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FHWA Home / Safety / Roadway Departure / Methods for Maintaining Traffic Sign Retroreflectivity

Methods for Maintaining Traffic Sign Retroreflectivity

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

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CHAPTER 4. MANAGEMENT METHODS

This chapter describes the management methods that can be used for maintaining traffic sign retroreflectivity. The methods described in this chapter include

EXPECTED SIGN LIFE

In this method, signs are replaced before they reach the end of their expected service life. The expected service life is based on the time required for the retroreflective material to degrade to the minimum retroreflectivity levels. The expected service life of a sign can be based on sign sheeting warranties, test deck measurements, measurement of signs in the field (control signs) and measurement of signs taken out of service, or information from other agencies. The key to this method is being able to identify the age of individual signs. This is often accomplished by placing a sticker or other label on the sign that identifies the year of fabrication, installation, or planned replacement or by recording the date of installation in a sign management system. Various approaches or algorithms can be used to trigger an indication of the need to replace a sign. For example, one software system uses sign material type, color, age, and direction the sign faces in a model that predicts the level of retroreflectivity at any point in time. When the minimum levels are approached, the sign is flagged for replacement. The process must, however, be geared to flag signs that need replacement early enough to assure that the process of physical replacement can be completed before the signs drop below the minimum retroreflectivity levels.

Procedures

Although there are variations to this method, the basic idea is that the installation date of every sign in an agency’s jurisdiction is known, along with the type of retroreflective sheeting material used on the sign face. It is also necessary to define an expected sign (i.e., service) life for each type of retroreflective sheeting material. This can be done for individual signs or as a general parameter for the types of material used by the agency. Other information may also be of interest to the agency such as sign color, direction the sign is facing, and sign construction (silk-screening versus electro-cut (EC) film). This information is used in a systematic manner to “flag” signs that need to be replaced before their sign life expires.

One way to use this method is through a computerized sign management system to keep track of an agency’s sign inventory and periodically extract information on signs that are reaching the age at which they need to be replaced. The degree of sophistication of the sign management system will dictate the options available to the agency. For example, most systems can generate lists of signs needing replacement, but some allow specific categories of sign type, size, or color to be focused upon. These systems may be able to generate individual work orders for each sign that needs to be replaced or can group replacements in a manner that provides an effective work schedule for sign crews.

If an agency has a computerized sign management system, it should be possible to query the sign database at regular intervals for a list of signs that are nearing the end of service life. Actual readings of sign retroreflectivity can be taken to determine if the degradation is occurring as expected. If the degradation is not occurring as fast as expected, then signs of that type could be left in the field longer (and an update to the planned replacement date subsequently made in the database). Conversely, if the deterioration is occurring faster than expected, the signs can be scheduled for replacement sooner. Monitoring changes in degradation can help ensure better nighttime visibility and increase the overall life cycle of an agency’s signs, resulting in cost savings.

Another way this method can be used is by placing an installation or replacement date sticker on each sign to allow field crews to know when specific signs reach their replacement age. If a sign is found to be older than indicated by the maximum life noted on the sticker, then the sign should be replaced. This method can be time consuming if signs along a roadway vary significantly in age, but it can be executed during the day and requires no inspection or measurement of the sign. A complication of this method is related to the placement of the date stickers. When placed on the front of the sign, field crews can more readily view the date information. However, the information must be limited so as not to distract from the message on the sign. More information can be included on stickers placed on the back of the sign, but it is harder for field crews to see this information as they drive by, particularly on wide roadways. Figure 3 shows the Wyoming coding system that can be seen from the front of the sign. The number indicates the year the sign was installed. For instance, the stop sign was installed in 1995. Wyoming uses a 10-year replacement cycle that allows the Wyoming agency to use a one-digit coding system. (Note: The photograph was taken before 2005.) Minnesota uses a color-coded sticker on the back of the sign, as shown in figure 4, with a different color each year to make inspections easier.

Stop Sign
Parking Directional Sign
Figure 3. Examples of Wyoming DOT Signs.

Minnesota DOT Sign Sticker
Figure 4. Example of Minnesota DOT Sign Sticker.

Current Practice

The use of expected sign life as a maintenance method is widely used because of its ease of implementation. Most agencies use the warranty period provided by the manufacturer to determine when a sign should be replaced. However, some agencies, like Indiana, are beginning to extend their expected sign life levels beyond the warranted sign life as a result of research documenting the durability of sign materials in their area.(18) This may create some complexity in sign management, as the expected sign life will vary across a jurisdiction depending on the geographic location of the sign, the amount of direct sun exposure, and other environmental factors that affect service life. The use of a single value for expected service life might result in some signs failing to meet the minimum maintained retroreflectivity levels prior to scheduled replacement or might result in replacement of signs that exceed the minimum levels.

Delaware, Kansas, Maine, Missouri, and North Dakota all use a 10- to 12-year life cycle for all of their beaded high-intensity retroreflective materials (as of 2005). Some agencies such as Indiana(18), Michigan, and North Carolina(19) are learning that they can expect their in-service life expectancy to be 15 years for beaded high-intensity materials.

Concerns

The main concern with this method is that there are little data on how different types of sheeting deteriorate over time in a given climate. It can be a complex process to determine how long signs of a certain sheeting type and color will last in a given region of the country. Also, there are no definitive results on the role that the orientation of the sign face plays in the deterioration of the sign and whether or not signs facing different directions deteriorate at significantly different rates. While there have been many studies, these studies do not come to the same conclusions about the relationship between sign face orientation and deterioration rates. (See references 18, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, and 31.)

One of the easiest ways to assign expected sign life to retroreflective sheeting materials is to use the manufacturer’s warranty. However, these warranties obviously include a certain factor of risk on the part of the manufacturer and therefore are often conservative. They may also vary depending on the region of the country. In general, however, it can be expected that retroreflective sheeting materials will have a warranty provided for the ASTM Type-designated materials as shown in table 5. Additional information on sign sheeting durability can be found in several research reports. (See references 1831.)

Table 5. Typical Warranty Life.
ASTM D4956 Type Years of Warranty*
I and II 7
III and IV 10
VII, VIII, IX, X 12
* May be different for fluorescent sheeting materials

Linking Expected Sign Life to Minimum Retroreflectivity Levels

The minimum retroreflectivity levels provide the initial basis for the expected life criteria, but an understanding of the actual degradation rates of in-service signs is required to set appropriate triggers as retroreflectivity levels approach the minimum requirements. Degradation rates differ by region of the country, type and color of material, and orientation. Furthermore, under this method, the actual retroreflectivity of a sign is not assessed—only the age of the sign is monitored.

There is a potential need to gather sample data on the true service life of signs to adjust the expected life measures. Some agencies accomplish this by the measurement of a sample of the removed signs; some monitor the performance of a small number of signs; and others measure the retroreflectivity of in-service signs with known installation dates.(19)

Advantages and Disadvantages

This method requires that agencies track the installation date of their signs. For the field replacement approach to this method, there is the benefit of associating the condition of a sign to its age. The use of a computerized sign management system may eliminate the need for a date sticker, but it also limits the means that may be used to analyze actual service lives because of the need for bar-code reading equipment or other technology-dependent equipment that might be used to code information on a sign.

The expected sign life method allows agencies to help develop local service life requirements based on actual end-of-service-life retroreflectivity measurements and comparisons to minimum required levels. These comparisons can provide useful information on service life under local conditions, product performance, sign fabrication processes, and analysis of replacement strategies. This method requires that the type of sheeting used to fabricate a sign be known. Other pertinent information may also be necessary to take advantage of sophisticated sign life prediction algorithms.

One drawback to this method is that it can be fairly time consuming to check date stickers if the stickers are not easily viewable or identifiable on the sign. Another possible difficulty relates to marking signs that need to be replaced, although immediate replacement is possible for some sign types. If an agency uses a sign management system and functions with the use of portable computers in the field, the inspectors can easily note the signs that need to be replaced, and even generate work orders.

BLANKET REPLACEMENT

The blanket replacement method is essentially the expected sign life method executed on a spatial or strategic basis. On a spatial basis, all the signs in a specific area or corridor get slated for replacement at the same time, when the effective service life is reached. On a strategic basis, all the signs of a specific type get slated for replacement at the same time. Depending on the size of the jurisdiction, it may be possible to plan sign replacements that consider both geographic and strategic criteria. The blanket replacement is being used by various agencies around the country such as the City of Glendale, AZ.

This method is probably the simplest of the management methods in that tracking the age of individual signs, either by physical labeling or in a database, is not necessary. It is only necessary to maintain a record of when the blanket actions were undertaken and when they need to be repeated. Usually this method is repeated after a set number of years, depending on the expected life of the signs.

Procedures

At set time periods, a sign maintenance crew will go to a specific area or corridor and replace all the designated traffic signs under its jurisdiction. This might be done such that regulatory signs are replaced in one cycle, warning signs in another cycle, and guide signs in a third cycle. The time interval between replacements is usually based on the expected sign life as discussed in the previous section. Under this method, all signs are replaced regardless of the amount of time they have been in the field or the condition at the time of replacement. Blanket replacements can be scheduled to coincide with major roadwork or repaving, resulting in the least impact on traffic. This is especially beneficial on routes with high traffic volumes.

Current Practice

This maintenance method is popular with State DOTs. Of the agencies that were contacted and that use a blanket replacement method, most replace their Type I signs every 7 to 10 years; Type III signs every 10 to 15 years; and Types VI, VIII, and IX signs every 15 years. The vast majority of the agencies contacted use Type III sheeting for the majority of their traffic signs, although some agencies are testing the use of Types VII, VIII, and IX for use on a wider scale. Some agencies currently use the microprismatic materials for signs that they believe require higher visibility.

Replacement time depends on the region of the country as well as on past experience of how long signs last in the field. For example, Maine, Kansas, and New Hampshire replace signs with Type III sheeting every 10 years, while Minnesota replaces Type III signs every 12 years and Michigan every 15 years. Minnesota also uses Type IX sheeting on guide signs and replaces these signs every 15 years.

Concerns

One of the issues with this method is that the replacement times can vary depending on the region of the country in which the agency is located, or even across a jurisdiction for large agencies. The replacement time also depends on the types of sheeting that are used to make the agency’s traffic signs. Therefore, an agency needs to have relevant data on the in-service life of all the sheeting materials it has in the field. Another concern is that this method potentially wastes resources by removing signs before their useful life has been reached. This is particularly true where signs have been added or replaced in an area after the last replacement cycle. When the replacement cycle comes around, these signs will be replaced regardless of their age. They can be reused if handled properly, but that would require that each sign that is replaced be inspected to determine the amount of useful sign life remaining.

Linking Blanket Replacement to Minimum Retroreflectivity Levels

The minimum retroreflectivity levels provide the initial basis for the expected life criteria, but an understanding of the actual degradation rates of in-service signs is required to set appropriate triggers as retroreflectivity levels approach the minimum requirements. Under this method, retroreflectivity levels of signs are not measured, and opportunities are limited for capturing data that may be useful in adjusting service lives, trigger points, or sign maintenance strategies.

Advantages & Disadvantages

The major benefit of using this method is that all signs are replaced; there is a low likelihood of a given sign being skipped over or not being replaced. This ensures that all replaced signs are visible and meet minimum retroreflectivity levels.

The major drawback to this method is the potential amount of waste than can be generated if signs that are relatively new are removed during a normal replacement cycle. This can be particularly expensive when a blanket replacement method is first implemented. Follow-up replacement cycles can also be wasteful if signs are replaced between the expected service life periods because of knockdowns, graffiti, etc.

CONTROL SIGNS

The control sign method is based on measurements made of a subset of signs that represent an agency’s inventory. The subset of signs represents a population of signs made with the same material for which the retroreflectivity performance over time is monitored by actual measurements. As the retroreflectivity levels of the control signs approach the minimum levels, it triggers action to begin replacement of the entire associated population. The control signs can be located at one or more of the agency’s maintenance yards or can be traffic signs that are deployed at various locations in the jurisdiction. The control signs are measured periodically to monitor actual degradation of retroreflectivity. This method requires only the management of the control sign information and the retroreflectivity measurements of those signs over time.

Procedures

The use of this method requires the installation of signs in a maintenance yard or the definition of specific control signs from the population of deployed signs. Periodic measurements of control signs are made following ASTM E1709 or other accepted procedures. Measurements or other observations are tracked over time to monitor changes in retroreflectivity and nighttime visibility. Once these signs, as a whole, start to approach the minimum retroreflectivity levels, all the traffic signs in the field that these control signs represent are replaced.

Current Practice

None of the agencies contacted reported using this method to maintain their traffic signs. However, some agencies do take retroreflectivity readings on a sample set of signs to estimate how the overall sign population is performing. This is used primarily as a verification method for agency sign management polices and practices.

Concerns

The effectiveness of this method is dependent upon the size of the control sign sample. The larger the sample, the better the estimation of the retroreflectivity levels of the sign populations it represents. There is no specific guidance on the number or percentage of the population the sample represents. However, a minimum of three signs per type of sheeting and color should be monitored.

Another question relates to how often a set of control signs is needed. Each new sign material or deployment of a major product order would warrant a set of control signs, as there are likely to be differences in retroreflectivity performance. It may be appropriate to install controls when new sign fabrication processes are implemented or other major changes in the sign management process occur. It may also be appropriate for a large agency that deploys signs continually to set up control signs as materials age on the shelf and personnel change. Too short a time period between adding control signs may cause the agency to have a large number of control signs to monitor, which negates the simplicity of this method. Too much time between control signs could result in errors estimating the service life of signs installed in the time interval between the control signs.

Another consideration is how often the control signs should be checked for their retroreflectivity levels and appearance. If the time interval between measurements is too short, then this may needlessly waste time and personnel resources. On the other hand, if the time interval is too long, signs may be left in the field that are not adequate for continued use and may pose a possible safety risk. An annual inspection of the signs, including retroreflectivity measurements, may be appropriate.

Linking Control Signs to Minimum Retroreflectivity Levels

The control signs must be measured at given intervals with a retroreflectometer to determine how they are performing. These values are then compared to the minimum retroreflectivity levels in order to trigger sign replacement actions. The precise retroreflectivity levels of the majority of deployed signs are not known using this method.

Advantages and Disadvantages

The main benefit of this method is that it is not nearly as labor intensive as taking retroreflectivity readings on every sign in an agency’s jurisdiction. Because a sample set of signs is used to monitor the retroreflectivity levels, it is easier and less labor intensive to get an estimate on how the traffic signs, represented by the control signs, are performing in the field. Another benefit of using this method is that signs that do meet the required minimum retroreflectivity levels are not removed prematurely, allowing for an efficient use of the signs and their material. This may be particularly advantageous when the life of a new sign material exceeds the warranties provided by the manufacturer.

This method requires agencies to have the capability to measure the retroreflectivity of the control signs. Without an appropriate sampling process, the control signs may not be representative of the larger sign population they are intended to represent. This could lead to replacing signs that do not need replacement or not replacing signs that do need replacement. Therefore, agencies must evaluate the number of signs of each type within their jurisdiction and establish guidelines on the number of control signs that are needed to appropriately represent signs in the field.

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Page last modified on October 17, 2012.
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