Methods for Maintaining Traffic Sign Retroreflectivity

November 2007
Publication No. FHWA-HRT-08-026

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CHAPTER 3. ASSESSMENT METHODS

This chapter contains information about the evaluation or assessment methods for maintaining traffic sign retroreflectivity. The methods described in this chapter include

  • Nighttime Visual Inspections.
  • Measuring Traffic Sign Retroreflectivity.

The basic concept of an assessment method is that the condition of each individual sign is assessed or evaluated on a periodic basis. The MUTCD does not set specific intervals, but many agencies currently assess their signs every one to two years.

VISUAL NIGHTTIME INSPECTIONS

Visual inspections are perceived to be the most likely means to find nighttime visibility problems with signs. Using this approach, it is possible to assess more than just the retroreflectivity of a sign. Damage, obstructions, poor placement, and other factors that might detract from the nighttime visibility of the sign can be observed. The MUTCD currently includes language that encourages agencies to undertake periodic daytime and nighttime visual inspections. Many agencies already perform some type of periodic sign inspection, although not all inspections are performed at nighttime. This method requires a minimal investment of resources on the part of the agency, although there is a need for a record-keeping system for inspection data and the potential for higher labor costs where overtime pay is required. While visual inspections will reveal night visibility problems not discernable under any other method, they are subjective and hence more difficult to tie to a benchmark value of retroreflectivity. Agencies using visual inspections must establish procedures to provide consistency in inspections. This implies the need for training programs and certification of inspectors to assure consistency of inspections. Inspection procedures should address the type of vehicle used, type of headlamps on the inspection vehicle, headlamp aiming, and age and visual acuity of the inspector(s).

Background

Probably the most common type of sign maintenance program is the visual inspection method. Guidelines have been available for at least 50 years concerning the details of how to conduct a proper nighttime sign inspection.(16, 17) While there are some concerns about the reliability of the visual nighttime inspection, research has shown that trained inspectors can do a reasonable job of determining which signs need to be replaced because of inadequate retroreflectivity.(14, 15)

The visual inspection technique uses trained personnel to observe traffic signs during the nighttime to assess the overall appearance of a sign and determine if it meets the required minimum retroreflectivity level. The observation is typically done through the windshield of the vehicle at or near the speed limit of the roadway.

The key to this method is having trained inspectors. While there is no nationally-recognized training course or certification for sign inspectors, agencies should provide some form of training before sign inspections are performed. One way to perform the training is to have the inspectors observe sample signs at a variety of known retroreflectivity levels before conducting the inspections. Training helps facilitate an inspector’s ability to discern sign retroreflectivity levels that are at the minimum levels prior to conducting inspections. Preferably, there should be sample signs that are at or near the minimum retroreflectivity levels associated with each sign type and color. The inspector should view the sample signs under similar conditions to those under which inspections will be performed. This includes using the appropriate vehicle and placing the sample signs at typical positions that will be encountered during an inspection. For this method to be effective, the training must prepare the inspector in advance, using correct sample signs that represent retroreflectivity levels at or near the MUTCD minimum retroreflectivity levels.

Procedures

The usual method of inspecting signs at night is to use a two-person crew. While the driver focuses on the driving task, the passenger evaluates the signs and records the appropriate information. An alternative to a two-person crew is to use one person with a tape recorder or camcorder. If an inventory is available, signs that have been knocked down or missing for some other reason can be identified during the nighttime inspection. If no inventory exists, an inventory of existing signs can be created while conducting the nighttime inspection, but it may not account for missing signs. A nighttime inspection procedure can be performed without a sign inventory.

The nighttime visual inspection method should only use the low-beam headlamps of the vehicle as the source of illumination for the signs. The interior light of the vehicle should remain off to the extent feasible. The inspection should be performed at highway speeds and from the travel lanes and not the shoulder. As the vehicle approaches the sign, the sign’s overall appearance in terms of brightness and legibility is assessed. Usually the sign is given a rating defined by the agency. At a minimum, the scale should include three designations: good, fair, and poor. The inspector records the information for each sign and the rating that it is given. Signs rated as poor should be scheduled for replacement as soon as possible. Depending on the inspection schedule, signs rated as fair can be noted as requiring attention during the next set of scheduled inspections or can be identified for additional assessment, such as measurement at a later date using a handheld retroreflectometer.

The vehicle and inspector combination should be selected to provide a conservative estimate of sign retroreflectivity. The increased sales of pickup trucks and sport utility vehicles, which result in larger observation angles, make these types of vehicles appropriate for use in many regions. Relatively new vehicles, with visually/optically aimable (VOA) headlamps, should be considered. Ideally, the inspector should be older, with nighttime visual capabilities similar to older drivers. The vision of the inspector should be tested to ensure that it is within the legal limits of the State. It is important that an agency develop consistent guidelines to decrease the subjectivity of inspections. For instance, some items to consider are procedures to clean the headlamps and windshield before each night of inspections and to periodically check the headlamp aiming. A procedure to check the headlamp aim of VOA headlamps is provided in table 4.

Table 4. Headlamp Aiming Procedure.

What you will need:

  • A level area with a distance of approximately 7.625 m (25 ft) plus the length of the vehicle from a flat lightly colored wall
  • A tape measure
  • Masking tape

Instructions:

  • Park the vehicle so that the headlamps are precisely 7.625 m (25 ft) from a flat lightly colored wall. The vehicle should have at least ½ of a tank of gas and should be loaded as it would be when inspecting signs. This includes the weight of the driver (and passenger present).

  • Measure the exact middle of both the windshield and rear window, and mark them with strips of tape, creating vertical centerlines, front and rear.

  • Standing behind the car, sight along the centerlines, and have an assist mark the position of the vehicle centerline on the wall with a vertical strip of tape.

  • Measure the distance between the vehicle centerline and the headlamp lenses. Mark that distance to the right and left of the centerline on the wall with vertical strips of tape.

  • Measure the height of each headlamp from the ground (measuring to the center of the lens). Using those measurements, place horizontal strips of tape on the wall where the vertical strips have been applied. There should now be two crosses on the wall, with centers that correspond to the center of each headlamp lens.

  • For headlamps with a left-side cutoff (VOL), mark a horizontal line that is 53.34 mm (2.1 inches) below the headlamp centers with a horizontal strip of tape. For headlamps with a right-side cutoff (VOR), mark a horizontal line that runs through the headlamp centers.

  • Turn the vehicle headlamps on low beam. The left edge of the bright spots on the wall should just touch the vertical bars of the crosses. The top edge of the strongest gradient of light should just touch the horizontal line. Adjust the headlamp aim per manufacturer’s instructions, if required.

Probably the most important element of the nighttime inspection is documenting the process and results. This can be done with a voice or video recorder, or even with paper and pencil. Whichever method is selected, it is important that inspections are properly documented and archived to provide tort protection.

Current Practices

Visual nighttime inspections are typically used in conjunction with a signage replacement schedule to make sure that the signs are legible and to find signs that may have been passed over or accidentally skipped during the last replacement schedule. Inspections are usually performed every one to two years and rotate between predefined sections of roads under the agency’s jurisdiction. The inspection plans should include specific routings to ensure full coverage of the road network and that the inspections can be safely conducted with the levels of traffic on the road. A variety of practices exist for documenting inspection results and initiating actions to replace signs that are at or near the minimum levels.

Concerns

One concern associated with nighttime visual inspections is that it is the most subjective of all the methods. Another concern is funding overtime pay to conduct the inspections during late-evening or early-morning hours. It is also important that inspectors are properly trained.

Linking Nighttime Visual Inspections to Minimum Retroreflectivity Levels

Minimum retroreflectivity levels are incorporated into this method by training the inspectors and using procedures that allow them to correlate their observations through the use of sample signs. A good practice is for inspectors to observe the sample signs prior to each inspection run. The use of appropriate sample signs at or near minimum retroreflectivity levels is a key element to training that links the nighttime visual inspection method to the minimum retroreflectivity levels.

Advantages and Disadvantages

One of the major benefits of using the visual inspection method is that it has the least administrative and fiscal burden of all the methods. Many agencies already perform some type of periodic sign inspection, although not all inspections are performed at night. This method also has a unique feature in that the signs are viewed in their natural surroundings. Thus, the overall appearance of the sign and the ability of the sign to provide information to the driving public can be assessed.

Another advantage of the visual inspection method is that it has the lowest level of sign replacement and sign waste. Only those signs identified as needing to be replaced because of low retroreflectivity levels are replaced, assuming that the inspection frequency is appropriate. With management methods, it is probable that some signs will be replaced before their full life is achieved. This may imply that the visual inspection method (as compared to the measured retroreflectivity method) maximizes sign life.

While this method may be more subjective than other methods, research has shown that trained observers can reasonably and repeatedly detect signs with marginal retroreflectivity. There is some risk involved while doing these inspections, particularly if the driver is also the evaluator and recorder. Ideally, nighttime inspections should be conducted with two people for safety reasons.

MEASURED SIGN RETROREFLECTIVITY

In general, there are two ways that sign retroreflectivity can be measured in the field: with handheld contact instruments or with non-contact instruments. Contact instruments require the measurement device to be in physical contact with the sign surface. Non-contact instruments, which measure the retroreflectivity from a distance, include both a hand-held device and vehicle-based systems. The use of the measurement method as an exclusive process to maintain sign retroreflectivity has not historically appealed to agencies, as will be discussed in the following sections. However, when combined with another method, the measured sign retroreflectivity method adds an element of accuracy to the overall program. This combination of methods may maximize maintenance budgets and provide additional protection from tort claims.

Background

There are several commercially available hand-held retroreflectometers that can be used to measure sign retroreflectivity. While the contact instruments are believed to provide relatively low levels of uncertainty for a given measurement, using contact instruments can be time consuming. Non-contact devices offer flexibility and speed-up the measurement process, but the trade-off is a higher level of uncertainty. The uncertainty associated with field measurement of sign retroreflectivity has not been well established. ASTM procedures for the measurement of sign retroreflectivity require the averaging of multiple measurements on the face and legend of the sign. The selection of the measurement points and the calibration of the device can lead to different results, even when measuring the same sign. This can create an issue if there are small differences between measured values and the required minimum levels—while the uncertainty in the precise value may not be significant from a performance standpoint, it has some potential to create tort liability for marginal signs.

More information about retroreflectometers can be found on the following Web sites:

Contact Devices, Hand-Held

Non-Contact, Hand-Held

Non-Contact, Mobile

Note that the FHWA does not endorse the use of any specific instrument. While the above list is, to the best knowledge of the authors, complete as of the date of publication, other instruments may be available or may become available in the future. Each agency is encouraged to review the specifications for the various instruments and determine for themselves which instrument is most appropriate for their application.

Procedures

Measuring retroreflectivity using a contact instrument should be performed as specified in ASTM Standard Test Method E1709-00e1, which requires a minimum of four retroreflectivity measurements to be taken of the sign background and legend, if applicable. The four measurements for each color are averaged to obtain an overall measurement of the retroreflectivity for each color on the sign. These values are compared to the minimum retroreflectivity values to determine whether or not the sign should be replace.

Hand-Held Contact Instruments

One important distinction needs to be made between the hand-held contact instruments: ASTM E1709 describes two types of sign retroreflectometers: point instruments and annular instruments, which are defined in the test method as follows:

  • “The instrument may be either a “point instrument” or an “annular instrument,” depending on the shape of the receiver aperture. Point and annular instruments make geometrically different measurements of RA, which may produce values differing on the order of 10 percent. Both measurements are valid for most purposes, but the user should learn the type of his instrument from its specifications sheet and be aware of certain differences in operation and interpretation. For both instrument types, the “up” position should be known. The point instrument makes an RA measurement virtually identical to an RA measurement made on a range instrument following the procedure of Test Method E810. The annular instrument makes an RA measurement similar to an average of a great number of RA measurements on a range instrument with presentation angle (γ) varying between -180 ° and 180 °.”

The geometries for the two types of hand-held contact instruments are illustrated in figure 1, while figure 2 provides an illustration of the instrument layout.

Annular and Point Aperture Instrument Angles
Figure 1. Annular and Point Aperture Instrument Angles.1


1 Reprinted, with permission, from ASTM E1709-00e1 Standard Test Method for Measurement of Retroreflective Signs Using a Portable Retroreflectometer, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the complete standard may be obtained from ASTM ( http://www.astm.org).

Upright Optical Schematics
Figure 2. Upright Optical Schematics.2

Differences in Point and Annular Instrument Measurements

Glass bead sheeting materials tend to be rotationally insensitive. Therefore, point and annular instruments should produce similar RA values for these materials. The RA values for prismatic sheeting, however, are rotationally sensitive, and the RA values produced by point and annular instruments can differ on the order of 10 percent, with differences of up to 25 percent possible. Annular instruments cannot accurately gauge how the RA of prismatic sheeting varies with rotation angle.

Current Practice

Few agencies have reported making retroreflectivity measurements of traffic signs on a regular basis. Most of those agencies use the retroreflectivity measurements to supplement visual or other inspection methods. The remainder use measured retroreflectivity values from a sample set of signs as an assessment of their total sign inventory.

There are private companies that specialize in retroreflectivity measurements. These companies offer retroreflectivity measurements for both signs and pavement markings, but the majority of the work that has been performed by private companies pertains to retroreflectivity measurements of pavement markings.

Concerns

The main concern with the measured sign retroreflectivity method is that retroreflectivity only accounts for one aspect of a sign’s appearance. Other factors should be considered when determining whether or not a sign is adequate for continued use at a particular location. These factors include ambient light levels, presence of glare, location relative to the road, and the


2 Reprinted, with permission, from ASTM E1709-00e1 Standard Test Method for Measurement of Retroreflective Signs Using a Portable Retroreflectometer, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the complete standard may be obtained from ASTM ( http://www.astm.org).

complexity of the visual background. A sign that is acceptable in a rural environment may not be acceptable in a complex urban environment.

Another concern with this method is the amount of time it takes to measure the retroreflectivity of a traffic sign using hand-held devices. Given the current methods and technology available to obtain a sign’s retroreflectivity, the time commitment required to take retroreflectivity readings of all signs within an agency’s jurisdiction may be labor intensive and cost prohibitive.

Linking Measurements to Minimum Retroreflectivity Levels

This method uses measured retroreflectivity as the basis for the decision of whether or not a sign meets the required minimum level of retroreflectivity. The measured retroreflectivity values are compared to the minimum retroreflectivity levels specified in the MUTCD. A sign should be scheduled for replacement if the measured retroreflectivity is at or very close to the minimum required level. This method provides the most direct comparison of the sign’s in-service retroreflectivity relative to the minimum maintained retroreflectivity levels.

Advantages and Disadvantages

Measured retroreflectivity provides the most direct means of monitoring the maintained retroreflectivity levels of deployed traffic signs. This removes all subjectivity that exists in other methods. However, a limit must be established on how close a sign’s retroreflectivity levels can be to the required minimum levels before they are replaced. Measurement uncertainty and the variance between the retroreflectivity at the prescribed measurement geometry versus the retroreflectivity at the actual observation geometry may result in a sign that meets the minimum requirements but does not meet the needs of the driver, and vice versa.

The main disadvantage of using this method is that measuring all of the signs in a jurisdiction is time consuming. Measured sign retroreflectivity may be best used to support one of the other methods or as a means of evaluating marginal signs. Another disadvantage is that using the retroreflectivity of the sign as the only indicator of whether or not a sign should be replaced may end up neglecting other attributes of the sign’s overall appearance. Other factors should be considered, including the overall appearance and legibility of the sign, as well as environmental concerns, such as areas with high levels of visual clutter or glare, that may require a brighter sign. Agencies need access to instruments and trained personnel to use this method.

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