a requirement that new pavement markings had a minimum initial retroreflectivity of 250 mcd/m2/lx for white and 175 mcd/m2/lx for yellow.(13) However, ASTM D6359 was last updated in 1999 and was withdrawn in 2006 because of onerous sampling requirements. In 2010, ASTM D6359 was replaced with ASTM D7585,(11) which includes a new, easier to use sampling protocol but eliminates the retroreflectivity requirements for newly installed markings.

Agencies have started to move toward warranty and performance-based pavement marking specifications with specific thresholds for retroreflectivity levels. The benefits of these types of contracts are better pavement marking performance and quality, protection against premature failure, reduced lane-occupancy for repairs, and life-cycle savings. Some warranty contracts include performance criteria out to 6 years. Most include minimum requirements for retroreflectivity. More information about warranty and performance-based pavement marking specifications can be found in NCHRP Synthesis 408.(14)

Report Organization

The FHWA has outlined maintenance methods that are intended to provide agencies with a flexible means of conformance with the proposed MUTCD requirements for minimum retroreflectivity of pavement markings. The primary purpose of this report is to describe the methods that can be used to maintain minimum retroreflectivity levels.

Chapter 2 covers the objectives of the maintenance methods. Approved maintenance methods are introduced and defined. In addition, other pavement marking inspection techniques are identified and described.

Chapter 3 describes the calibrated pavement markings procedure for the visual nighttime inspection method. A description of the visual nighttime inspection method using this procedure is included as well as identified advantages and disadvantages.

Chapter 4 describes the consistent parameters procedure for the visual nighttime inspection method for pavement markings. Instructions for conducting the visual nighttime inspection method using this procedure are described as well as the advantages and disadvantages.

Chapter 5 includes a description of the measured pavement marking retroreflectivity method. This chapter includes information an agency needs to be familiar with if they are planning to use retroreflectivity measurements of their pavement markings to be in compliance with the proposed MUTCD language.

Chapter 6 describes the expected service life method and how an agency can use this method to maintain their pavement marking retroreflectivity levels. The information an agency needs to know in order to use expected service life to maintain their markings can be found in this chapter.

Chapter 7 describes the blanket replacement method, which is a form of the expected service life method with less administrative requirements, but perhaps more waste.

Chapter 8 includes a list of questions and answers to support the pavement marking retroreflectivity methods.

Appendix A contains the language that the FHWA has proposed for the Supplemental Notice of Proposed Amendment for the 2009 MUTCD to add language regarding minimum pavement marking retroreflectivity levels.

1U.S. Department of Transportation, U.S. Department of Transportation Strategic Plan, 2006-2011. This document can be viewed at the Internet Web site: https://www.transportation.gov/mission/budget/dot-strategic-plan-fy2006-fy2011

2National Highway Traffic Safety Administration, "Traffic Safety Facts, 2012 Motor Vehicle Crashes: Overview," November 2013. This document can be viewed at http://safety.nhtsa.dot.gov/

3Federal Highway Administration and The Bureau of Transportation Statistics, "2001 National Household Travel Survey." This document can be viewed at the Internet Web site: http://nhts.ornl.gov

4Federal Highway Administration, Roadway Departure Safety, "Nighttime Visibility" web page. Available at http://safety.fhwa.dot.gov/roadway_dept/night_visib/ (Accessed Sept 22, 2011)

5Federal Highway Administration, Manual on Uniform Traffic Control Devices (Washington, D.C.: 2009).

6Carlson, P.J., E.S. Park, and C.K. Andersen, "The Benefits of Pavement Markings: A Renewed Perspective Based On Recent and Ongoing Research," Transportation Research Record: Journal of the Transportation Research Board, 2107 (2009): 56-68.

7Carlson, P.J., E.S. Park, and D.H. Kang, "An Investigation of Longitudinal Pavement Marking Retroreflectivity and Safety" (Paper Number 13-2512, Pending Publication in the Proceedings from the 92nd Annual Transportation Research Board Meeting, Washington, D.C., January 2013).

8Avelar, R.E. and P.J. Carlson, "Characterizing the Combined Association of Edge Line and Center Line Retroreflectivity to Nighttime Crashes on Michigan Rural Two-Lane Highways," Transportation Research Record: Journal of the Transportation Research Board, publication pending.

9Falk, K.W. and P.J. Carlson, Pavement Marking Retroreflectivity Workshop Report, FHWA-SA-08-003 (Washington, D.C.: FHWA, 2008). Accessed at: http://safety.fhwa.dot.gov/roadway_dept/night_visib/pavement_visib/fhwasa08003/fhwasa08003.pdf

10National Highway Traffic Safety Administration, Older Driver Program: Five-Year Strategic Plan 2012-2017, DO-HS-811-432 (Washington, DC: December 2010).

11ASTM International, ASTM D7585 / D7585M-10, Standard Practice for Evaluating Retroreflective Pavement Markings Using Portable Hand-Operated Instruments (West Conshohocken, PA: 2010). Available at: www.astm.org

12Deballion, C., P. Carlson, Y. He, T. Schnell, and F. Aktan. Updates to Research on Recommended Minimum Levels for Pavement Marking Retroreflectivity to Meet Driver Night Visibility Needs. FHWA-HRT-07-059 (Washington, D.C.: FHWA, 2007)

13ASTM International, ASTM D6359-99, Standard Specification for Minimum Retroreflectance of Newly Applied Pavement Marking Using Portable Hand-Operated Instruments, (Withdrawn).

14M.J. Markow. NCHRP Synthesis 408: Pavement Marking Warranty Specifications, (Washington, D.C.: Transportation Research Board, 2010). Available at : http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_syn_408.pdf

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