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

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13 Table 2 Black on White Regulatory Signs Comparison ASTM ASTM New MUTCD Difference in Retroreflective Sheeting RA Typical Manufacturers' Minimum RA Warranty and Sheeting Type* Values* Warranty RA Values Level Minimum RA I 70 56 50 6 II 140 112 50 62 III 250 200 50 150 IV 360 288 50 238 VIII 700 560 50 510 IX 380 304 50 254 XI 580 464 50 414 Note: All retroreflectivity RA values are in units of cd/lx/m2 for an observation angle of 0.2 and an entrance angle of - 4.0. *ASTM information originated from ASTM D4956-11a (18). Type VIII, Type IX, and Type XI. The warranty periods for ranty were above the minimum requirements. Of those signs these prismatic materials may range from 10 to 12 years past the warranty period, 43% were in compliance. depending on the sheeting type, color, and signing applica- tion. These warranty periods may be different, but the peri- A study at Purdue University by Bischoff and Bullock (21) ods mentioned previously were common industry lengths at applied a similar approach; however, their primary objective the time. Besides warranty periods, service life may be ascer- was to determine if Indiana's current Type III 10-year service tained from past regional studies. life needed to be shortened or could be extended. A total of 1,341 Type III roadway sign retroreflectivity measurements One of the first studies to assess sign service life and deterio- were recorded, and sheeting colors included red, yellow, ration rates was conducted in 1992 by Black et al. for FHWA and white. Many of the signs exceeded the 10-year warranty (19). The objective of the study was to determine factors that period and installation ages were as high as 16 years. Overall, contributed to sign retroreflective degradation and to formu- the analysis found that only seven signs were not in compli- late models based on significant factors to accurately estimate ance with the minimum requirements and signs past 10 years retroreflectivity. The researchers collected retroreflective read- were performing adequately. Linear prediction models were ings from 5,722 signs in 18 different locations throughout the created that showed that red Type III sheeting produced the United States. In addition to the measurements, the collection highest R-squared value at 0.32, and white Type III sheeting process identified sheeting color, type, contrast ratio, sign direc- displayed the lowest at 0.02. There was a great deal of dis- tion, ground elevation, area type, and sheeting age. The mea- parity in the regression models and differences became more surements revealed that Type III signs performed adequately evident as sign age increased. Ultimately, researchers could for up to 12 years. The analysis determined that sheeting age, not fully support the prediction models, but did recommend ground elevation, and temperature were significant factors in that the service life of white and yellow Type III sheeting be sign deterioration. It also showed that the sign direction and extended to 12 years and that the service life of red Type III solar radiation variables were not acceptable predictors of in- sheeting remain at 10 years. service sign retroreflectivity. The researchers also created dete- rioration models for projecting service life periods in certain The last and most recent expected service life study was conditions. Despite weak correlation in some of the models, the conducted in 2006 by Rasdorf et al. for the North Carolina deterioration equations predicted that most Type III sign sheet- DOT (13). There were similar objectives and a comparable ing could last well past the manufacturers' warranty periods. approach to the earlier studies. Measurements were compiled from 1,057 Type I and Type III signs in North Carolina and Ten years later, the Louisiana Department of Transporta- included the four different colors. Models were generated from tion and Development produced a study that generated retro- linear, logarithmic, polynomial, power, and exponential func- reflectivity deterioration models (20). The objectives of the tions. The majority of the models exhibited poor correlation Wolshon et al. study were to assess current compliance rates, and the R-squared values ranged from 0.01 to 0.48. Within the determine influential factors, and create statistical models to sign sheeting types, white had the weakest relationship, while predict retroreflectivity relative to age. The data collection red showed the strongest, which was similar to the Bischoff and measured 237 signs in Louisiana and identified key envi- Bullock study (21). Despite the poor correlation, the majority ronmental factors that might affect sign deterioration. The of the Type III signs performed well and the models projected results showed that 92% of the signs under the 10-year war- long-term retroreflective compliance beyond 10 years.