U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
The calibrated visual inspection method can be used by agencies to conduct nighttime visual inspections as long as a few initial steps are taken to calibrate the inspector’s perception of the pavement marking visibility threshold. The MUTCD currently includes language that encourages agencies to undertake periodic daytime and nighttime visual sign inspections (see Section 2A.22). In addition, Section 2A.09 lists visual inspection as an approved method to maintain traffic sign retroreflectivity. Adding pavement markings to a nighttime sign inspection program is an effective method as long as the necessary steps are followed.
Using this approach, it is possible to assess more than just the retroreflectivity of pavement markings. Other damage may be identified such as excessive wear from turning movements or loss of presence (i.e. when some of the pavement marking material is missing), which may not be identified with spot retroreflective measurements alone.
This method requires little 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 (because the inspection is performed at night). The significant up-front resource needed is a retroreflectometer to measure the calibrated pavement markings to assure they are at the intended retroreflectivity levels. However, agencies do not necessarily need to purchase their own instruments. Some agencies share devices or use loaners from Local Technology Assistance Program (LTAP) offices.
While nighttime visual inspections will reveal visibility problems not discernable under daytime conditions, they are subjective and hence more difficult to tie to a benchmark value of retroreflectivity. As a result, agencies using visual inspections must establish procedures to provide consistency in inspections. This implies the need for training programs and inspector certification.
Probably the most common type of assessment method used to evaluate pavement marking retroreflectivity has been some form of the nighttime visual inspection method. Despite the subjectivity and reliability concerns of the visual nighttime inspection method, recent research has shown visual assessment techniques can be used to assess the relative brightness of pavement markings, but not necessarily the retroreflectivity level of pavement markings.(13) Therefore, it is important to have trained inspectors who follow the process outlined in this section when conducting nighttime visual inspections of pavement markings. While there is no nationally recognized training course or certification for pavement marking inspectors, agencies should provide some form of training before nighttime inspections are performed. The FHWA will provide inspection training tools to the Local Technical Assistance Program (LTAP) and Tribal Technical Assistance Program (TTAP) Centers
One way to perform the training is to have the inspectors observe sample pavement markings at a variety of known retroreflectivity levels, including levels near the minimum levels, before conducting the inspections. This type of training helps the inspector understand the differences between various retroreflectivity levels. Training is also necessary for the inspector to understand what the objectives are for the inspection and to ensure an understanding of the critical inspection elements, cautions for improper techniques, and safety procedures. Inspectors should view the sample pavement markings under conditions similar to those under which they will perform inspections. This includes using the low beam headlamps of the inspection vehicle so that the calibration pavement markings are located in positions that replicate most typical field applications. The inspector should also be trained on agency guidelines and procedures for conducting nighttime inspections, including any necessary documentation.
The calibrated visual inspection procedure uses trained personnel to observe pavement markings during nighttime conditions to assess the overall appearance of the markings and determine if they meet the required minimum retroreflectivity level. If the inspector believes a marking appears to be less bright than the calibration marking viewed earlier, then the markings should be replaced. The observation is typically made through the windshield of the vehicle at or near the speed limit of the roadway.
The preferred technique for inspecting pavement markings at night is to use a two-person crew. While the driver focuses on the driving task, the trained passenger evaluates the pavement markings and records the appropriate information. An alternative to a two-person crew is one person using a tape recorder or mounted camcorder for recording notes (to review later for determining the condition of the markings).
To get started, an agency should develop a step-by-step set of instructions for consistency of inspections. This procedure requires a sample of pavement markings at or near the appropriate proposed minimum retroreflectivity levels in the MUTCD. These markings will be designated as the calibration markings. Depending on the agency specifications, the retroreflectivity levels of the calibration markings may be at the minimum levels outlined in Appendix A, or they may be higher than the minimum levels but not lower than the minimum levels. It is possible to install pavement markings at a desired level of retroreflectivity or to use in-service pavement markings. It is also possible that pavement marking manufacturers may produce sections of pavement markings with retroreflectivity levels consistent with the minimum levels. In any case, the pavement marking retroreflectivity will need to measured and documented to ensure it is near the desired level
The calibration markings should be both yellow and white and positioned accordingly (e.g., for a two-lane two-way roadway, white on the right and yellow on the left). If the calibration pavement markings are installed specifically for this purpose, the contractor or installer will have to make special provisions to apply pavement markings near the desired level. This may take some trial and error but can generally be achieved by controlling truck speed and bead load rates. The retroreflectivity can also be lowered by applying a clear coat polyurethane or other similar product.
The calibration markings should be evaluated before the inspection begins. Since the markings need to be seen for a preview distance of 2.2 seconds, the length of the calibration markings will vary by speed. The calibration markings must be at least 10 feet long. They must be viewed from the inspection vehicle but they can be viewed in a static position or a moving position. If they are to be viewed in a static position, Table 1 shows the preview distances that should be used depending on the posted speed limit of the roadways to be inspected.
When viewing the calibrated the markings, the inspector should try to commit to memory the appearance of the calibrated markings. The inspector will use the appearance of the calibrated markings as a benchmark to determine if in-service markings are brighter or less bright than the calibrated markings. Those that are less bright, and those at about the same brightness level, should be scheduled for replacement.
Table 1 . Calibrated pavement marking viewing distances.
|Speed (mph)||Distance (ft)||Number of Lane Lines|
Once a set of pavement markings that are representative of those installed within the agency’s jurisdiction has been installed or identified, the retroreflectivity levels should be measured and documented. ASTM Test Method D7585(11) includes a description of the appropriate test method for hand-held pavement marking retroreflectometers. A mobile pavement marking retroreflectometer can be used, as well.
Any type of vehicle can be used for the calibrated pavement marking visual inspection procedure. The low-beam headlamps of the inspection vehicle should be checked for proper alignment, and inspections should only be conducted with the low beam headlamps The ambient conditions must be dark, at least 30 minutes beyond sunset. The conditions of the pavement must be dry during the calibration and during the inspection. At least one pass of the markings should be made, but more than one may be beneficial. It is helpful either to locate the calibrated pavement markings in a centralized area or to plan the night inspection route so that the calibrated markings can be observed several times throughout the night.
Minimum retroreflectivity levels are incorporated into this method by training the inspectors and using procedures that allow them to correlate their inspection observations through the use of calibrated pavement markings with known retroreflectivity levels (at or above the minimum levels in the MUTCD). A good practice is for inspectors to observe the calibrated pavement markings prior to and intermittently throughout each nighttime inspection. The use of appropriate calibrated pavement markings at or near minimum retroreflectivity levels is a key element that links the nighttime visual inspection method to the minimum retroreflectivity levels.
One of the major benefits of using the calibrated nighttime visual inspection procedure is that it has low administrative and fiscal burdens. Many agencies already perform some type of periodic pavement marking inspection, although not all inspections are performed at night, and few are actually linked to any retroreflectivity level. This procedure also has a unique feature in that the pavement markings are viewed in their natural surroundings. Thus, the overall appearance of the pavement marking and the ability of the pavement markings to provide information to the driving public can be assessed.
Another advantage of the calibrated nighttime visual inspection method is that it has a low level of unnecessary pavement marking replacement and waste. Only those pavement markings identified as needing to be replaced because of low retroreflectivity levels are replaced, assuming that the inspection frequency is appropriate.
While this procedure may be more subjective than other methods, research has shown that subjective ratings can be made of pavement marking visibility that can be used as surrogates for retroreflectivity (i.e., using qualitative ratings such as poor, marginal, and desired rather than specific retroreflectivity levels such as 110 or 115 mcdm2/1×). There is some risk involved while performing these inspections, particularly if the driver is also the evaluator and recorder. Ideally, nighttime inspections should be conducted by two people for safety reasons. Including additional inspections by different inspectors has been shown to increase the reliability of visual inspections.
Using this procedure will require agencies to establish a protocol that fits their conditions, including the frequency of inspections and the frequency of replacing calibrated markings. Calibrated markings will need to be monitored so that they do not fall below the minimum levels established in the MUTCD. Since pavement markings tend to lose their retroreflective performance over time, it is important to measure the calibrated markings periodically to ensure that they are at or above the minimum levels outlined in the MUTCD.
Visual inspections of pavement markings supplemented with raised pavement markers (RRPMs) can be difficult. The brightness of the RRPMs is usually much greater than that of the pavement markings, and therefore it can be difficult to judge the pavement marking retroreflectivity.
Pavement markings on either side of the inspection vehicle can be evaluated during a visual inspection. However, pavement markings that are not adjacent to the inspection vehicle cannot be evaluated during a visual inspection. As a result, for multilane highways, more than one pass is needed to inspect all of the longitudinal markings (per direction).
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