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Nighttime Visibility


Methods for Maintaining Pavement Marking Retroreflectivity



Traditionally, it has been up to agencies to manage and maintain their pavement markings in accordance with the MUTCD standards. As a result, agencies have implemented different methods to manage pavement markings that reflect local conditions, needs, and priorities. These management methods, as well as the ones described herein, are understood to incorporate both monitoring and replacing the markings.

The management process begins with agency policies and practices regarding the use of pavement marking materials. Agency policies have often been driven by the costs of the various marking materials. Once new pavement markings have been installed, there is a need to monitor the markings to ensure they provide the intended delineation in both daytime and nighttime conditions. By and large, the most common method used to trigger the upgrade of pavement marking retroreflectivity by restriping or replacement has been visual inspection. However, other methods have been tested and implemented including measuring retroreflectivity and re-applying markings based on anticipated service life (e.g., waterborne paint is typically thought to have an expected service life of one year and is therefore commonly re-applied on an annual basis).

The proposed MUTCD language regarding minimum maintained pavement marking retroreflectivity levels is similar to the MUTCD language for minimum sign retroreflectivity in that it requires agencies to adopt one or more acceptable methods designed to maintain retroreflectivity at or above the appropriate threshold. This provision was intended to assure that agencies use methods that will be effective in maintaining nighttime visibility for their pavement markings.

The concept of a minimum pavement marking retroreflectivity standard has raised concerns among State and local agencies. One of the main concerns is associated with the potential increase in tort exposure once numerical values are established. The FHWA sponsored two workshops in the summer of 2007.(9) The workshops included participants from State and local agencies from around the country. The goal of the workshops was to obtain input from public agencies regarding efforts to establish a minimum retroreflectivity requirement for pavement markings. Similar to the comments expressed during the sign retroreflectivity rulemaking, the stakeholders at the workshops in 2007 expressed concern over tort liability claims.

In order to minimize the risk to an agency of being found negligent in meeting the requirements for minimum pavement marking retroreflectivity, FHWA proposes a pavement marking retroreflectivity maintenance method be developed and implemented in order to ensure the nighttime visibility of markings. The FHWA has also addressed special circumstances where maintenance of pavement marking retroreflectivity would be naturally difficult and provided an explanation of how compliance with the MUTCD can be achieved.
There have also been concerns that the implementation of new methods would impose new burdens on agencies. Workshop participants noted that the MUTCD should provide flexibility for agencies in terms of complying with minimum maintained pavement marking retroreflectivity levels. The maintenance methods described in this report are intended to provide that flexibility for agencies to minimize their burdens while remaining compliant with the forthcoming standards.

Objectives of Retroreflectivity Maintenance Methods

The intent of these methods is to provide a systematic means for agencies to maintain longitudinal pavement marking retroreflectivity at or above minimum retroreflectivity levels. The FHWA has determined that agencies that use an approved method to maintain pavement marking retroreflectivity are in conformance with the minimum maintained retroreflectivity requirements as proposed in the MUTCD.

Compliance with the proposed MUTCD language is achieved by having a method in place to maintain the minimum retroreflectivity levels. This implies the use of the method(s) chosen to monitor, schedule, and replace deficient markings in a timely manner. Compliance does not require or guarantee that every inch of pavement marking will meet or exceed the minimum retroreflectivity levels at every point in time. However, agency methods and replacement schedules should be designed to replace markings before they fall below the minimum levels.

For example, if an agency chooses to implement the calibrated pavement marking visual nighttime inspection method, there is no guarantee that the retroreflectivity of all pavement markings will be satisfied during the entire period that they are in service. Assuming that an agency successfully completes periodic nighttime visual nighttime inspections in accordance with guidelines set forth in this report, and that markings failing the subjective evaluation or markings rated as marginal are scheduled for replacement within a reasonable time period, then there is clearly a period of time when these markings (or some portion thereof) might be below the minimum retroreflectivity levels while the markings are awaiting replacement or reassessment. Having a method in place to maintain the minimum retroreflectivity levels is a valuable way for agencies to prioritize how to spend limited resources on those markings that should be replaced sooner, ultimately contributing to improved safety for the motoring public.

There are other conditions where markings might be rated as being satisfactory while temporarily falling below the minimum retroreflectivity levels. For example, water and snow on pavement markings can significantly reduce their visibility. In addition, while research has shown that the visual nighttime inspection is a reasonable method in terms of identifying pavement markings that need to be replaced because of inadequate retroreflectivity, the nighttime visual inspection method is not 100 percent reliable(15) nor is any other method.

Regardless of which maintenance method is adopted by an agency, documentation of the pavement marking retroreflectivity maintenance process is important in assisting agencies to achieve conformance with the proposed MUTCD standard to maintain minimum retroreflectivity levels. Written procedures ensure that agency personnel properly follow the selected method, while maintenance records provide the agency with a systematic process for scheduling replacements and justification for the allocation of limited resources. As an example, measurements of pavement marking retroreflectivity might show that certain markings are near or below the MUTCD thresholds of minimum retroreflectivity before they reach the end of their expected life. The records provide documentation that an appropriate maintenance method was followed and permit the agency to assess and revise, if necessary, the expected service life for a given type or group of markings. As long as an agency has a reasonable method in place to manage or assess its markings and establishes a reasonable schedule for marking replacement as needed, the agency will be considered to be in conformance.

Documentation of the pavement marking retroreflectivity maintenance process can include a variety of information and levels of detail. It is understood that a diverse array of resources often contribute to maintenance marking processes including, but not limited to, on-call service contracts for restriping, consultant services for monitoring, material performance contracts, resurfacing contract work with marking replacement, and bartering services/materials/equipment with neighboring agencies/public officials. The form and extent of documentation are up to the discretion of the individual agency. Moreover, FHWA does not intend to hold official review and approval processes for agency policy or documentation regarding implementation of minimum pavement marking retroreflectivity programs. However, some of the items an agency may want to consider in a program or documentation are described below.

  • The proposed SNPA language provides an exception for maintained marking retroreflectivity for roadways with less than 6,000 ADT per day. The minimum retroreflectivity levels are also based on the speed limit of the roadway. Therefore, agencies should include a feature in their pavement marking retroreflectivity maintenance program that can adapt to volume and speed changes as needed.

  • The proposed SNPA language provides an exception for maintained marking retroreflectivity for roadways where ambient illumination assures that the markings are adequately visible. Intersection or safety lighting, where isolated areas of a roadway are lighted, do not qualify for this exemption. The intended type of lighting that qualifies is continuous roadway lighting or high-mast lighting, typically used at interchanges. A recent report from Alaska shows that when continuous lighting is used along stretches of highway with pavement marking retroreflectivity levels less than those proposed in the SNPA, the pavement marking visibility remains adequate.(16)

    There are two primary documents used in the United States to describe roadway minimum lighting criteria: ANSI/IESNA RP-8-00 and the AASHTO Roadway Lighting Design Guide (AASHTO Guide).(17),(18) The American National Standard Practice for Roadway Lighting published by the Illuminating Engineering Society of North American (IESNA) is commonly used by public agencies as the basis for establishing the appropriate lighting level design values for roadway lighting. This publication has been approved by the American National Standards Institute (ANSI) and is commonly referred to as RP-8. The RP-8 lighting design criteria parallels the lighting criteria found in the AASHTO Guide, with the exception of the general land-use parameter. The RP-8 criteria are based on an assessment of the roadway classification and pedestrian conflict area classification rather than the general land-use classification found in the AASHTO Guide.

  • There may be occasions when a roadway or roadway segment is planned for rehabilitation or resurfacing but the existing pavement markings are known to have inadequate retroreflectivity levels. In such cases, a decision needs to be made regarding the effectiveness of restriping the roadway given that it will soon be resurfaced. In its documentation of the pavement marking retroreflectivity maintenance process, an agency may choose to set a maximum time-frame between identification of inadequate pavement marking retroreflectivity and resurfacing. If more time than the maximum is expected, one or more of the following may be needed: a low-cost temporary pavement marking (e.g. conventional waterborne paint and AASHTO M247 Type I beads) restriping contract or a provision within the resurfacing contract which conveys the maintenance of the markings to the contractor once the project is let for construction. Agencies can use a wide array of resources to maintain their pavement marking retroreflectivity.

    Resources will change over time and unexpected events will occur. It will be important to establish documentation and make revisions as necessary. When extraordinary resources are used or events and circumstances outside the control of the agency occur, documentation can be advantageous. Repetitive use of such resources or events impacting the ability to comply with the minimum retroreflectivity levels should be addressed by revision to the documented method. Examples of documentation needs that address these uses or occurrences could be 1) revisions to standard operating procedures to accelerate replacement schedules in high-wear areas, 2) inclusion of new resources in lieu of or in combination with agency resources, and 3) characterizing procurement rule changes that impact material deliveries.

    If sections of roadways are found to have less than the minimum retroreflectivity levels, agencies will be considered to be in compliance provided they are actively implementing a method as described herein and they take a reasonable course of action to restore the markings in a timely manner. There is no official definition of "in a timely manner." The limits can be set by each agency or set by precedence if no definition is established prior to legal actions.

Pavement Marking Visibility and Retroreflectivity

The ability to see a pavement marking at night is not solely a function of the retroreflective characteristics of the pavement marking, but is dependent upon several factors, including the:

  • Amount and pattern of light produced by a vehicle’s headlights,
  • Amount of light reaching the pavement marking,
  • Retroreflective characteristics of the pavement marking, and
  • Visual characteristics of the observer.

A key factor in determining the visibility of a pavement marking is the contrast between the marking and the adjacent pavement surfaces. During daylight hours, marking visibility is achieved through ambient light striking the marking surface and scattering in all directions, some of which reaches the driver’s eyes. However, in dark environments at night (without roadway lighting), vehicle headlamps produce most of the light striking a pavement surface, and therefore the retroreflective properties of the pavement marking govern the amount of light that reaches the driver’s eyes. While the retroreflective performance of pavement markings is primarily provided by optics such as glass beads, there are also other factors that contribute to the retroreflective performance. These include the properties of the binder material (color, pigment type and amount, type, thickness), characteristics of the optics, which are usually glass beads, pavement surface roughness, and the amount of debris and dirt on the marking.

Pavement Marking Retroreflectivity Maintenance Methods

During the agency’s initial efforts to develop recommended MUTCD language for the minimum retroreflectivity levels, the FHWA hosted workshops around the country to solicit city, county, and State transportation agency input regarding minimum pavement marking retroreflectivity levels.(7) One of the main points was that a "one-size-fits-all" policy would not be practical and that the FHWA needed to provide flexibility in terms of meeting the minimum retroreflectivity levels. Therefore, the FHWA established methods that agencies could choose from to maintain their markings at or above the minimum levels. Again, though mainly descriptive of the evaluation and monitoring processes, all methods below and described in Chapters 3-7 include the work of replacing or restriping longitudinal markings that do not meet the minimum retroreflectivity criteria. The methods and a brief description of each are provided below.

Visual Nighttime Inspection Methods

There are two visual nighttime inspection procedures allowed by the FHWA. Both procedures are meant to be conducted during dry nighttime conditions. The procedures have common elements such as:

  • The use of low beam headlamp illumination,
  • Inspections to be conducted at prevailing nighttime speeds,
  • The use of trained inspectors, and
  • The dependence on subjective evaluations.

Calibrated Pavement Markings Procedure of the Visual Nighttime Inspection Method

With this procedure, a trained inspector views "calibrated pavement markings" at night prior to conducting a nighttime visual inspection. Calibrated pavement markings have known retroreflectivity levels at or above minimum levels. These pavement markings are set up where the inspector can view them in a manner similar to the conditions of the nighttime visual inspections. The markings can be in a maintenance yard, along a service road, or on a road open to public travel. The inspector uses the visual appearance of the calibrated pavement markings to establish an evaluation threshold for that night’s inspection activities. An example of calibrated markings is shown in Figure 1.

Figure 1. Photo pavement markings with known retroreflectivity levels

(This photo shows an example of markings with known retroreflectivity (shown in yellow). It is not necessary to include multiple markings like shown when using the calibrated marking method. These markings are 30-meters from the observer, representing the standard 30-meter measurement geometry used as a standard for pavement marking retroreflectivity)

Chapter 3 provides additional details pertaining to the calibrated nighttime visual inspection procedure.

Consistent Parameters Procedure of the Visual Nighttime Inspection Method

The consistent parameter inspection procedure is based on factors similar to those that were used in the research to develop the minimum retroreflectivity levels. It is similar to the calibrated pavement marking visual nighttime inspection procedure described above except that no calibrated pavement markings are needed and therefore no retroreflective measurements are needed either. Instead, this method relies on the judgment of an inspector who is aged 60 years or older.

Chapter 4 contains more details about this procedure.

Measured Retroreflectivity Method

In this method the pavement marking retroreflectivity is measured and directly compared to the minimum levels as shown in Appendix A. The retroreflectivity measurements can either be made with handheld devices or mobile devices, as long as they are measured using the standard 30-meter geometry. Inspectors should follow the instructions provided by the manufacturer to obtain reliable retroreflectivity readings, including periodically calibrating the equipment.
Chapter 5 contains procedures to implement the measured retroreflectivity inspection method.

Expected Service Life Method

In this method, pavement markings on a given stretch of roadway are replaced before they reach the end of their service life (i.e., the length of time the markings can remain in service before they reach the minimum retroreflectivity levels and need to be re-applied). Service life is typically established through research or monitoring of pavement marking test decks. Alternatively, if an agency does not know the service life of its markings, it can begin to establish service life values by measuring a representative sample of pavement markings. Service life must be determined using the replacement retroreflectivity levels at or above the minimum retroreflectivity levels shown in Appendix A. This method should include a system for tracking similar groups of pavement markings based on installation date, color, type of materials, and other characteristics such as traffic volume. Chapter 6 contains procedures to implement the service life method.

Blanket Replacement Method

With this method, an agency replaces all of the pavement markings in an area, corridor, and/or of a given marking material type, at pre-selected time intervals based on the relevant expected service life (using the MUTCD minimum levels shown in Appendix A). The replacement intervals are based on historical retroreflectivity data for specific roadways and types of marking material. The replacement intervals are based on when the shortest lived marking in that group/area/corridor approaches the minimum retroreflectivity levels. This method typically requires that all of the designated pavement markings within a replacement area, or of the particular pavement markings type, be replaced, even if segments of markings were recently installed–following a resurfacing project, for instance.

Chapter 7 contains procedures to implement the blanket replacement method.

Other Methods

Agencies can choose from the methods described in this chapter, combine them, or develop other methods based on engineering studies. It is important, however, that if an agency develops a different method, it must be based on an engineering study and must be tied to the minimum levels shown in Appendix A.

Pavement Marking Retroreflectivity Inspection Techniques Not Included as Approved Maintenance Methods

During the 2007 workshops and upon further review of the literature (referenced accordingly), several pavement markings assessment techniques were identified and considered. These have not been included with the recommended methods in this document because they could not be tied to minimum retroreflectivity levels or for some other concern. These techniques are described below. Future technologies and research may demonstrate that the techniques listed below or other innovative techniques can be successfully used to maintain pavement marking retroreflectivity levels to the proposed levels for the MUTCD.

Figure 2. Photo showing sun over the shoulder technique

Sun Over the Shoulder Technique

The sun-over-shoulder test (Figure 2) is an evaluation of pavement marking retroreflectivity normally used to assess the general quality of freshly installed pavement markings. This test is conducted during daylight hours and is a quality control method that is often used during the pavement marking installation process. The sun-over-shoulder test is not considered a suitable pavement marking management method to determine if and when longitudinal pavement markings have reached the end of their useful service life. This is noted in Test Method Tex-828-B "Determining Functional Characteristics of Pavement Markings," where the sun-over-shoulder method is recommended for use during striping operations "only as a guide to determine the nighttime appearance of the markings, not for final acceptance." (19)

Comparison Panel Technique

The comparison panel technique involves placing a comparison panel with a retroreflective level at or above the minimum retroreflectivity level next to an in-service pavement marking. An inspector views the combination at a specified distance (e.g., 30 meters). If the comparison panel appears brighter than the pavement marking, the marking needs to be replaced. This technique would have to be conducted at night and with traffic control for safety reasons. While this method may be acceptable for the airfield markings for which it was developed, workshop participants deemed it unsafe for roadways as it requires too much risk for the inspectors.

Figure 3. Daytime comparison samples for visual inspection of pavement marking.

Lane Line Count Technique

To use this technique, trained inspectors must count the number of lane lines visible from the driver seat of a static test vehicle. This count of visible lane lines is multiplied by the lane line length and spacing to calculate visibility distances. While it is possible to tie the minimum retroreflectivity levels to a visibility distance, the need to have the inspection vehicle in a static position on the road places the inspectors at risk when properly performing the test for in-service markings. It is not feasible to count the lane lines from a moving vehicle.

Control Markings Technique

A maintenance method described in the MUTCD for maintaining traffic sign retroreflectivity is called the "control sign method" and involves monitoring a subsample of traffic signs to determine their service life based on minimum sign retroreflectivity levels. As the control signs near the end of their retroreflective life, they are scheduled for replacement along with the other in-service signs of the same age and materials. The control signs can be signs in a maintenance yard or in-service signs.
The pavement marking version of this maintenance technique is limited to only in-service markings. It is classified under the expected service life method in Chapter 6. The expected service life method allows an agency to use the method if they already have the necessary supporting data or if they choose to start monitoring a sample of their markings to determine the most appropriate service life based on their local conditions.
Because pavement marking retroreflectivity is so closely tied to pavement marking type, pavement surface types, and traffic volumes, the control method is not appropriate when the control markings are installed at a low volume facility such as a maintenance yard. This is acceptable for sign degradation, but not for pavement marking degradation.

Windshield Marking Technique

With this technique, a mark (using tape) is placed on the windshield at the line of sight for the particular inspector. This mark coincides with a visibility distance derived from a preview time of 2.2 seconds and the posted or prevailing nighttime speed of the roadway. The inspector then drives the roads at appropriate speed (the mark would need to be adjusted for speed) and disqualifies any segments where the pavement marking cannot be seen at the appropriate distance. This technique is not directly tied to the minimum pavement marking retroreflectivity levels. In addition, there is concern that minor changes in the driver position (e.g. slouching) would affect the accuracy of this system. This technique would require a research study to determine if it would be an effective technique.

Comparison Light Box Technique

A comparison light box is a hand-held device that performs a daytime check of pavement marking retroreflectivity. The device is similar in size to a pavement marking retroreflectometer, but significantly less expensive. The device is composed of a box with a mirror and a light to show the appropriate geometry as the inspector looks directly down into the box. The image that the inspector sees is a side-by-side comparison of the in-service marking and a calibrated marking (placed within the device) that is set to a specific retroreflectivity level. While this device provides a good side-by-side comparison, it needs to be used the same way as a hand-held pavement marking retroreflectometer. While both devices are used by placing them on the pavement marking to be inspected, the retroreflectometer can be operated with a trigger pull and does not require the inspector to glance away from the roadway. The comparison light box, on the other hand, requires that the inspector look down into the device, focus, and then make a decision, demanding the inspector to look away from the roadway for a significant period. The comparison light box technique will work if it is used in a safe location such as with the appropriate traffic control. However, the hand-held retroreflectometer provides a more objective measure of retroreflectivity and reduces user risk by allowing the user to scan traffic. The hand-held retroreflectometer is more expensive, but it is preferred over the comparison light box.

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).

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

15Pike, A., "Quantitative Versus Qualitative Pavement Marking Visibility Assessment," (paper 10-1287, presented to the 89th Annual Meeting of the Transportation Research Board, Washington, D.C., January 10-14, 2010).

16Carlson, P.J. and J.D. Miles, Nighttime Visibility of In-Service Pavement Markings, Pavement Markers, and Guardrail Delineation in Alaska (with and without Continuous Lighting), FHWA-AK-RD-11-04, (Fairbanks, AK: Alaska Department of Transportation, 2011).

17Illuminating Engineering Society of North America, American National Standard Practice for Roadway Lighting, ANSI/IESNA RP-8-00 (New York: 1999).

18American Association of State Highway and Transportation Officials, Roadway Lighting Design Guide (Washington, D.C.: AASHTO, 2005).

19Texas Department of Transportation, Pavement Marking Handbook. (Austin, TX: 2004).

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