National Academies Press: OpenBook

Practices to Manage Traffic Sign Retroreflectivity (2012)

Chapter: Chapter Two - Description of Sign Retroreflectivity Maintenance Methods

« Previous: Chapter One - Introduction
Page 6
Suggested Citation:"Chapter Two - Description of Sign Retroreflectivity Maintenance Methods." National Academies of Sciences, Engineering, and Medicine. 2012. Practices to Manage Traffic Sign Retroreflectivity. Washington, DC: The National Academies Press. doi: 10.17226/14663.
×
Page 6
Page 7
Suggested Citation:"Chapter Two - Description of Sign Retroreflectivity Maintenance Methods." National Academies of Sciences, Engineering, and Medicine. 2012. Practices to Manage Traffic Sign Retroreflectivity. Washington, DC: The National Academies Press. doi: 10.17226/14663.
×
Page 7
Page 8
Suggested Citation:"Chapter Two - Description of Sign Retroreflectivity Maintenance Methods." National Academies of Sciences, Engineering, and Medicine. 2012. Practices to Manage Traffic Sign Retroreflectivity. Washington, DC: The National Academies Press. doi: 10.17226/14663.
×
Page 8
Page 9
Suggested Citation:"Chapter Two - Description of Sign Retroreflectivity Maintenance Methods." National Academies of Sciences, Engineering, and Medicine. 2012. Practices to Manage Traffic Sign Retroreflectivity. Washington, DC: The National Academies Press. doi: 10.17226/14663.
×
Page 9
Page 10
Suggested Citation:"Chapter Two - Description of Sign Retroreflectivity Maintenance Methods." National Academies of Sciences, Engineering, and Medicine. 2012. Practices to Manage Traffic Sign Retroreflectivity. Washington, DC: The National Academies Press. doi: 10.17226/14663.
×
Page 10
Page 11
Suggested Citation:"Chapter Two - Description of Sign Retroreflectivity Maintenance Methods." National Academies of Sciences, Engineering, and Medicine. 2012. Practices to Manage Traffic Sign Retroreflectivity. Washington, DC: The National Academies Press. doi: 10.17226/14663.
×
Page 11
Page 12
Suggested Citation:"Chapter Two - Description of Sign Retroreflectivity Maintenance Methods." National Academies of Sciences, Engineering, and Medicine. 2012. Practices to Manage Traffic Sign Retroreflectivity. Washington, DC: The National Academies Press. doi: 10.17226/14663.
×
Page 12
Page 13
Suggested Citation:"Chapter Two - Description of Sign Retroreflectivity Maintenance Methods." National Academies of Sciences, Engineering, and Medicine. 2012. Practices to Manage Traffic Sign Retroreflectivity. Washington, DC: The National Academies Press. doi: 10.17226/14663.
×
Page 13

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

6 The 2009 MUTCD states that an agency “shall use an assess- ment or management method” (1) to maintain sign retroreflec- tivity. The manual does not dictate the method, but provides agencies the flexibility to implement one or more methods that best fit their needs, expertise, and level of resources. The intent of the methods and guidance outlined in the MUTCD is to provide support to the agencies and offer them sys- tematic procedures to maintain traffic sign retro reflectivity. Again, compliance is achieved by having a method in place and being able to document active implementation. Confor- mance does not require or guarantee that every individual sign will meet or exceed the minimum retro reflectivity levels at every point in time. This chapter describes each method and concludes with a section on sign service life for different sheeting materials. Sign RetRoReflectivity Maintenance MethodS Section 2A.08 in the MUTCD offers five traffic sign meth- ods for maintaining nighttime sign visibility and an “Other” method, which must be supported by an engineering study (1). The five methods are categorized as either assessment or management. Assessment methods evaluate the retrore- flectivity of individual signs and include visual nighttime inspection and measured sign retroreflectivity. Management methods incorporate an expected retroreflective life period of individual sheeting materials within the sign inventory. The retroreflective life of signs can originate from manufactur- ers’ warranties, demonstrated performance, or control sign assessments. The management methods include expected sign life, blanket replacement, and control signs. Assess- ment and management methods may be combined in many different ways to accommodate an agency’s needs and objec- tives. The MUTCD description of each method is provided here and additional details of each method are provided in this chapter. • Visual Nighttime Inspection—The retroreflectivity of an existing sign is assessed by a trained sign inspector conducting a visual inspection from a moving vehicle during nighttime conditions. Signs that are visually iden- tified by the inspector to have retroreflectivity below the minimum levels are to be replaced. • Measured Sign Retroreflectivity—Sign retroreflectivity is measured using a retroreflectometer. Signs with retro- reflectivity below the minimum levels are to be replaced. • Expected Sign Life—When signs are installed, the installation date is labeled or recorded so that the age of a sign is known. The age of the sign is compared with the expected sign life. The expected sign life is based on the experience of sign retroreflectivity deg- radation in a geographic area compared with the mini- mum levels. Signs older than the expected life need to be replaced. • Blanket Replacement—All signs in an area or corridor, or of a given type, are replaced at specified intervals. This eliminates the need to assess retroreflectivity or track the life of individual signs. The replacement inter- val is based on the expected sign life, compared with the minimum levels, for the shortest-life material used on the affected signs. • Control Signs—Replacement of signs in the field is based on the performance of a sample of control signs. The control signs might be a small sample located in a maintenance yard or a sample of signs in the field. The control signs are monitored to determine the end of retro reflective life for the associated signs. It is impor- tant that all field signs represented by the control sam- ple be replaced before the retroreflectivity levels of the control sample reach the minimum levels. It can also be pointed out that FHWA has a full report detailing each of the sign retroreflectivity methods listed in the MUTCD. The FHWA report also includes a useful descrip- tion of how to conduct the assessment methods. Finally, it also specifies the advantages and disadvantages of each of the sign retroreflectivity methods listed in the MUTCD. The FHWA report can be found at the following web address: http://safety.fhwa.dot.gov/roadway_dept/night_visib/policy_ guide/fhwahrt08026/. visual nighttime inspection Visual nighttime inspection is a common method for main- taining traffic sign retroreflectivity and guidelines for the inspection procedure have been documented for approxi- mately 50 years (10). The method is simple and requires a trained or experienced inspector to view traffic signs from a moving vehicle during nighttime conditions. The inspector subjectively concludes if a given sign passes or fails. This is a broad overview, but effective implementation does require expertise and attention to detail. chapter two deScRiption of Sign RetRoReflectivity Maintenance MethodS

7 Visual nighttime inspection requires one individual, but is more effective with two; a dedicated inspector monitoring and recording sign failures and a focused driver following a predetermined inspection route. It is important that visual inspection take place during typical nighttime conditions and that viewing not be affected by adverse or inclement weather such as fog or rain. Interior vehicle lighting should be minimized so that the inspector’s vision is not affected. The inspection can emulate how a normal driver would view a typical sign: at normal roadway speeds, from an appropri- ate travel lane, and at an adequate viewing distance. Sign failures and noteworthy comments are to be documented in a standardized procedure. The inspector can document his or her evaluations by means of written notes on an agency form, audio recording, or laptop computer. The duration of a nighttime inspection session must not exceed a period where inspector fatigue becomes an issue or where roadway condi- tions change, such as frost forming on a sign. Throughout the inspections, it is important to be consistent with agency procedures and be able to document when the nighttime sign inspections have been completed. There have been several research studies that have evalu- ated the visual nighttime inspection method. One of the first research studies to assess and document the accuracy of visual sign inspection was conducted in the state of Washington in 1987 (11). The first part of the study surveyed state DOTs and determined that 35 of 44 responding states used some type of visual inspections in the daytime and/or nighttime. The prac- tices varied between DOTs, but all states replaced signs if there were visual physical defects or inadequate retroreflectivity. The second part of the Washington study evaluated the accu- racy of 17 trained sign observers (11). The researchers trained the observers to rate STOP and warning signs in two environ- mental settings: a controlled gymnasium and a stationary car on a simulated road. After training, the observers were driven on two highway courses where they rated a total of 130 traf- fic signs. Overall, the observers made correct ratings for 75% of the signs. Within the total incorrect responses, observers were more likely to replace an adequate sign than to accept a sign with insufficient retroreflectivity. Despite the incorrect responses, replacing signs that are questionable or borderline is a more cautious but preferable approach for drivers. The researchers concluded that “trained observers can make accu- rate and reliable decisions to replace traffic signs” (11). In 1996, Hawkins et al. (12) conducted a similar study that built on the Washington study’s survey. In a statewide survey of Texas DOT (TxDOT) district sign maintenance offices, the researchers found that 80% of the districts con- ducted nighttime visual inspections and 65% also performed daytime inspections. Approximately 83% of the districts would implement visual inspection training when the pro- posed FHWA requirements took effect. The researchers also conducted a cost–benefit analysis of several different sign maintenance methods and determined that at that time visual inspection was one of the least expensive methods. In 2001, to build on the previous studies findings, Hawkins and Carlson evaluated the accuracy of experienced TxDOT sign personnel (10). In this study, TxDOT staff subjectively assessed 49 test signs during nighttime conditions and rated them as “Acceptable,” “Marginal,” or “Unacceptable.” TxDOT observers viewed test signs on a closed-course track at speeds of 30 to 40 mph. Only one test sign within the sample failed to meet the MUTCD minimums; however, the TxDOT observers rejected a total of 26 signs. The research- ers determined that for sign assessment the overall appear- ance and uniformity of the sign face were as important as the retroreflectivity levels. The TxDOT observers identified sign inconsistencies and blemishes that rendered the sign un acceptable despite meeting the retroreflective minimums. The researchers concluded that “visual nighttime sign inspec- tions should be a critical component of any process that evalu- ates the nighttime visibility of traffic signs” (10). Another visual sign inspection study was conducted in North Carolina in 2006. Rasdorf et al. (13) evaluated the accu- racy of North Carolina DOT (NCDOT) staff evaluations by comparing the visual nighttime inspection pass or fail deci- sions with retroreflectivity measurements. The study collected retroreflectometer measurements of 1,057 inspected signs on various types of state roadways in five different counties. Overall, the analysis determined that the NCDOT sign inspec- tors were effective in identifying and removing signs that were below the minimum values, and that accuracy levels ranged from 54% to 83%. The incorrect inspection decisions included a mix of both type I errors (failing adequate signs) and type II (passing inadequate signs). Despite the wide accuracy range, the researchers concluded that the NCDOT inspectors were proficient and that nighttime visual inspection was reliable. Finally, Kilgour et al. (7) at the Indiana LTAP Center com- pleted a study similar to the North Carolina study during the same time period. Again, researchers compared the pass or fail decisions of sign inspectors with the infield retro reflectometer measurements. There were 1,743 signs measured on road- ways that were recently inspected by local agency personnel throughout different counties, cities, and towns in Indiana. Overall, the study determined that the inspectors were accurate in 88% of the pass or fail decisions. Within the type I errors, inspectors failed 1.2% of the signs despite adequate retrore- flectivity values. Type I errors were most common for signs with red sheeting material. The red color in the sign would fade before the retroreflectivity resulting in inspectors failing the signs as a result of poor appearance. The overall type II error rate was 10.8% and the highest occurrence of this type of error was observed for yellow warning signs. Ultimately, the study acknowledged that visual nighttime inspection was a “reasonably accurate” method with “minimally trained personnel” (7). Despite the high accuracy nighttime retroreflectivity inspection rates noted previously, the visual nighttime inspec- tion method is still a subjective process and dependent on the

8 experience and knowledge of the sign inspectors. FHWA offers three different sign inspection procedures to assist inspectors, reduce the subjective nature of inspections, and develop a link to the minimum retroreflectivity requirements. The details are contained in the 2007 publication Maintaining Traffic Sign Retroreflectivity (14), which is referenced in the MUTCD and contained in Appendix A. The procedures are a recommended practice to comply with the MUTCD standard. If an agency chooses not to use at least one of the supportive techniques, it is important that they be able to justify the deviation with an engineering study that describes another procedure linked to the minimum MUTCD levels. One or more of the following procedures can be used to support visual inspections: • Calibration Signs: An inspector views a calibration sign each time before conducting a nighttime field review. The calibration signs have known retroreflectivity lev- els at or above the specified minimums. The calibration signs are set up in a maintenance yard where the inspec- tor can view the signs in a manner similar to nighttime field inspections. The inspector uses the visual appear- ance of the calibration sign to establish the evaluation threshold for that night’s inspection activities. • Comparison Panels Procedure: This procedure involves assembling a set of comparison panels that represent retroreflectivity levels above the specified minimums. Inspectors then conduct a nighttime field review and when a marginal sign is found a comparison panel is attached and the sign/panel combination is viewed. Signs found to be less bright than the panel would then be scheduled for replacement. • Consistent Parameters Procedure: The nighttime inspec- tions are conducted under conditions similar to those used in the research to develop the minimum retro reflectivity levels. These factors include: – Using a sport utility vehicle or pick-up truck to con- duct the inspection. – Using a model year 2000 or newer vehicle for the inspection. – Using an inspector who is at least 60 years old with 20/20 vision (corrected). With the aid of one or more of these techniques, visual nighttime inspection can be an effective method for maintain- ing sign retroreflectivity and monitoring sign quality. When an agency is considering strategies, this is one method that might be closely examined. Before making a decision, there are some advantages and issues to consider: • Advantages: – Evaluates more than sign retroreflectivity, such as face uniformity, message legibility, sign support integrity, damage, knockdowns, vandalism, obscuring vegeta- tion, genera sign visibility, etc. – Provides the opportunity to observe other roadway items such as raised pavement markers, pavement striping, delineators, and object markers. – Does not require advanced equipment or sophisti- cated computer programs. – Limits the low amount of waste because only failed signs are targeted for replacement. • Issues to consider: – Sign evaluation is subjective. – Inspectors need to be properly trained and one of the three supportive techniques be used correctly. – Because nighttime inspection occurs during non- regular work hours, overtime and next-day schedul- ing may be a concern. – There are outside aspects that are difficult to control such as weather, moisture in the air, and oncoming vehicles headlights. – Agency procedures need to be followed consistently. Measured Retroreflectivity The measured sign retroreflectivity method directly obtains retroreflectivity values with specialized equipment. Sign mea- surements remove the subjective nature by acquiring a specific retroreflectivity value. Repeatable and adequate measurements require both a calibrated instrument and a knowledgeable oper- ator. As with the visual nighttime inspection method, standard operating procedures need to be established. There are two types of devices that measure sign retro- reflectivity in the field: contact instruments, which require the operator to place the device in direct contact with the sign face, and noncontact instruments, which can measure sign retro- reflectivity from a distance and where devices can be either hand-held or vehicle-based systems. Noncontact instruments can expedite the sign measurement process and offer a sig- nificant amount of flexibility; however, the trade-off is higher levels of uncertainty. The current technology of vehicle-based systems is not yet at the level of practical implementation; therefore, agencies must use hand-held contact units. There are two common types of hand-held retroreflectom- eters and both instruments express measurements, the co- efficient of retroreflection (RA), in units of candelas per lux per meters squared (cd/lx/m2). Measurements need to be taken at an observation angle of 0.2 degrees and an entrance angle of -4.0 degrees to be comparable to the minimum levels in the MUTCD. Sign retroreflectivity measurement procedures are rela- tively straightforward; however, it is important that proce- dures be followed consistently. ASTM Standard Test Method E1709 outlines the procedures for operating and taking mea- surements with a retroreflectometer (15); it specifies that a retroreflectometer operator acquire a minimum of four retro- reflectivity measurements per retroreflective sign color. The measurement locations are in different parts of the sign and the readings can be averaged when compared with the MUTCD minimum levels.

9 The measured sign retroreflectivity method can be expen- sive and time-consuming. Individual retroreflectometer units can cost between $10,000 and $12,000; therefore, assigning one to each sign technician is typically not feasible. Also, some measurements can be difficult to obtain because the lower edge of many signs is 7 ft above the road surface. Readings may require the use of a ladder, and overhead signs may call for a truck with a boom lift. The readings may also expose sign technicians to more potential roadway hazards and place them in undesirable locations. Widespread imple- mentation of this method at a large agency may not be prac- tical because of the cost, time requirements, and roadway exposure. The advantages and issues to consider are: • Advantages: – Readings can be directly compared with MUTCD minimum levels. – A retroreflectometer removes the subjective nature of the visual nighttime inspection. – Data collected throughout the years can provide sheeting material deterioration rates for localized conditions. – Sign compliance can be thoroughly documented and there is a minimal amount of waste because only failed signs are targeted for replacement. – The MUTCD minimum contrast ratios for red/white signs can be obtained. – Measurements can be obtained during normal day- time work hours. • Issues to consider: – Retroreflectometers can cost between $10,000 and $12,000. – Signs may be difficult to access because of physi- cal barriers, sign height, and certain roadway condi- tions. Obtaining some measurements with hand-held contact units can be difficult and time-consuming. – Dew, light rain, and moisture on a sign can impede the data collection process. – Agencies must decide if sign measurements are to be collected when the signs are washed or unwashed. – Units only account for retroreflectivity readings and this method does not consider overall sign appear- ance and uniformity. – Obtaining measurements may expose sign techni- cians to potential roadway hazards and place them in undesirable locations. In general, sign retroreflectivity measurements appear to be best suited to complement another method. expected Sign life The expected sign life method is the first of the three different management methods. The main aspect of the expected sign life method is that it documents and tracks individual signs to be replaced before the service life period expires. Sign service life represents the length of time that a certain sign sheeting material will be used in the field while remaining in com- pliance with the minimum retroreflective requirements. Sign service life can be based on sign sheeting warranties, test deck or field measurements, or empirical data from other regional studies. The key is being able to identify the age of individual signs, which may be accomplished through a scientific sys- tem and/or advanced technology. The level of complexity and sophistication depends on an agency’s needs and available resources. Implementation of the expected sign life method can vary significantly; however, there are three main com- ponents to most successful systems. These components, in a hierarchical order, are establishing sign installation dates, identifying signs for replacement, and organizing sign data. The first component is establishing a sign’s installation dates, which is the foundational base to the expected sign life method. The majority of the agencies employing this method use installation date stickers to track sign age (16). An installation date sticker may contain the fabrication and/ or installation dates, sheeting type, unique sign identifica- tion number, and/or other agency specific information. The stickers are typically placed on the back of signs in a visible area. Figure 1 shows several examples of sign installation date stickers. Barcode labels can also be used and serve the simple purpose of linking important information physically to the sign. Another simple technique to establish sign age is through digital images. Most digital cameras record the date and time stamp noting when a picture was taken, and some cameras can also associate latitude and longitude coordinates with the image, which transitions into the next component. The second component is identifying and locating individ- ual signs that require replacement. Large agencies with size- able sign inventories need to be able to identify the locations of signs slated for replacement. Two effective forms of sign location information are spatial data and benchmark-based data. Spatial coordinates could be collected with a hand-held Global Positioning System (GPS) unit and benchmark-based data could be measured with a Distance Measurement Instru- ment from the nearest cross street or mileage marker. The third component deals with organizing and managing the sign data. A large sign inventory may generate a signifi- cant amount of data and agencies need to be able to access information in a timely and efficient manner to schedule sign replacement. The sign location and installation data can be linked and stored in either a spreadsheet form or a geo- graphic information system (GIS)-based platform. Microsoft Excel is one type of spreadsheet software and Google Earth and ArcGIS are two examples of spatial mapping platforms. A small agency could use Google Earth and the latitude and longitude coordinates from the images of a digital camera to populate an expect sign life system.

10 Agency needs and objectives vary considerably and there are many different options and levels of sophistication for expected sign life systems. These systems may have a sign inventory component that allows agencies to query specific sign information or asset management features that allow for enhanced planning, work scheduling, and budgeting capabilities. In all systems, the overall objective is to expe- dite and streamline maintenance operations through the effective organization and management of the sign data. Expected sign life systems or inventory programs could be developed in-house or acquired through an outside vendor. Several LTAP centers offer systems at a reasonable cost and there are many commercial companies that have developed packages that include data-gathering equipment and sophis- ticated software. Agencies considering this method need to thoroughly research the many different options available before select- ing a system or program. An agency could take into consid- eration its level of resources, funding, staff demands, and technical expertise. A system is selected that is compatible with both short- and long-term agency goals. There also needs to be general acceptance from all involved users and parties. If users are not willing to fully support the system and keep the sign information up-to-date and accurate, then the investment into the system could be wasted. The advan- tages and issues to consider are: • Advantages: – Sign replacement can be thoroughly documented. – There may be only a small amount of waste because only those signs near the service life period are tar- geted for replacement. – This method can expedite and streamline signing operations. – This method keeps accurate records of the sign inventory and is easily able to access specific sign information. – This method provides asset management capabili- ties and enhanced tools for planning, scheduling, and budgeting purposes. • Issues to consider: – Service life periods need to account for the different types of sheeting materials and environmental condi- tions that may affect retroreflectivity. – This method relies on accurate and up-to-date infor- mation of individual signs. – Sophisticated and advanced systems may require a high level of technical support and expertise. – Collecting sign inventory data and initially creating an expected sign life system can be an expensive and time-consuming process. – Administrative, maintenance, and upkeep cost can be high. – Computer-based systems are susceptible to technical problems and information loss. Blanket Replacement The blanket replacement method uses service life periods and is similar to the expected sign life method; the fundamental dif- ference derives from targeting a large group of signs as opposed to identifying individual signs. The replaced signs can be based on either spatial or strategic data. The spatial sign replacement removes all signs in a certain geographic area. The scale of the spatial area can vary widely between agencies. The area could be limited to a single road or corridor or as large as all signs in a county. The strategic approach replaces all signs of a common characteristic such as sheeting type, sign classification, and sign content. Upgrading sign sheeting from Type I to Type III is an example of strategic replacement. STOP signs are a major concern and may have a strategic priority for replacement over warning and guide signs. Blanket replacement could incorpo- rate both spatial and strategic characteristics by removing spe- cific sign types in a certain area. FIGURE 1 Images of sign installation date stickers.

11 The major advantage of the blanket replacement method is that it is relatively easy and straightforward to implement. Operations and resources are minimal; it does not require advanced personnel training, high administrative cost, or time- consuming maintenance procedures. A computer-based sign inventory and management system may not be a requirement, but it could greatly benefit this approach. When implemented, agencies typically stagger the blanket replacement schedule to simplify planning and budgeting. Consider an agency using Type III High-Intensity Beaded Sheeting, which has a warranty period of 10 years. The agency divides its jurisdiction into ten different areas. Each year, that agency will replace all the signs in one of the ten different areas and the replacement rate for each area is based on a regu- lar 10-year cycle. Planning, scheduling, and budgeting can be simplified when an agency knows that it will have to replace around 10% of the sign population each year. Figure 2 is a map of a blanket replacement schedule and the divided areas. The blanket replacement method documentation is simple and an agency can draft a short policy memo justifying the ser- vice life period, defining the area boundaries, and outlining the yearly sign replacement procedures. Because all of the signs in a specific area are replaced on a regular cycle, the chances of hav- ing signs that are below the MUTCD minimum requirements are small. An agency can easily show that it is implementing its method and working toward compliance through work orders and sign replacement schedules. Overall, the blanket replace- ment method has simple procedures, removes subjectivity, and can simplify sign replacement documentation. Conversely, the blanket replacement method can lead to premature sign replacement and waste. Signs are sometimes replaced before the retroreflectivity falls below the minimum levels and reasons could be attributed to vandalism, vehicle knockdowns, road reconstruction, and changes in standards. As a result, signs in a specific blanket replacement area will not always have the same installation period. When the replace- ment cycle is reached, there may be many signs with adequate retroreflectivity that are removed. Not only do signs not reach full potential in the field, but maintenance costs for replacing adequate signs in the long term may be a substantial drain on agency resources. The advantages and issues to consider are: • Advantages: – Identifying signs and formulating the replacement schedule is simple and straightforward. – Administrative costs are low. – Regular replacement cycles can help with planning, scheduling, and budgeting. – There is the capacity to target certain sign types such as placing a greater priority on STOP signs or removing all Type I signs from the roadway. – Sign inventory and management systems may not be necessary; however, they could provide support. • Issues to consider: – There is a high possibility of premature sign replacements. – There remains the need to determine the replacement cycles. – Routine daily inspection and maintenance is still needed. – Operating costs and additional sign installation labor could be higher than with other methods. control Signs The control signs method is the third sign management strat- egy and it may utilize both sign assessment and management techniques to maintain sign compliance. The MUTCD states that sign replacement in the field is based on the performance of a sample set of control signs (1). Specific sheeting types in the controlled sample set represent the retroreflective val- ues of a sign population in the field. The control signs may be a sample in a secure maintenance yard or selected signs on the roadway. Control signs are monitored and assessed to determine retroreflective performance. When the control signs approach the retroreflective minimums, all correspond- ing signs in the field are replaced. The control signs method requires a means of establishing a creditable sample set, sign evaluation techniques, and a system to locate corresponding signs in the field. The first step is to establish an acceptable and effective sample size. An agency should select a sample size that it determines is appropriate and justifiable. The National Trans- portation Product Evaluation Program conducts sign dete- rioration studies for new sheeting products for AASHTO. It tests two panels for each new sheeting type in an accelerated FIGURE 2 Blanket replacement map and schedule.

12 experiment to determine minimum levels of outdoor durabil- ity (16). Carlson and Lupes recommend testing a minimum of three signs per sheeting type continually installed at stra- tegic intervals (17). Another aspect of the control signs method is determin- ing adequate sign sample locations and arrangements. The unprotected signs on an open roadway are exposed to vandal- ism, knockdowns, and other forms of premature damage. A protected facility greatly lessens the likelihood of the control signs being harmed, and may provide a limited and biased sample that does not fully represent roadway conditions. Unprotected sample signs can encompass a large geographic area and cover a wide range of roadway conditions. It is important that the unprotected sample size is large enough to compensate for signs that are removed or damaged during the evaluation period. It may be an effective strategy to establish control signs in both a protected area and on the open road. Unlike the previous two management methods, this approach requires the periodic use of a retroreflectometer. Measuring the retroreflectivity of control signs should follow the same procedures outlined in ASTM Standard E1709-00e1 (15). An average of four readings per retroreflective sign color is recorded to document the retroreflectivity levels throughout the life of the sign. The time intervals between consecutive measurements depend on an agency’s objectives and desired level of precision. Carlson and Lupes (17) rationalized that too little time between measurements of control signs may lead to the misuse of labor and resources, whereas long peri- ods between readings may lead to inaccuracies in predicting service life in the field. This method not only indicates when corresponding signs in the field require replacement, but can also help to establish regional specific service life periods for different sheeting materials. The control signs method allows an agency to document and verify the extension of service life periods past the manufacturer’s warranty. The control signs method is a desirable option for agencies that want to monitor regional sign performance, but do not want to spend the time and resources to measure every sign in the field. This approach could be used when an agency wants to extend or examine service life of a specific sign sheeting material. Because sign measurements are periodic, an agency may be able to borrow a retroreflectometer from a LTAP center or rent a unit from a vendor once per year instead of spending between $10,000 and $12,000 to purchase one. The advantages and issues to consider are: • Advantages: – The ability to monitor regional specific year-to-year sign retroreflectivity performance without having to measure every sign in the field. – A means to validate the extension of service life for a specific sign sheeting material past the manufac- turer’s warranty with the purpose of minimizing cost and resources. • Issues to consider: – Agencies need to purchase or obtain a retro- reflectometer. – Installing control signs, collecting measurements, and analyzing the data can be time-consuming and costly. – This method requires continuous monitoring of con- trol signs and regular upkeep. Sign SeRvice life The sign retroreflectivity management methods have a com- mon theme of being based on knowledge of the sign service life, or the length of time that a certain sign sheeting material will remain compliant with the minimum retroreflective require- ments (without being subjected to bullet holes, graffiti, or other sources of damage that would result in premature removal). The retroreflectivity of a sign will degrade and deteriorate over time as it is exposed to regional environmental conditions. When a sign reaches or approaches the end of its service life, it is then replaced. Different sheeting materials, regional condi- tions, and maintenance practices are some of the major factors that can significantly affect service life periods. The sign service life that an agency selects can be based on several different options such as sign sheeting warranties, test deck or field measurements, or empirical data from other regional studies. The most basic and rudimentary approach would be using sign sheeting manufacturers’ warranty peri- ods as a substitute for service life for one of the management methods. A typical manufacturer’s warranty period guaran- tees that a sign will retain 80% of the original retroreflectivity levels within a certain time period and does not necessarily represent a sign’s true service life. Most warranty periods are fairly conservative because the same warranty period needs to cover all signs whether they are in Arizona or Alaska. Some signs may fail before the end of the warranty period, but most will surpass it. Table 2 provides an example of how conservative war- ranty periods can be for certain sign sheeting types. The last column in the table shows the difference between the manu- facturers’ warranty values and the MUTCD minimum main- tained retro reflectivity level for black on white regulatory signs. The table contains the typical manufacturers’ warranty values, which are 80% of the ASTM new sheeting values. It can be noted that the 80% threshold in new sheeting retro- reflectivity is typical. Besides the Type I and Type II sheeting, it may be inferred that most of the sheeting types’ service lives may extend well past the typical warranty periods. Manufacturers’ warranty retroreflectivity values may deviate from the typical 80% thresholds, which mean that the warranty service periods may also vary. Typical and common warranty periods are seven years for Type I and ten years for Type II and Type III sheeting materials. There is a wider range for prismatic materials, which include Type IV,

13 ranty were above the minimum requirements. Of those signs past the warranty period, 43% were in compliance. A study at Purdue University by Bischoff and Bullock (21) applied a similar approach; however, their primary objective was to determine if Indiana’s current Type III 10-year service life needed to be shortened or could be extended. A total of 1,341 Type III roadway sign retroreflectivity measurements were recorded, and sheeting colors included red, yellow, and white. Many of the signs exceeded the 10-year warranty period and installation ages were as high as 16 years. Overall, the analysis found that only seven signs were not in compli- ance with the minimum requirements and signs past 10 years were performing adequately. Linear prediction models were created that showed that red Type III sheeting produced the highest R-squared value at 0.32, and white Type III sheeting displayed the lowest at 0.02. There was a great deal of dis- parity in the regression models and differences became more evident as sign age increased. Ultimately, researchers could not fully support the prediction models, but did recommend that the service life of white and yellow Type III sheeting be extended to 12 years and that the service life of red Type III sheeting remain at 10 years. The last and most recent expected service life study was conducted in 2006 by Rasdorf et al. for the North Carolina DOT (13). There were similar objectives and a comparable approach to the earlier studies. Measurements were compiled from 1,057 Type I and Type III signs in North Carolina and included the four different colors. Models were generated from linear, logarithmic, polynomial, power, and exponential func- tions. The majority of the models exhibited poor correlation and the R-squared values ranged from 0.01 to 0.48. Within the sign sheeting types, white had the weakest relationship, while red showed the strongest, which was similar to the Bischoff and Bullock study (21). Despite the poor correlation, the majority of the Type III signs performed well and the models projected long-term retroreflective compliance beyond 10 years. Type VIII, Type IX, and Type XI. The warranty periods for these prismatic materials may range from 10 to 12 years depending on the sheeting type, color, and signing applica- tion. These warranty periods may be different, but the peri- ods mentioned previously were common industry lengths at the time. Besides warranty periods, service life may be ascer- tained from past regional studies. One of the first studies to assess sign service life and deterio- ration rates was conducted in 1992 by Black et al. for FHWA (19). The objective of the study was to determine factors that contributed to sign retroreflective degradation and to formu- late models based on significant factors to accurately estimate retroreflectivity. The researchers collected retroreflective read- ings from 5,722 signs in 18 different locations throughout the United States. In addition to the measurements, the collection process identified sheeting color, type, contrast ratio, sign direc- tion, ground elevation, area type, and sheeting age. The mea- surements revealed that Type III signs performed adequately for up to 12 years. The analysis determined that sheeting age, ground elevation, and temperature were significant factors in sign deterioration. It also showed that the sign direction and solar radiation variables were not acceptable predictors of in- service sign retroreflectivity. The researchers also created dete- rioration models for projecting service life periods in certain conditions. Despite weak correlation in some of the models, the deterioration equations predicted that most Type III sign sheet- ing could last well past the manufacturers’ warranty periods. Ten years later, the Louisiana Department of Transporta- tion and Development produced a study that generated retro- reflectivity deterioration models (20). The objectives of the Wolshon et al. study were to assess current compliance rates, determine influential factors, and create statistical models to predict retroreflectivity relative to age. The data collection measured 237 signs in Louisiana and identified key envi- ronmental factors that might affect sign deterioration. The results showed that 92% of the signs under the 10-year war- ASTM Retroreflective Sheeting Type * ASTM New Sheeting R A Values* Typical Manufacturers’ Warrant y R A Values MUTCD Minimum R A Leve l Difference in Warrant y and Minimum R A I 70 56 50 6 II 140 112 50 62 III 250 200 50 150 IV 360 288 50 238 V III 700 560 50 510 IX 380 304 50 254 XI 580 464 50 414 Note : All retroreflectivity R A values ar e i n units of cd/lx/m 2 for an observation angle of 0.2° and an entrance angle of−4.0°. *ASTM information originated from ASTM D4956-11a ( 18 ). TABLE 2 BLACK ON WHITE REGULATORy SIGNS COMPARISON

Next: Chapter Three - Range of Practices »
Practices to Manage Traffic Sign Retroreflectivity Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 431: Practices to Manage Traffic Sign Retroreflectivity includes examples of practices that illustrate how different types of transportation agencies might meet federal retroreflectivity requirements for traffic signs.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!