Analysis of Three Sign Management Program Case Studies

Abstract

Traffic signs play a major role on the national highway system because they provide users with important information such as warnings, regulations, and directions. To ensure sign visibility at night, the Manual on Uniform Traffic Control Devices (MUTCD) requires transportation agencies to meet minimum sign retroreflectivity levels through a sign maintenance program. To better understand current trends, the researchers conducted an extensive literature search that showed that expected sign life and nighttime inspection are the most used methods, followed by blanket replacement. The literature does not typically discuss how these methods are implemented. Thus, the research team also contacted three of the four largest state-maintained highway systems in the United States (North Carolina, Virginia, and South Carolina) to discuss sign maintenance programs, implementation issues, and best practices. The authors describe in this article the findings and conclusions drawn from these case studies that may help other agencies improve their sign maintenance practices.

Keywords

sign maintenance sign service life blanket replacement sign management practices transportation asset management

Introduction

Traffic signs play an important role in transportation systems because they provide drivers with valuable roadway safety information (Wisconsin Transportation Information Center [WTIC], 2013) and are classified by the Manual on Uniform Traffic Control Devices (MUTCD; Federal Highway Administration [FHWA], 2009) as regulatory (e.g., speed limit and stop), warning (e.g., right curve and road closed), and guide signs (e.g., distance guide sign and Interstate route). In a study conducted in 2009, Rasdorf, Hummer, Harris, and Sitzabee (2009) pointed out the importance of traffic signs as being a critical part of the transportation system and having a major impact on road safety.

Given their importance, it is extremely relevant that transportation agencies develop sign management programs to ensure that signs are visible and legible to drivers. The Federal Highway Administration (FHWA; 1999) describes a generic asset management program as having the following components: goals and policies, budget allocations, asset inventory, condition assessment and performance modeling, evaluation of alternatives, short- and long-term plans, program implementation, and performance monitoring. Thus, an efficient sign management program is expected to contain most of these components.

While, during the day, the sunlight makes signs visible (even on a cloudy day), transportation agencies have to find alternative ways to ensure that signs are visible at night. Schertz (2005) reported that although only one fourth of all travel occurs at night, almost 50% of all traffic fatalities happen in these same hours. To address the nighttime visibility issue, the MUTCD (FHWA, 2009) requires transportation agencies to adopt one of two options: artificial illumination of signs or signs manufactured with retroreflective sheeting. In the United States, most agencies opted for using retroreflective sheeting instead of artificial illumination of signs (Carlson, Park, & Andersen, 2009).

In 2009, the FHWA published a revised edition of the MUTCD that describes five sign maintenance methods, from which agencies can choose one or more methods, to ensure proper sign legibility, minimum retroreflectivity levels, and performance. Those methods are classified as assessment (nighttime visual inspection and measured retroreflectivity) or management (blanket replacement, expected sign life, and control signs). State Departments of Transportation (DOT) have autonomy to choose sign retroreflectivity maintenance methods that best fit their resources, needs, and overall sign management program.

By improving the nighttime visibility through retroreflectivity compliance, the FHWA expects that drivers will “better navigate the roads at night and thus promote safety and mobility” (FHWA, 2007). In addition, maintaining signs at or above minimum retroreflectivity levels is also part of “FHWA’s efforts to be responsive to the needs of older drivers whose visual capabilities are declining” (FHWA, 2007). Another reason to meet the minimum retroreflectivity requirements of the MUTCD is to reduce liability risk (McCarthy, Liang, Park, McFadden, & Trieu, 2013).

While sign visibility at night is a critical sign attribute, it is also critical that signs be comprehensively managed to meet all performance measures. Thus, since 2012, many DOTs have improved their sign maintenance programs, and many seem to be transitioning from assessment methods to management methods. To better understand the current trends, the research team contacted three of the four largest state-maintained highway systems in the United States (behind only Texas): North Carolina, Virginia, and South Carolina (FHWA, 2017). The research team met traffic and sign engineers from these DOTs to observe, document, and assess which sign maintenance methods they have in place, as well as their practices, benefits, and challenges. This article describes the findings and discussions drawn from these case studies enabling other DOTs and transportation managers to gain insights into problems and solutions that may help them improve their sign maintenance programs. In addition, this article refers to the different types of sign sheeting (e.g., Type I, Type III, etc.) based on the American Society for Testing Materials (ASTM) D4956-17 standards (ASTM, 2017).

Literature Review

Carlson et al. (2009) stated that nighttime crash rates are much higher (up to three times) than daytime crash rates. This statement makes clear why sign retroreflectivity is so important for all those who use the transportation network. According to Markow (2007), one of the major considerations on a sign management program is the selection of the sign sheeting type, “which plays a strong role in initial appearance and long-term performance of the sign, and affects its life-cycle costs” (Markow, 2007).

A portion of the literature focused on the development of retroreflectivity deterioration models. In addition, researchers also focused on retroreflectivity compliance, sign service life, and methods to maintain signs above the minimum retroreflectivity levels required by the MUTCD (FHWA 2009). The following subsections summarize these studies.

Sign Retroreflectivity Measurement and Deterioration

Retroreflective sign sheeting contains either prismatic reflectors or glass beads that reflect a portion of the light incident on it back to the source with a minimum of scattering (Delaware T² Center [DT2C], n.d.). It is the retroreflective sheeting that enables a driver to see signs at night (Carlson & Picha, 2009; WTIC, 2013).

The level of retroreflectivity of a sign sheeting is known as coefficient of retroreflectivity (RA). The RA is calculated as the ratio of light that strikes the sign and the portion of this light that is reflected back to the source. The unit of measurement is candela per lux per meter square (cd/lx/m²; Re, Miles, & Carlson, 2011), which is defined by Immaneni, Hummer, Rasdorf, Harris, and Yeom (2009) as “the ratio of light a sign reflects to a driver (candela (cd)) to the light that illuminates the sign (lux (lx)), per unit area (square meter (m²)).” The RA is often measured by a handheld retroreflectometer even though it is time consuming and exposes inspectors to road hazards. An alternative method is to measure retroreflectivity at night using vehicles equipped with retroreflectometers. However, this method is expensive because of the nighttime labor cost (Balali, Sadgehi, & Golparvar-Fard, 2015). Trying to mitigate the disadvantages of this measurement method, Balali, Sadgehi et al. (2015) proposed the use of a car equipped with a camera and a flash device that is capable of measuring sign retroreflectivity during the day. In doing so, the authors concluded that the proposed technique was efficient in reducing costs and improving safety when compared with the former methods.

Many studies were conducted to assess how retroreflectivity deteriorates over the years (Clevenger, Colello, & Quirus, 2012; Evans, Heaslip, Boggs, Hurwitz, & Gardiner, 2012; Immaneni et al., 2009; Kipp & Fitch, 2009; Preston, Atkins, Lebens, & Jensen, 2014; Re et al., 2011, etc.) and, although many of them suggested that sign retroreflectivity deteriorates as signs age, only a few obtained successes in proving the extent of this relationship. An understanding of how sign retroreflectivity deteriorates and which factors are involved in that process is necessary to develop deterioration models, and based on them, to estimate sign service life. Figure 1 illustrates how sign retroreflectivity deterioration affects nighttime visibility through the years.

Figure 1.

How retroreflectivity deterioration affects sign visibility at night.

Most past studies conducted field surveys to collect sign data, in which the number of signs surveyed varied from 137 to 5,722 (Clevenger et al., 2012; Evans et al., 2012; Kirk, Hunt, & Brooks, 2001; and others). All these studies used retroreflectometers to measure sign retroreflectivity. In addition, they collected other data such as sign age, and, in some cases, sign color, sheeting type, location, orientation, and visual assessment (overall sign condition; e.g., poor, adequate, and good).

Most studies suggested a strong correlation between sign age and retroreflectivity deterioration (Clevenger et al., 2012; Kipp & Fitch, 2009; Kirk et al., 2001; Pike & Carlson, 2014; Preston et al., 2014; Wolshon, Degeyter, & Swargam, 2002; and others). However, the mathematical models developed based on the field data had low R² values, which indicated that the real data were not close to the curve obtained from the mathematical model. Only a few studies were successful in showing a close relationship between sign age and retroreflectivity (Boggs, Heaslip, & Louisell, 2013; Jiang & Zhou, 2012; Pulver, Huynh, & Mullen, 2018; Re et al., 2011; Wolshon et al., 2002).

Sign Retroreflectivity Compliance

In relation to sign retroreflectivity compliance with the minimum levels required by the MUTCD (FHWA, 2009), most studies found compliance rates above 90% (Boggs et al., 2013; Clevenger et al., 2012; Dumont et al., 2013; Evans et al., 2012; Hawkins & Carlson, 2001; Kipp & Fitch, 2009; Kirk et al., 2001; Pike & Carlson, 2014; Pulver et al., 2018). One of the few studies that found noncompliance rates greater than 10% was Immaneni, Rasdorf, Hummer, and Yeom (2007) but the authors noted that most of the noncompliant signs were ASTM Type I sheeting (also known as Engineering Grade). Wolshon et al. (2002) found that 57% of the signs that were over the warranty period were noncompliant. However, a portion of those signs were also Type I sheeting. Thus, the literature indicates that compliance is generally good.

Sign Service Life

Sign service life (also known as life expectance) is the time between the installation (or manufacturing) of an asset and its replacement (or removal). In the case of signs, their service life can be determined by age rather than by routine inspections with the objective of tracking retroreflectivity and damage (Thompson et al., 2012). Based on a survey of 39 transportation agencies, Markow (2007) reported a sign service life ranging from 10 to 30 years depending on the sign sheeting type and color.

Many retroreflectivity studies concluded that the use of a sheeting manufacturer’s warranty period to define sign service life is very conservative, and it is not considered to be good practice. Most studies found the signs out of warranty performed well above the minimum retroreflectivity levels required by the MUTCD (FHWA 2009). Preston et al. (2014) cited that one of the explanations for older signs performing well is the fact that sheeting quality has increased. Thus, signs seem to satisfactorily outlive their warranty.

Researchers found that, although the practice of using the warranty period to define sign service life may assure compliance with MUTCD, it often results in replacing signs well before retroreflectivity deteriorates below the minimum. This increases the cost to maintain signs (Pike & Carlson, 2014; Preston et al., 2014; Re & Carlson, 2012; Re et al., 2011). Thompson et al. (2012), based on a literature review and Markow’s (2007) work, found a typical sign service life of 5 to 7 years for ASTM Type I, 10 to 15 years for ASTM Types III and IV, and 15 to 20 years for ASTM Types V and X.

Most studies that developed deterioration models recommended sign service life to be 15 years and above for ASTM Type III sheeting (Clevenger et al., 2012; Dumont et al., 2013; Immaneni et al., 2009; Kipp & Fitch, 2009; Pike & Carlson, 2014). Pulver et al. (2018) was the only study that recommended the adoption (South Carolina Department of Transportation [SCDOT]) of a sign service life (Type III sheeting) that is the same as the 10-year sheeting warranty.

Sign Maintenance Methods

There were also studies that focused on the analysis or implementation of different sign maintenance methods recommended by the MUTCD (FHWA, 2009). A description of each method is provided before describing the literature reviewed.

Visual nighttime inspection consists of trained sign inspectors riding along the roads at night and visually inspecting all signs to identify those that are below minimum retroreflectivity levels. The deficient signs identified during the nighttime inspections are then replaced. This method is widely used because it is inexpensive when compared with others, but is subjective and exposes inspectors to unsafe conditions. The measured retroreflectivity method consists of measuring the retroreflectivity of all signs using a handheld or mobile retroreflectometer during daytime inspections, which is extremely labor intensive. This measurement procedure follows the “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” (FHWA, 2007). This is the most objective of the methods, but it is also the most labor intensive.

The expected sign life method replaces only the signs that have achieved the end of their service life. This method requires an updated sign inventory database to keep track of sign age and location. However, a sign inventory database may not be feasible for all transportation agencies. Rasdorf et al. (2009) studied the challenges involved in the development and maintenance of a high-volume and low-cost asset (e.g., signs) database inventory. Balali, Rad, and Golparvar-Fard (2015) stated that most agencies do not have a management system for signs because of the high cost associated with the traditional data collection methods. As an alternative to the traditional methods, Balali, Rad et al. (2015) studied the effectiveness of creating (or updating) a sign inventory database by using Google Street View images. The authors proposed a system that analyzed 6.2 miles of a highway and found an accuracy of almost 95% of sign classification, which would be useful in maintaining an up-to-date sign database inventory. He, Sonf, and Liu (2017) conducted a study to assess the feasibility of building or updating highway asset inventory using airborne Light Detection and Ranging (LiDAR). However, the proposed method was not able to collect data on ground mounted signs.

The control signs method consists of measuring the retroreflectivity of control signs, which are considered to be representative of all other signs of a given type installed on the same date. Although this method is not as time consuming as the measured retroreflectivity method, it still consumes a significant number of labor hours depending on the sample size. One of the major issues with this method is the determination of an adequate sample size, which has not been well defined in the literature so far.

The blanket replacement method is similar to the expected sign life method in that signs are replaced based on their service life. The difference is that signs are replaced by group (e.g., red signs) or by geographical area (sections or corridor). As a result, there is no need to keep track of the age of individual signs. It is noteworthy that when adopting the blanket replacement method, agencies can either do the field sign replacement themselves or through contractors. An agency may opt to have a warranty contract with a private company, in which contractors warrant the performance and quality of their products for a specified period (Hancher, 1994). One of the major advantages of warranty contracts is to share with or shift to the contractor the burden of quality control of the asset during the validity of the contract (Singh, Oh, Labi, & Sinha, 2007). In addition, warranty contracts might have a higher initial investment than traditional contracts; however, from a long-term perspective, they lead to a lower life cycle cost than do similar traditional contracts (Singh et al., 2007).

An overview of these five methods is provided in Table 1 (FHWA, 2009, 2013; WTIC, 2013). In addition, advantages and disadvantages for each method are listed (Clevenger et al., 2012; Dumont et al., 2013; FHWA, 2007; Re & Carlson, 2012). Most of the researchers opted for studying the five sign maintenance methods recommended by MUTCD. Previous studies showed that all methods have advantages and disadvantages (Clevenger et al., 2012; Dumont et al., 2013; Re & Carlson, 2012). In some cases, researchers concluded that the combination of two or more methods was advantageous because then it is possible to reduce weakness of individual methods (Dumont et al., 2013; Re & Carlson, 2012).

Table 1.

Sign Retroreflectivity Maintenance Methods: Description, Advantages, and Disadvantages.

Method	Description	Advantages	Disadvantages
Visual nighttime inspection	Trained sign inspectors ride along the roads at night and visually inspect all signs to identify those that are below minimum retroreflectivity levels. These are then replaced.	Does not require retroreflectometers; other aspects of signs are assessed (e.g., damage and knockdown); inspectors can evaluate more than signs (e.g., pavement markings and shoulders); development of a sign inventory while driving roads.	Need for trained inspectors; highly subjective; overtime labor cost; dependent on weather.
Measured retroreflectivity	Sign inspectors measure retroreflectivity levels of all signs using a retroreflectometer and replace those below the minimum.	Objective evaluation; data collection can be used to generate deterioration models; sign retroreflectivity can be measured during the day.	High retroreflectometer cost; inspectors exposed to roadway hazards; some signs are located in areas of difficult access; highly labor intensive.
Expected sign life	Sign crews replace all signs that exceed their expected life. DOTs often estimate expected sign life based on field experience, warranty, or retroreflectivity deterioration rates. To track expected sign life, agencies stamp the installation date on the back of the sign. Doing so allows sign crews to identify and remove signs that are beyond their expected life.	Reduced material waste; accurate record; possible extension of sign service life; provide data for planning, scheduling, and budgeting.	Signs may fade before the end-of-service life; sign service life may be overestimated or underestimated; high administrative and management cost; requires a detailed inventory database.
Blanket replacement	Sign crews replace all signs in a corridor or area (section). Those signs are replaced at regular long-term intervals that are based on the expected sign life.	Simple and straightforward; regular replacement cycles; sign inventory may not be required if agency keeps track of when the signs in an area/corridor are replaced.	High chances of replacing signs before the end of their service life; daytime inspections still needed to detect damaged signs; determination of the replacement cycles is required.
Control sign	Instead of checking the retroreflectivity level of all field signs, inspectors monitor the retroreflectivity of control signs, which are representative of all other signs of a given type installed on the same date.	Data collection throughout the year; data collection can be used to generate deterioration models; centrally located; less costly.	Requires a retroreflectometer; there is no guidance on what is considered an adequate sample size; high installation and maintenance cost of a control sign facility.

Note. DOT = Department of Transportation.

Kipp and Fitch (2009) conducted a study for the Vermont Transportation Agency and analyzed three sign maintenance methods: measured retroreflectivity, blanket replacement, and control signs. At the end of the study, the research team recommended that the transportation agency adopt blanket replacement because it does not require retroreflectivity measurements of individual signs nor a sophisticated inventory database, which can be costly to an agency. For instance, North Carolina Department of Transportation (NCDOT) would have to invest between $1.6 million and $4.1 million to create a sign inventory, which cost does not consider inventory maintenance costs through the years (Vereen, Hummer, & Rasdorf, 2002). In addition, even if an agency decides to create a sign control facility to collect data instead of a statewide inventory, it still could be costly and time consuming. Harris, Rasdorf, and Hummer (2009) estimated that it would require an initial investment of $104,000 (building, signs, fences, equipment, and software) and an annual maintenance cost of $25,000. In addition, it would be necessary to hire personnel with some level of expertise to analyze the data collected in the facility.

Re and Carlson (2012) also assessed previous studies and surveyed transportation agencies across the United States to determine which sign maintenance methods they were adopting. They found that expected sign life was the most used method, followed by visual nighttime inspection and blanket replacement. Similarly, Clevenger et al. (2012) stated that 13 out of 27 interviewed states were planning to adopt the expected sign life method. In addition, five of the 12 states that were already using expected sign life indicated that they combined it with another method, often with blanket replacement (Indiana, Mississippi, New York, Ohio, and Wisconsin). In addition, Rasdorf and Machado (2017, 2018a, 2018b) held meetings with NCDOT, SCDOT, and Virginia Department of Transportation (VDOT) and found that all three DOTs are currently adopting management methods (expected sign life and/or blanket replacement).

Table 2 lists a summary of sign maintenance methods adopted by 40 of the 50 states. The information was obtained from various sources in the literature (second column). The literature clearly shows that nighttime visual inspection and expected sign life are the most commonly used methods, followed by blanket replacement. The literature does not typically discuss how these methods are implemented, which becomes crucial to their success.

Table 2.

Sign Maintenance Method Adopted by State.

State DOT	Authors	Nighttime inspection	Measured retroreflectivity	Expected sign life	Blanket replacement	Control signs
Alabama	Re and Carlson (2012)	x
Alaska	Clevenger et al. (2012)	x	x
Arizona	Clevenger et al. (2012)	x		x
Arkansas	Clevenger et al. (2012)				x
California	Clevenger et al. (2012)	x
Colorado	Re and Carlson (2012)				x
Delaware	Clevenger et al. (2012)	x		x
Florida	Re and Carlson (2012)	x
Idaho	Re and Carlson (2012)	x
Illinois	Re and Carlson (2012)				x
Indiana	Clevenger et al. (2012)			x	x
Iowa	Clevenger et al. (2012)	x
Kansas	Re and Carlson (2012)				x
Kentucky	Clevenger et al. (2012)	x		x
Louisiana	Clevenger et al. (2012)			x
Maine	Clevenger et al. (2012)			x
Massachusetts	Clevenger et al. (2012)	x			x
Michigan	Clevenger et al. (2012)			x	x
Minnesota	Huynh et al. (2018)				x
Mississippi	Clevenger et al. (2012)			x	x
Missouri	Re and Carlson (2012)	x
Nebraska	Kipp and Fitch (2009)	x
New Hampshire	Re and Carlson (2012)	x
New Jersey	Re and Carlson (2012)			x
New York	Clevenger et al. (2012)			x	x
North Carolina	Rasdorf and Machado (2018a) Rasdorf and Machado (2018b)	x			x
North Dakota	Clevenger et al. (2012)	x
Ohio	Clevenger et al. (2012)			x	x
Oklahoma	Clevenger et al. (2012)
Oregon	Kipp and Fitch (2009)	x
Pennsylvania	Re and Carlson (2012)			x
South Carolina	Rasdorf and Machado (2017)	x		x
South Dakota	Clevenger et al. (2012)			x
Texas	Re and Carlson (2012)	x
Utah	Huynh et al. (2018)				x
Vermont	Clevenger et al. (2012)			x		x
Virginia	Rasdorf and Machado (2017)	x		x	x
West Virginia	Kipp and Fitch (2009)			x
Wisconsin	Clevenger et al. (2012)			x	x
Wyoming	Clevenger et al. (2012)	x
TOTAL	40	20	1	18	15	1

Note. DOT = Department of Transportation.

Method

The first step was to conduct the literature review and then to conduct three case studies. To do so, the research team met with traffic engineers from transportation agencies located in North Carolina, South Carolina, and Virginia on 11 occasions between October 19, 2017 and May 29, 2018.

The meetings were held in each DOT facility and had an average duration of 2 hr. Once the research team arrived at the meeting location, a brief introduction about the research and the main literature review findings were presented to the engineers and personnel who were attending the meetings. Then, the research team asked questions about their sign management program (e.g., which sign maintenance method they used; what were the challenges, what were the benefits, etc.). In addition to the meetings, two facility tours were also taken at Bunn Sign Shop (Bunn, North Carolina) and Central Virginia Sign Shop (South Chesterfield, Virginia).

Practices and Procedures

The following subsections describe the various state DOTs practices and procedures. Table 3 shows a summary of the sign maintenance methods adopted by the three case study DOTs.

Table 3.

Sign Maintenance Method Summary.

DOTs	State-owned mileage	Maintenance method	Sign sheeting	Sign service life	Sign inventory
NCDOT	79,637	Blanket replacement (10-year cycle) combined with expected sign life and nighttime and daytime visual inspections	Minimum prismatic Type III	10 years(Warranty: 12 years)	No
SCDOT	41,340	Expected sign life (10 years) combined with nighttime visual inspection	Minimum prismatic Type III	10 years(Warranty: 10 years)	Yes (statewide)
VDOT	58,821	Blanket replacement (10-15 year cycle) combined with expected sign life and nighttime and daytime visual inspections	Prismatic Type IX	15 to 30 years(Warranty: 10 years)	In progress

Note. DOT = Department of Transportation; NCDOT = North Carolina Department of Transportation; SCDOT = South Carolina Department of Transportation; VDOT = Virginia Department of Transportation.

North Carolina Case Study

NCDOT has 14 divisions and is the second largest state-maintained highway system in the United States, with a total roadway network of almost 80,000 miles, which includes Interstates (2%), primary roads (17%), and secondary roads (81%; North Carolina Department of Transportation [NCDOT], 2018).

Sign service life

Since 2006, NCDOT has been using signs manufactured with Type III and above prismatic sheeting, which has a warranty period of 12 years starting from the date the sign was manufactured. The NCDOT Routine Maintenance Improvement Plan (RMIP; NCDOT, 2016) specifies the sign service life in North Carolina as 10 years (less than the sign warranty).

Sign maintenance method

Up to 2017, NCDOT used the nighttime visual inspection method to ensure compliance with minimum retroreflectivity levels required by the MUTCD (FHWA 2009). Sign crews conducted nighttime inspections every other year on primary roads and every 3 years on secondary roads. In addition, daytime inspections were also conducted to identify damaged and missing signs.

NCDOT started implementing the RMIP in July 2017. RMIP is a long-term sign management program that covers the following roadway assets: ditches, shoulders, pipes, pavement markings, and signs. In relation to signs, the RMIP adopts the blanket replacement method (mostly by area) based on an expected sign service life of 10 years (NCDOT, 2016).

According to the “2016 Maintenance Operations and Performance Analysis Report (MOPAR)” (NCDOT, 2016), the objective of the RMIP is to encourage NCDOT divisions to adopt planned maintenance practices and also to “hold divisions accountable for production levels.” NCDOT divisions are now required to forecast their future budgets based on the plan that they submitted in July 2017. In addition, at the end of the year, they are required to report their progress in meeting their planned work goals. Doing so allows the NCDOT Maintenance Office to identify areas that demand more investment, better forecast future budgets, and more efficiently distribute its resources throughout the state. Overall, NCDOT divisions (visited by the research team) believe that blanket replacement is a preferred sign maintenance method.

Although NCDOT does not have a sign inventory, the department maintains a Maintenance Condition Assessment Program (MCAP) that is used by NCDOT to monitor and evaluate assets’ conditions within North Carolina. MCAP data include the number of signs inspected and the number of signs to be replaced for any reason. Such data are available in MCAP by county and by type of road (Interstate, primary, and secondary). In addition to the numbers of signs, MCAP also stores cost data by work function.

From the meetings with the NCDOT divisions, one sign maintenance practice stood out as follows:

Area-based approach for blanket replacement implementation

Area-based replacements (often referred to as sections by some agencies) create a routine that helps laborers better understand the processes involved in sign maintenance and replacement. It also gives personnel a sense of “ownership,” which results in a set of benefits such as the following:

Increased depth of knowledge about a specific area.

Reduction of idle time.

Increase in labor productivity.

Reduction in distance traveled to accomplish work.

Decrease of sign unit cost per square foot (this unit cost includes labor, material, and equipment).

Improved employee morale.

A clear definition of areas promotes efficiency (reduces idle time and increases productivity) because it prevents sign crews from randomly driving division roads without having a set of established goals. In addition, upper management can make it clear to personnel what the expected productivity level for each crew is. Then, crew member’s evaluations can more fairly be based on their productivity. Thus, an area-based approach to sign work is highly beneficial and is currently a key component of RMIP success.

Sign recycling

One approach to sign management is a practice of recycling signs by reusing them as spot replacement signs when they are younger than 5 years (relatively new) and are in a good condition. That is, during a blanket replacement in a specific area, if a relatively new sign is replaced, it is saved and used in some other area to replace a bad or damaged sign there. Thus, when blanket replacement occurs, no signs are replaced that are less than 5 years old.

To illustrate this better, consider that a county may be divided into 10 areas (Area 1, Area 2, Area 3, etc.) and that the blanket replacement is per area. Consider also that sign crews are conducting blanket replacement in Area 1 during the first year of the cycle. When doing so, sign crews might identify signs in good condition and younger than 5 years. Thus, instead of discarding them, the sign crews will stock and use these signs for spot replacements in other areas that are scheduled to be replaced in the following years.

Sign management program

The NCDOT sign management program is in a process of improvement with the implementation of the RMIP. The department has defined a set of goals and policies that considers both short- and long-term plans. With respect to condition assessment, both nighttime and daytime inspections are still conducted to assess sign condition. To define a more cost-efficient budget allocation, the NCDOT recently initiated a sign replacement research study conducted by the North Carolina State University (NCSU) to investigate the trade-offs between different sign replacement strategies. The next step is to monitor the performance of the RMIP through the years. However, an aspect that the FHWA (1999) considers important for an asset management program is an asset inventory, which the NCDOT does not maintain for signs. Instead, the RMIP describes the development of a sign inventory; however, this task will take some years to initiate as priority is being given to the inventory of other assets first.

South Carolina Case Study

SCDOT has seven districts and is the fourth largest state-maintained highway system in the United States, with a total roadway network of 41,340 miles, including Interstates (2%), primary roads (17%), and secondary roads (81%; South Carolina’s Information Highway, 2014).

Sign service life

Around 2005, SCDOT initiated a program to replace Type I sheeting with Type III or above on primary and secondary roads. In 2015, SCDOT adopted an Engineering Directive Document “ED-4” that requires districts and counties to use a minimum of Type III (high-intensity grade or prismatic high-intensity) sheeting. According to the “ED-57” document, the sign service life is 10 years, which is based on the sheeting manufacturer warranty of 10 years.

Sign maintenance method

SCDOT has a statewide sign management program with a standardized expected sign life method, daytime and nighttime inspections, and a sign inventory database. To maintain control of the sign maintenance process, SCDOT uses a Highway Maintenance Management System (HMMS). One of the modules of the management system is sign inventory and maintenance. This module contains all relevant sign information, including location, type of sign, manufacture date, sheeting type, installation date, and a record of inspections.

To ensure sign retroreflectivity compliance with MUTCD (FHWA, 2009), SCDOT adopted ED-57 in 2012, which specifies the expected sign life method to maintain its signs. In addition to the expected sign life, nighttime inspections are still conducted as a control method to verify whether signs meet the retroreflectivity requirements described in the MUTCD (FHWA, 2009).

All this is possible because each sign has a unique identification number. This identification number (barcode) is placed on the back of each sign when it is manufactured. Thereafter, any action or data related to the sign (e.g., replacement and maintenance) uses the identification number to enter it into the HMMS, enabling SCDOT districts to identify signs that are older than the expected life. In addition, the system has the signs’ exact global positioning system (GPS) location, allowing sign crews to go directly to the locations where signs need to be replaced.

SCDOT also uses its HMMS in the extreme case of a natural disaster (hurricanes, earthquake, flooding) when many signs can be totally lost or severely damaged. The HMMS, containing a complete statewide sign inventory, helps to identify all signs that were lost, the type of signs, their specification, and their exact location, enabling districts to plan the work necessary to replace them.

Sign management program

The SCDOT contains a mature sign management program when considering the components described by the FHWA (1999). The agency has a well-defined set of goals and policies that also considers short- and long-term plans. To evaluate sign condition assessment, the agency conducts both daytime and (not so often) nighttime inspections. In addition, the SCDOT contains a robust and statewide HMMS that includes sign inventory. The agency also monitors the performance of its program by evaluating data contained in the HMMS. Looking for improvement areas in their program, the SCDOT recently sought to determine the sign service life in South Carolina with the objective of evaluating their practices.

Virginia Case Study

VDOT, with nine districts, is the third largest, state-maintained highway system in the United States, with a total roadway network of 58,821 miles. These include Interstates (2%), primary roads (14%), and secondary roads (84%).

Sign service life

Since 2010, VDOT has adopted prismatic sheeting (ASTM Type IX) with a warranty period of 10 years starting from the date the sign was manufactured. However, according to VDOT’s Sign Maintenance and Retroreflectivity Compliance Plan (SMRC Plan), the service life of the Type IX sheeting actually ranges from 15 (minimum) to 30 (maximum) years. Thus, VDOT believes that with this sheeting, they will significantly reduce their sign replacement frequency.

Sign maintenance method

In 2017, the VDOT Traffic Engineering Division developed an SMRC Plan to be used as a guideline for maintaining minimum retroreflectivity levels. An important observation is that “the Plan does not present a centralized, or standardized statewide approach to sign retroreflectivity; rather, it allows each unit (district) within VDOT to allocate resources in a way that best serves the area’s needs.”

According to VDOT’s SMRC Plan, the sign replacement rate in the state can be estimated based on the number of signs annually manufactured by the Central Virginia Sign Shop (CVSS), which is approximately 90,000 signs per year. This number of signs is equivalent to 10% of all VDOT signing inventory. The annual cost to manufacture these signs is about $2.9 million or $32 per sign, not including installation.

VDOT uses a combination of blanket replacement based on sign service life and daytime and nighttime visual inspections. The blanket replacement is conducted by corridor in cycles of 10 to 15 years, which ensures that signs will be below the minimum sign service life of 15 years found in previous studies. Daytime inspections identify damaged and missing signs and are conducted during maintenance activities and routine inspections. Nighttime inspections focus on retroreflectivity levels and occur at a lower frequency than daytime inspections. Signs on primary roads follow the pavement maintenance cycle and are inspected every 8 or 10 years. On other roads, nighttime inspections occur near the end of the replacement cycle (which is between 10 and 15 years) to ensure that signs in good condition will not be replaced, thus, increasing the sign service life observed in the field.

VDOT planned to use a new HMMS system and start loading data into it in 2018 so that districts can rely on the HMMS to accurately determine when to perform blanket replacements based on expected sign life simply by knowing the location and age of the signs. VDOT believes that once signs are blanket replaced, the districts can drastically reduce the number of inspections during the sign warranty period because all replaced signs are expected to be in new condition.

Sign management program

The VDOT sign management program was improved with the implementation of the SMRC Plan. Similar to the NCDOT and SCDOT, the VDOT has a set of goals and policies that considers short- and long-term plans. To evaluate sign condition assessment, the agency conducts both daytime and nighttime inspections. In addition, VDOT is in the process of creating a sign database inventory, which will be part of its HMMS. The agency also evaluates alternatives to improve its program and, after selecting one alternative, monitors its performance. An example was the selection of Type IX sheeting that, according to VDOT and the literature, has a longer service life than Type III.

Discussion

The case study results are analyzed in the subsections below. Key best practices related to sign service life, sign maintenance methods, and sign inventory are identified.

Sign Service Life

Among the three DOTs that the research team visited, only VDOT has a less conservative sign service life. By using a more advanced type of sheeting (Type IX), the agency adopted a sign service life of 15 years, which resulted in a reduction in both inspection frequency and labor hours. According to VDOT, improving the quality of sheeting was the factor that brought the most positive impact to their sign maintenance program.

Both NCDOT and SCDOT use a sign service life of 10 years for Type III sheeting, which is considered by the majority of the literature to be a conservative approach (Clevenger et al., 2012; Dumont et al., 2013; Kipp & Fitch, 2009; Preston et al., 2014; Re & Carlson, 2012; Re et al., 2011). Most of the previous studies recommended 15 years or above for Type III sheeting (Clevenger et al., 2012; Dumont et al., 2013; Immaneni et al., 2009; Kipp & Fitch, 2009; Pike & Carlson, 2014).

Most NCDOT and SCDOT traffic engineers believed that, although a 10-year service life is conservative, it would protect the agencies from lawsuits and liability. As a result, they would be able to make a stronger case that their sign maintenance procedures are adequate and that nearly all signs are younger than 10 years old and within the warranty period. Thus, it is less likely that an agency would be found to be legally at fault.

In contrast, some engineers defended the idea that because Type I sheeting was phased out, meeting the minimum retroreflectivity requirements from the MUTCD is no longer a problem and that signs are expected to last more than 10 years in the field, perhaps significantly more so. Clevenger et al. (2012) obtained data from sheeting manufacturers and an interesting statement made by one was that “warranties protect public agencies against manufacturing defects, but the goal is to create products that far outlast the warranty period.”

Complementing this statement, there were many field survey studies that concluded that most signs were compliant with MUTCD minimum retroreflectivity levels. In some cases, the noncompliance rate was less than 1% of all signs surveyed (Kipp & Fitch, 2009; Kirk et al., 2001; Pike & Carlson, 2014; Pulver et al., 2018; Re et al., 2011), further confirming that it is no longer retroreflectivity that is the governing factor in sign safety. Thus, adopting a 10-year sign service life is a conservative approach that often results in a premature replacement of signs in good condition. An option for those DOTs that desire to be conservative would be to use a higher quality of sheeting (as VDOT did) that would provide them with a longer sign service life and still protect them from lawsuits.

Sign Maintenance Method

All of NCDOT, SCDOT, and VDOT use management methods to maintain sign retroreflectivity above the minimum required. Those methods are based on sign expected life with some variations. In addition, some DOTs use a combination of methods, usually coupling nighttime visual inspection with another method.

For its management method, SCDOT developed a statewide sign inventory (integrated into HMMS) to enable them to know when a sign needs to be replaced and where the sign is located. Because SCDOT had been using the expected sign life method for many years, the sign crews were already accustomed to the process and to HMMS data entry. As a result, productivity increased. In addition, SCDOT still conducts nighttime inspections on all roads of the state at least once a year to verify that signs are in a good condition.

VDOT uses a combination of sign service life, blanket replacement, and both daytime and nighttime inspections. However, those inspections occur at a much lower rate than in other DOTs. Given that the sign replacement cycle adopted by VDOT is 10 to 15 years, nighttime inspections will start on the 10th year of the replacement cycle. The daytime inspections have the objective of identifying damaged and stolen signs while the objective of the nighttime inspections is to assess whether or not signs are still above the minimum retroreflectivity level. If so, inspection crews would return after 2 years (in the 12th year of the cycle) for the same assessment. By doing so, the agency can potentially extend the sign service life based on field observation and assessment.

NCDOT adopted a blanket replacement method based on a sign service life of 10 years. The objective is to replace one tenth of the state-owned signs per year. However, NCDOT is just beginning the transition (begun in 2017) from nighttime inspection to blanket replacement. Currently, the divisions must conduct both blanket replacement and nighttime inspections, which lead to a debate about whether or not NCDOT should eliminate the nighttime inspections. Consideration is being given to eliminating nighttime inspections because the blanket replacement method already ensures that all signs would be above minimum retroreflectivity levels. Others consider nighttime inspections to be valuable and important to maintaining the signs in good condition, primarily by identifying damage.

Considering the literature reviewed and Table 2, it is possible to note that most of the DOTs adopted either assessment methods (mostly visual inspections) or management methods (mostly sign expected life). However, some studies showed that combinations of two or more methods were advantageous (Dumont et al., 2013; Re & Carlson, 2012). All of NCDOT, SCDOT, and VDOT adopted a combination of assessment and management methods. However, not all of the three achieved a balance among the different methods.

SCDOT and VDOT reduced the frequency of their nighttime inspections because signs are expected to perform above the minimum required retroreflectivity levels when adopting a management method. Hence, the primary sign maintenance method used by VDOT and SCDOT is either expected sign life or blanket replacement while nighttime inspection is a secondary method. NCDOT is a different case in which it is not clear whether the primary maintenance method is blanket replacement or nighttime visual inspection.

Sign management program implementation

NCDOT has just begun the transition from nighttime inspections to blanket replacement. The North Carolina case study shows that the transition can result in problems. The major problems that arose were shortages of sign material and labor and a larger scope of work. These are discussed below.

Sign material shortage

In North Carolina, one of the problems has been a sign material shortage. The problem lies in the fact that NCDOT requires all divisions to replace about one tenth of their signs per year. As a result, there is a higher sign demand from all divisions that the sign shop had not previously faced. Because there was little to no advance notice given on the rollout of the RMIP implementation, the sign shop was initially unable to meet all sign demand because they were having difficulty in obtaining ink and aluminum sheeting to manufacture the signs. As a consequence, there was initially not enough sign material available to meet the new demand.

Personnel shortage

In some cases, there are not enough personnel to handle all the sign work. There are even some sign crews that consist of a single person, which has a negative impact on productivity. In addition, because of the limited manpower, divisions are unable to conduct both daytime inspections and blanket replacement. During the meetings with the divisions, it was noted that shortages require personnel to work late (past 5:00 p.m.) and on Saturdays to complete the added work.

Scope of work

Another challenge faced while implementing sign blanket replacement is that sign personnel have a variety of duties to meet the varied work demands of the agency. For instance, when there is road construction, or a utility company is working on a road, it is often necessary to close a lane or determine a detour. In such cases, sign crews need to install detour signs and barricades. Then, when the service (or construction) is concluded, the sign crew needs to go back to collect the signs and barricade. All this work takes time and disrupts the sign inspection and replacement process.

The engineers suggested that perhaps all sign replacement activities could be coordinated with the replacement of other roadway features and could be done at one time using a corridor approach (e.g., by performing that work in combination with resurfacing a road). Instead of replacing and maintaining signs in an area, paving another road, replacing the ditches in another area, and so on, all road features could be replaced and/or maintained in a specific corridor while the agency is resurfacing that part of the road. However, it is important to note that different features have different life cycles. For instance, pavement marking (paint) may be redone every 4 years while signs may be replaced every 10 years.

Beneficial Sign Maintenance Practices

Based on this study’s discussions through the case studies and on the literature reviewed, the research team selected a set of practices that can be considered by other DOTs to improve their sign maintenance programs. Practices that can be used independent of the sign maintenance method adopted include the following:

Train personnel to conduct daytime inspections and observe signs while conducting other work activities.

Track both sign manufacture and installation dates to determine sign life and age.

Use a combination of sign maintenance methods to optimize the maintenance program.

Consider using a higher quality of sheeting to increase sign life (as VDOT does).

When using expected sign life (such as SCDOT does), agencies could consider the following practices:

Use bar codes to identify signs.

Maintain a sign inventory that contains sign installation date, age, and GPS location.

Utilize an integrated system of software and hardware (bar code reader, tablet, computer, GPS).

Automation of sign inventory data collection (e.g., image-based, LiDAR technology, Google Street View images, etc.).

When using a blanket replacement method (NCDOT and VDOT), agencies could consider the following practices:

Replace signs by areas (counties or sections) that are delineated by roads (corridors).

Reuse replaced signs that are less than 5 years old and in good condition.

Do a nighttime inspection near the end of the expected sign life (as VDOT does) to determine whether or not the expected sign life in an area can be increased. Alternatively, evaluate a set of control signs near the end of the expected sign life for the same purpose.

Provide sign shop support for increased sign manufacturing load when first implementing a blanket replacement method.

If resources are available, create a sign inventory. However, blanket replacement can be done without an inventory.

Conclusion

This study covered an extensive literature search and three case studies. Based on all that was discussed so far, it was possible to present a set of beneficial practices that other state DOTs can consider in their management program that could result in cost reductions and safety improvements.

Final considerations include the fact that the expected sign life method does require a sign inventory, and a level of automation is required to know sign age and location because there is not necessarily statewide area or corridor uniformity in age. Combining blanket replacement with expected sign life, in contrast, enables the work to be accomplished without maintaining an inventory. However, the agency should retain knowledge of the boundaries of the areas and the years in which their signs were replaced.

Both expected sign life and blanket replacement also significantly reduce the need for daytime and nighttime inspections. Instead of these being conducted annually, they may be conducted toward the end of the expected sign life. If such inspections (full inspection, random sampling, or a set of control signs) reveal nearly full compliance, it may be the case that replacement may be delayed in that area, thus, effectively increasing sign life. In doing so, the replacement method is linked directly to field performance and MUTCD compliance levels rather than to a theoretical estimate of sign life. That is, the actual implementation of a replacement method can be fine-tuned based on performance over time, and adjustments and corrections can be made when necessary.

With respect to a sign management program, the SCDOT is the only one among the three DOTs that has a mature program in place, which consists of a statewide program that contains a robust sign inventory database. Both NCDOT and VDOT are improving their sign management program by adopting new plans (RMIP and SMRC Plan, respectively). In addition, while VDOT already started creating its sign inventory database, NCDOT will also create such an inventory to assist them in the maintenance and management of signs.

These findings have implications for maintaining and replacing other roadway assets. Furthermore, they could inform infrastructure asset management in general. Numerous agencies maintain civil infrastructure assets. The article, thus, directly addresses infrastructure system operations.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed the following financial support for the research, authorship, and/or publication of this article: this project was sponsored by NCDOT.

ORCID iD

William Rasdorf

Author Biographies

Patricia Machado is a PhD candidate at North Carolina State University. She earned her master’s of science in civil engineering degree from North Carolina State University in 2016. Her emphasis is in construction engineering and management. Her current research focuses on traffic sign maintenance and replacement.

William Rasdorf is a professor of civil engineering at North Carolina State University. His interests span construction, transportation, and computer-aided engineering. His research focuses on information technology and the modeling of engineering processes, facilities, and operations.

References

American Society for Testing Materials. (2017). ASTM standard specification for retroreflective sheeting for traffic control (ASTM D4956-17). West Conshohocken, PA: ASTM International.

Balali

Rad

Golparvar-Fard

(2015). Detection, classification, and mapping of U.S. traffic signs using Google street view imaged for roadway inventory management. Visualization in Engineering, 3, 1-18. doi:10.1186/s40327-015-0027-1

Balali

Sadgehi

Golparvar-Fard

(2015). Image-based retro-reflectivity measurement of traffic signs in day time. Advanced Engineering Informatics, 29, 1028-1040.

Boggs

Heaslip

Louisell

(2013). Analysis of sign damage and failure: Utah Case Study. Transportation Research Record: Journal of the Transportation Research Board, 2337, 83-89.

Carlson

Park

Andersen

(2009). Benefits of pavement markings: A renewed perspective based on recent and ongoing research. Transportation Research Record: Journal of the Transportation Research Board, 2107, 59-68.

Carlson

Picha

(2009). Sign retroreflectivity manual: How to meet the new national standard for small agencies, federal land management agencies, and tribal governments (Project FHWA-CFL/TD-09-005). Washington, DC: Federal Highway Administration.

Clevenger

Colello

Quirus

(2012). Retroreflectivity of existing signs in Pennsylvania (Report No. FHWA-PA-2012-003-E01041-W09). Philadelphia, PA: McCormick Taylor Engineers and Planners.

Delaware T² Center. (n.d.). Tech topic: Sign retroreflectivity. University of Delaware. Retrieved from https://cpb-us-w2.wpmucdn.com/sites.udel.edu/dist/1/1139/files/2013/12/Retroreflectivity-2fgj46c.pdf

Dumont

Johnson

Lechtenberg

Lott

McGraw

Moser

. . . Ficek

(2013). Evaluation of MnDOT’s sign replacement method (Project 15310.000). Saint Paul: Minnesota Department of Transportation.

10.

Evans

Heaslip

Boggs

Hurwitz

Gardiner

(2012). Assessment of sign retroreflectivity compliance for development of a management plan. Transportation Research Record: Journal of the Transportation Research Board, 2272, 103-112.

11.

Federal Highway Administration. (1999). Asset management primer. Washington, DC: U.S. Department of Transportation. Retrieved from https://www.mdt.mt.gov/publications/docs/brochures/research/toolbox/fhwa/asstmgmt.pdf?bcsi_scan_e881b9121136f8e3=0&bcsi_scan_filename=asstmgmt.pdf

12.

Federal Highway Administration. (2007). Methods for maintaining traffic sign retroreflectivity (Publication No. FHWA-HRT-08-026). Washington, DC: U.S. Department of Transportation. Retrieved from https://safety.fhwa.dot.gov/roadway_dept/night_visib/policy_guide/fhwahrt08026/fhwahrt08026.pdf

13.

Federal Highway Administration. (2009). Manual on uniform traffic control devices: 2009 edition. Washington, DC: U.S. Department of Transportation. Retrieved from https://mutcd.fhwa.dot.gov/pdfs/2009/mutcd2009edition.pdf

14.

Federal Highway Administration. (2013). Maintaining traffic sign retroreflectivity: Revised 2013 (Publication No. FHWA-SA-07-020). Washington, DC: U.S. Department of Transportation. Retrieved from https://safety.fhwa.dot.gov/roadway_dept/night_visib/sign_retro_4page.pdf

15.

Federal Highway Administration. (2017). Highway statistics 2016 Table HM-81: State highway agency-owned public roads. Washington, DC: U.S. Department of Transportation. Retrieved from https://www.fhwa.dot.gov/policyinformation/statistics/2016/hm81.cfm

16.

Hancher

(1994). Use of warranties in road construction: Synthesis of highway practice (NCHRP Synthesis 195). Washington, DC: National Cooperative Highway Research Program.

17.

Harris

Rasdorf

Hummer

(2009). A control sign facility design to meet the new FHWA minimum sign retroreflectivity standards. Public Works Management & Policy, 14, 174-194.

18.

Hawkins

Carlson

(2001). Sign retroreflectivity: Comparing results of nighttime visual inspections with application of minimum retroreflectivity values. Transportation Research Record: Journal of the Transportation Research Board, 2464, 11-20.

19.

Sonf

Liu

(2017). Updating highway asset inventory using airborne LiDAR. Measurement, 104, 132-141. doi:10.1016/j.measurement.2017.03.026

20.

Huynh

Mullen

Pulver

(2018). “Sign Life Expectancy,” Final Report FHWA-SC-17-11, Department of Civil and Environmental Engineering, University of South Carolina. Columbia, SC.

21.

Immaneni

Hummer

Rasdorf

Harris

Yeom

(2009). Synthesis of sign deterioration rates across the United States. Journal of Transportation Engineering, 135, 94-103.

22.

Immaneni

Rasdorf

Hummer

Yeom

(2007). Field investigation of highway sign damage rates and inspector accuracy. Public Works Management & Policy, 11, 266-278.

23.

Jiang

Zhou

(2012, August). Research of in-service regulatory sign sheeting, retro-reflectivity and deterioration characteristics. Paper presented at Proceedings of the 12th International Conference of Transportation Professionals, American Society of Civil Engineers, Beijing, China.

24.

Kipp

Fitch

(2009). Evaluation of measuring methods for traffic sign retroreflectivity (Final Report 2009-8). Montpelier: Vermont Agency of Transportation.

25.

Kirk

Hunt

Brooks

(2001). Factors affecting sign retroreflectivity (Final Report SR 514, Publication OR- RD-01-09). Salem: Oregon Department of Transportation.

26.

McCarthy

Liang

Park

McFadden

Trieu

(2013). Risk-based method for development and management of a traffic sign inventory for local agencies. Public Works Management & Policy, 18, 82-95.

27.

North Carolina Department of Transportation. (2016). 2016 Maintenance Operations and Performance Analysis Report (MOPAR). Raleigh, NC. Retrieved from https://connect.ncdot.gov/resources/Asset-Management/MSADocuments/2016%20Maintenance%20Operations%20and%20Performance%20Analysis%20Report%20(MOPAR).pdf

28.

North Carolina Department of Transportation. (2018). North Carolina official state mileages. Raleigh, NC. Retrieved from https://connect.ncdot.gov/resources/State-Mapping/Documents/Official_State_Mileage.pdf

29.

Pike

Carlson

(2014). Evaluation of sign sheeting service life in Wyoming. Transportation Research Record: Journal of the Transportation Research Board, 758, 88-94.

30.

Preston

Atkins

Lebens

Jensen

(2014). Traffic sign life expectancy (Final Report 2014-20). Mendota Heights: Office of Material and Road Research, Minnesota Department of Transportation.

31.

Pulver

Huynh

Mullen

(2018). Evaluation of expected traffic sign life in South Carolina. Transportation Research Record: Journal of the Transportation Research Board, 2258, 88-94.

32.

Rasdorf

Hummer

Harris

Sitzabee

(2009). IT issues for the management of high-quantity low-cost assets. Journal of Computing in Civil Engineering, 23, 91-99.

33.

Rasdorf

Machado

(2017). Sign replacement strategy (First Quarterly Research Progress Report 2018-25). Raleigh: North Carolina Department of Transportation.

34.

Rasdorf

Machado

(2018a). Sign replacement strategy (Second Quarterly Research Progress Report 2018-25). Raleigh: North Carolina Department of Transportation.

35.

Rasdorf

Machado

(2018b). Sign replacement strategy (Fourth Quarterly Research Progress Report 2018-25). Raleigh: North Carolina Department of Transportation.

36.

Carlson

(2012). Practices to manage traffic sign retroreflectivity: Synthesis of highway practice (NCHRP Synthesis 431). Washington, DC: National Cooperative Highway Research Program.

37.

Miles

Carlson

(2011). Analysis of in-service traffic sign retroreflectivity and deterioration rates in Texas. Transportation Research Record: Journal of the Transportation Research Board, 2258, 88-94.

38.

Schertz

(2005). The importance of sign retroreflectivity. Washington, D.C.: American Public Works Association Reporter. Retrieved from http://www3.apwa.net/Resources/Reporter/Articles/2005/7/The-importance-of-sign-retroreflectivity

39.

Singh

Labi

Sinha

(2007). Cost-effectiveness evaluation of warranty pavement projects. Journal of Construction Engineering and Management, American Society of Civil Engineers, 133, 217-224.

40.

South Carolina’s Information Highway. (2014). South Carolina fast facts: Transportation. Retrieved from https://www.sciway.net/facts/

41.

Thompson

Ford

Arman

Labi

Sinha

Shirole

(2012). Estimating life expectancies of highway assets (NCHRP Report 713). Washington, DC: National Cooperative Highway Research Program.

42.

Vereen

Hummer

Rasdorf

(2002). A sign inventory study to assess and control liability and cost (Report No. FHWA/NC/2002-17). Raleigh: North Carolina Department of Transportation.

43.

Wisconsin Transportation Information Center. (2013). Wisconsin transportation bulletin: Meeting minimum sign retroreflectivity standards. Madison, UW. Retrieved from http://epdfiles.engr.wisc.edu/pdf_web_files/tic/bulletins/Bltn_023_Retroreflectivity.pdf

44.

Wolshon

Degeyter

Swargam

(2002, June). Analysis and predictive modeling of road sign retroreflectivity performance. Paper presented at 16th Biennial Symposium on Visibility and Simulation, Iowa City, IA.