Transportation & Mapping Solutions

Retroreflectivity of Signs

MINIMUM RETROREFLECTIVITY LEVELS

The Federal Highway Administration (FHWA) issued The Manual on Uniform Traffic Control Devices (MUTCD) in order to establish “the guiding principles for the use of traffic control devices.” ¹ The MUTCD “is intended to promote national uniformity of traffic control devices.” ¹ Before the MUTCD was modified in 1993, it stated that traffic signs had to be reflectorized or illuminated, but it did not provide any guidelines. The standard minimum levels were revised in 1998 and further updated in 2003. The most recent MUTCD includes research recommendations for minimum maintained levels of sign retroreflectivity and proposes minimum allowable values for the retroreflectivity of traffic control devices. Additionally, the most recent MUTCD addresses sign visibility through factors such as design, placement, maintenance, operation, and uniformity. This will ensure that signs are designed, placed, and maintained in a way that will allow traffic control devices to be seen by drivers traveling at a reasonable speed down the road. Ultimately, the vehicle operator will be able to safely and efficiently navigate the roadway. The FHWA estimates that up to half of the 58 million traffic signs in the U.S. are beyond their useful lifespan (estimated at 10 years) from a reflectivity standpoint.² This is one major reason that approximately 42,000 people have been killed on the nation's highways each of the past 8 years.³ To view the most recent version of the MUTCD, follow (L1).

CURRENT METHODS OF MANAGING AND ASSESSING THE RETROREFLECTIVIY OF TRAFFIC SIGNS AND INHERENT DRAWBACKS

Since the MUTCD has defined and updated the standard minimum retroreflectivity level for traffic signs, transportation officials need a fast and efficient way of locating the approximate 58 million traffic signs in use and assessing their effectiveness. An overview of some of the current methods being used and a list of their drawbacks is provided below.

Methods for analyzing the retroreflectivity of traffic signs can be categorized as Management Methods and Assessment Methods. Assessment methods involve the evaluation of individual signs by way of routine inspections or measurements. Management methods provide an agency with the capability of sustaining sign retroreflectivity without having to spend time assessing individual signs. Traffic sign retroreflectivity is represented as R_A and pavement marking retroreflectivity is represented as R_L.

Management Methods*⁴

There are three basic types of management methods – replacing signs based on age, blanket replacement of large numbers of signs at appropriate intervals, and using a sample of control signs to determine when to replace equivalent signs.

Expected Sign Life Method*

In this method, individual signs are replaced before they reach the end of their expected service life, which is based on the time required for the retroreflective material to degrade to the minimum R_A levels. The following factors provide additional information about using this method:

• The expected service life of a sign can be based on several different sources of information, such as:
  
  • Sign sheeting warranties
  • Sign test deck measurements
  • Measurements of actual sign

• The following are potential methods that an agency can use to identify the age of individual signs:
  • A sticker or other label attached to the sign that identifies the year of fabrication, installation or replacement. See Figure 1.
  • A sign management system that can identify the age of individual signs.
    

 

Blanket Replacement Method*

In this method, an agency replaces all the signs in an area/corridor, or of a given type, at specified intervals. An agency that uses this method does not need to track the age or assess the retroreflectivity of individual signs. The following factors provide additional information about the use of this procedure:

• Replacement zones can be based on an area, corridor, or sign type.
• The replacement interval for the area/corridor, or sign type, is based on the expected sign life for the affected signs.
• All signs within a replacement area/corridor/type are typically replaced, even if the sign was recently installed.

Control Sign Method*

In this method, a control sample of signs is used to represent the total population of an agency’s signs. The R_A is monitored at appropriate intervals and sign replacement is based on the performance of the control signs. The following factors provide additional information about using this method:

• An agency develops a sampling plan to determine the appropriate number of control signs needed to represent the agency's sign population.
• Control signs may be actual signs in the field or signs installed in a maintenance yard to serve specifically as control signs.
• The R_A of the control signs should be monitored following the procedures outlined for one of the assessment methods.
• All field signs represented by the control sample need to be replaced before the R_A levels of the control samples reach the minimum levels.

The drawbacks of the management methods are as follows:

• Signs degrade at different levels depending on environmental forces such as corrosion caused by exposure to man-made contaminants and natural elements.
• Possibility of signs being replaced even if their R_A is still at or above the proposed MUTCD minimum standards.
• Methods are time consuming and expensive.

Assessment Methods

The two types of assessment methods are visual nighttime inspection and measured retroreflectivity. There are multiple practices for each of these assessment methods.

Visual Nighttime Inspection Method*

This method is likely the most consistent practice that current agencies are performing.
Agencies record which signs fail to exhibit the proper level of reflectivity with the following criteria in mind:

• Inspection is conducted at normal highway speeds with low beam headlights; if it is necessary to slow down or stop in order to read the sign, it is in need of replacement.
• Signs are viewed at a distance that allows a driver sufficient time for an appropriate response.

One or more of the subsequent procedures are used to conduct the visual nighttime inspection:

Calibration Signs Procedure*

Calibration signs, which are viewed prior to the nighttime inspections, are used as a comparative tool in order to establish a threshold for the visual inspection of the actual road signs. Additional information is provided in order to assist in the procedure:

• Calibration signs are needed for each color of sign for which there are minimum levels.
• The calibration signs are viewed at typical viewing distances and from the same vehicle that will be used for conducting the inspections.
• Field signs need to be replaced if the inspector judges a sign to be less bright than the appropriate calibration sign or if it is not legible.
• Signs are viewed from a typical viewing distance by someone age 60 or older.

Comparison Panel Procedure*

Comparison panels are used to assess the R_A of questionable signs. When the R_A is considered questionable, it is compared to the panel which is set equal or above the minimum R_A levels. If the comparison panel is brighter than the sign, the sign must be replaced.

The drawbacks of the visual nighttime inspection method and subsequent procedures are as follows:

• This method is highly subjective, impossible to quantify, and as a result, outcomes will vary greatly.
• Method is prone to data entry errors.
• Signs may be replaced that are still considered acceptable by the proposed MUTCD minimum standards.
• This method is a poor use of time and money.

Measured Retroreflectivity Methods

While current measured retroreflectivity methods are the most advanced the industry has seen, they are also prone to error and time-consuming. There are two approaches in the measured retroreflectivity methodology. In the first approach, the R_A is measured using a hand-held or stable retroreflectometer. These hand-held retroreflectometers are positioned flush against a sign in order to collect the R_A value. Other hand-held retroreflectometers using lasers can be fired at a traffic sign from a distance of about 100 feet away. The second approach for measuring R_A uses a vehicle-based retroreflectometer. These devices employ a high intensity flash source, multiple cameras, and a range sensing device.

Hand-held Retroreflectometers

The inspector using the hand-held retroreflectometer manually positions the instrument directly on the surface of the traffic sign in order to collect the RA value. See Figures 2 and 3. Most of the hand-held retroreflectometers are set to the standard geometry of .2/-4 (observation angle and entrance angle) currently proposed by the MUTCD and only measure that single value. See Figures 4 and 5 and (L1). Once the RA values are collected, they are compared to the MUTCD recommendations in order to determine if the sign is in need of replacement. If the measured sign has a RA value lower than the minimum level recommended, the sign is replaced. Since operators of the hand-held retroreflectometer must stop at every traffic sign they inspect, this method is very time-consuming, expensive, and tedious.

 

Other hand-held devices, such as the “Impulse RM” retroreflectometer by Laser Technology, Inc., are manually directed at a target object and then manually “fired.” When fired, the hand-held device bounces a laser beam off the target object and measures the reflected laser energy, which is then used to determine the R_A value. See Figure 6. If the measured sign has a R_A value lower than the minimum level recommended, the sign is replaced.

Drawbacks of the Impulse RM are as follows:

• The user of the hand-held Impulse RM must be stationary – either standing along the side of the road, sitting on top of an all-terrain vehicle that is stopped along the side of the road, or leaning out of a vehicle that is parked on the side of the road.
• Since operators must hold the device very steady while the R_A is taken, there are considerable opportunities to obtain values for objects other than the intended sign. The net result is that measurements made with the Impulse RM have considerable exposure to error-inducing procedures.

In addition to the problems mentioned above, there are several other drawbacks that traditional hand-held retroreflectometers and the Impulse RM share. They are:

• The majority of the hand-held devices can only measure a single R_A value for a single sign, and can only measure either foreground or background R_A with a single measurement. See Figure 7.
• Operators will typically make several measurements for a given sign and will report the average value, the most frequently occurring value, or use some other non-standardized method for manipulating the series of readings.
• In order to validate a measurement made by such devices, the device must be taken back to the precise location where the original measurement was made for a valid comparison of the measurements to be made.
• The crew performing the measurement along the side of a road is continually exposed to the possibility of being struck and seriously injured by traffic, even when personnel are sitting inside a stationary vehicle.

Vehicle-Based Retroreflectometer

Aside from Facet Technology Corp's RetroView™ , which will be discussed later, there is only one other traffic sign vehicle-based retroreflectometer in the industry. The Sign Management and Retroreflectivity Tracking System (SMARTS) is a vehicle introduced by the FHWA that is equipped with a mobile retroreflectometer. SMARTS contains:

• One high intensity flash source
• One color camera
• Two black and white cameras
• A range-sensing device
• A GPS unit

The SMARTS vehicle requires two people – a driver and a system operator – for proper operation. As the SMARTS vehicle travels down the road, the system operator “locks on” to an upcoming traffic sign by rotating the camera and light assembly to point at the sign. See Figure 8. At a distance of 60 meters, the system triggers the flash source to illuminate the traffic sign surface, an image of which is captured by one of the black and white cameras. A histogram (bar graph of a frequency distribution) of the traffic sign's legend and background is produced, which is then used to calculate the R_A. A GPS unit stores the location of the vehicle along with the calculated R_A in a computer database.

The drawbacks of the SMARTS system are:

• Like hand-held retroreflectometers, the SMARTS system can only determine the R_A for one traffic sign at a time and it can only determine the R_A for the discrete point on the roadway 60 meters from the traffic sign.
• Two people are required to operate the vehicle and measurement system.
• The SMARTS vehicle cannot make R_A determinations for traffic signs on both sides of the roadway in a single pass over the roadway and does not produce nighttime traffic sign visibility information for lanes on the roadway not traveled by the vehicle.
• Because the system operator in the SMARTS vehicle must locate and track the traffic sign to be measured while the vehicle is in motion, a high level of operational skill is required and the likelihood that a sign will be missed is significant.

Hand-held retroreflectometers and the SMARTS mobile retroreflectometer system don't lend themselves to increased processing throughput so as to more easily manage the monitoring and maintenance of the more than 58 million individual traffic signs in the U.S.

The current so-called automated data collection systems often require that normal traffic be stopped during data collection because either the acquisition vehicle moves very slowly or because the acquisition vehicle has to come to a full stop before recording data about the roadside scene. Furthermore, a human operator is required to point one or more measurement devices at a sign of interest, perform data collection for that particular traffic sign and then set up the device for another particular traffic sign of interest.

With such a large number of traffic signs that must be monitored, an automated R_A data collection system is needed that addresses these and other shortcomings of the existing techniques for determining R_A. More automation will enable higher throughput, which is a necessity for the widespread utilization of the R_A measurement systems.

BENEFITS OF FACET TECHNOLOGY CORPORTATION'S METHODOLOGY

Automated Determination of Retroreflectivity

As discussed in the previous section, traditional methods of measuring R_A in the field have been very manual and time-consuming, and they are not cost-effective processes. Facet Technology Corp's multi-wavelength, active sensor system is an integral part of the state-of-the-art RetroView™ data collection vehicles. R_A measurements that were traditionally made with hand-held devices (used by personnel standing on the side of the road) can now be made by a vehicle that moves at the speed of the surrounding traffic. The RetroView™ approach to R_A has many benefits over the traditional methods. They include:

• R_A values can be determined for multiple points along the roadway for each traffic sign.
• R_A values can be determined for multiple traffic signs along the roadway with a single pass of the vehicle.
• Digital imagery is captured at the same time as R_A information.
• Foreground and background R_A can be determined with the same information.
• There is no need to stop the vehicle or drive at less than posted speeds in order to collect R_A information.
• R_A values for clusters of traffic signs are determined with a single pass of the vehicle.
• R_L can be determined from the same information that is used to compute R_A.
• Sign sheeting type can be determined with the same information that is used to compute R_A.
• The personnel operating the RetroView™ vehicle require no prior knowledge of the location or existence of objects for which R_A measurements are desired.
• R_A values for all points on a multi-lane roadway can be determined from just a single pass of the RetroView™ capture vehicle.

The RetroView™ approach is designed to: 1) dramatically increase the accuracy of R_A measurements, 2) make the process much more automated, thus lowering costs, 3) combine R_A data gathering with digital imagery collection on a single mobile platform and 4) introduce new ways of reporting R_A results that give the transportation professional as-yet-unavailable information for planning and maintenance. For a more technical explanation of Facet Technology Corp's approach, consult (L2).

Expanding the Usability of Gathered Data

R_A measurements have traditionally been an underutilized aspect of sign management systems. Hand-held laser-based devices have made the gathering of R_A values somewhat more attainable. The use of R_A in predictive modeling algorithms has allowed R_A measurements to be utilized in multi-year maintenance programs.

Currently, most management techniques represent R_A values only at one specific point and for a given light source and observer geometry (observation angle). This single value is very limited for describing the R_A. As mentioned, current methods are prone to error and time-consuming. This is why Facet Technology Corp. created an innovative way to conduct a more thorough method of R_A collection. Our multi-point data gathering system will be implemented within our high-end RetroView™ capture platforms.

RetroView™

Because Facet Technology Corporation has determined a more thorough way of collecting multiple R_A points along a roadway, we must have a method to display and manage the data. Facet Technology Corp's RetroView™ provides a rich graphical viewing environment that is used to display the R_A values of road signs as well as process imagery. A transportation planner can utilize RetroView™ to visually understand a sign's performance over the entire roadway. Additionally, the RetroView™ system will produce a RetroCurve® for every sign.

RetroCurve®

Yet another innovative method for analyzing R_A is Facet Technology Corp's RetroCurve® . A RetroCurve® is a graphical comparison of R_A versus observation angle for a given entrance angle. For a typical distance to a sign a driver in a passenger car will typically see an observation angle of .2 degrees while the observation angle of light trucks (defined as pickups, sport-utility vehicles, and minivans) at the same distance would be approximately .3 to .4 degrees. Currently, most agencies measure R_A at an observation angle of .2 degrees. With an ever-increasing number of light trucks and commercial freight liners on the road, it is important to measure the R_A at an observation angle which will allow drivers to properly see the traffic signs. While a couple of hand-held retroreflectometers have the ability to measure R_A values at .2 and .5 observation angles, they do not measure the R_A values in between.

The FHWA reports that, “[while] the passenger car has traditionally been the best-selling vehicle type in the United States, for the 1999 model year, new trucks ... outsold new cars for the first time; trucks had about 50.1 percent of the new-vehicles market versus 49.9 percent for cars ... Furthermore, over the past decade the number of registered passenger cars decreased by 0.1 percent, while the percent of trucks has increased over 60 percent.” ⁵ These trends have continued since 1999. Moreover, Forbes.com reports that four of the six top selling vehicles in 2004 were classified as light trucks. ⁶ This fact further underscores the need for R_A measurements at multiple observation angles. In addition to the increase in light trucks on America's roadways, the volume of commercial freight liners is also growing.

The typical observation angle of a commercial truck is approximately .5 degrees. Because current assessment methods measure R_A at the standard observation angle of a car, operators of commercial truck drivers are at a disadvantage since a lower amount of light is being reflected back to them from the traffic signs. Not surprisingly, this has a direct impact on the quantity of nighttime fatalities involving commercial trucks. According to the Federal Motor Carrier Safety Administration, in 2002 the fatality rate per 100 million miles traveled was 2.3 for large trucks as compared to only 1.5 for all vehicles. ⁷

The transportation industry has been waiting for some time for a methodology that eases the measurement and reporting of R_A. Fortunately for the industry, Facet Technology Corporation has a thorough method of collecting R_A values at multiple observation angles with RetroView™, making the process easier and more accurate. Facet Technology Corporation also has a method for analyzing the values in RetroCurve® , and we have expanded the use of the raw data by way of a set of serious graphical planning tools.

Research Conducted at the Texas Transportation Institute (TTI)

The Texas Transportation Institute (TTI), which is a former Air Force base, was deeded to the Texas A&M University System Engineering Program for the purpose of transportation safety and research. See Figure 9. The TTI is currently being funded by the Texas Department of Transportation (TxDOT) as well as the Federal Highway Administration (FHWA) because they realize the TTI's importance in creating a safer transportation industry. The TTI has a worldwide reputation in safety research and development which explains why agencies such as the U.S. Departments of State, Energy and Homeland Security continue to rely on the facility for state-of-the-art research.

Facet Technology Corporation and business partner, Mandli Communications, Inc. based out of Wisconsin, recently spent two days at the TTI to test our innovative R_A methodology called RetroView™. RetroView™ is a multi-wavelength, active sensor system that has multiple benefits over current methods of testing R_A. A few of these benefits include more accurate testing, a more automated process, and on-the-fly measurements of multiple points for individual signs. RetroView™ also includes the GPS location for each sign, thus simplifying the asset management process for the end user. Data was collected while traveling around a predetermined two lane course equipped with traffic signs.

A diverse number of road sign attributes were analyzed in order to test road signs that correspond to those seen on America's public roads. Seven of the most widely utilized traffic sign colors were tested. Perhaps one of the most important attributes tested at the TTI was sign sheeting types. See (L3). Facet Technology Corp's RetroView™ was used to collect the R_A of four different sheeting types from both the inside and outside lanes of the course. R_A values were tested from both lanes because in a real-world situation our collection vehicles may not be able to gather data from the right lane, and secondly, it is important to know what the R_A values of the traffic signs are in all lanes of traffic. Lastly, drivers rely on traffic signs having high retroreflectivity values for every lane of the road. These values were later compared to those collected with a hand-held retroreflectometer.

A GPS location was recorded for each sign that was assessed. This allowed us to verify our method for calculating the sign's location from the van's sensor data. After the data was collected from multiple distances, Facet Technology Corp's RetroCurve® was used to analyze R_A values. As previously mentioned, a RetroCurve® is utilized for a graphical representation and it compares R_A to observation angle at a given entrance angle. See Figure 10. Again, this is of significant importance because traditional assessment methods only collect the R_A at a single observation and entrance angle, representing an average car at a single distance from the sign. With the recent increase in light truck sales and our dependence on the trucking industry for the movement of goods, the traditional single R_A value is insufficient.

Test runs conducted at the cutting-edge facility of the TTI provided Facet Technology Corporation the valuable opportunity to test our innovative and unique methodology while working with the leading transportation safety institute in the U.S.