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NCHRP Web-Only Document 146:
Replacement Processes for Light Emitting Diode (LED) Traffic Signals John D. Bullough Jeremy D. Snyder Aaron M. Smith Terence R. Klein Lighting Research Center, Rensselaer Polytechnic Institute Troy, NY
Contractor’s Final Report for NCHRP Project 20-07/Task 246 Submitted August 2009 National Cooperative Highway Research Program
ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies.
COPYRIGHT PERMISSION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FTA, Transit Development Corporation, or AOC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP.
DISCLAIMER The opinion and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the TRB, the National Research Council, AASHTO, or the U.S. Government. This report has not been edited by TRB.
NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
CONTENTS LIST OF FIGURES .................................................................................................................... iv LIST OF TABLES ....................................................................................................................... iv ACKNOWLEDGMENTS .............................................................................................................v ABSTRACT .................................................................................................................................. vi CHAPTER 1 Background ............................................................................................................1 CHAPTER 2 Research Approach ...............................................................................................2 CHAPTER 3 Findings and Applications ....................................................................................3 Research Review and Synthesis...........................................................................................3 Photometric Measurement Techniques ................................................................................9 LED Traffic Signal Failure Modes ....................................................................................17 CHAPTER 4 Conclusions, Recommendations, and Suggested Research .............................26 Conclusions ........................................................................................................................26 Recommendations for Replacement Strategies .................................................................27 Suggested Research ...........................................................................................................31 REFERENCES .............................................................................................................................33
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
LIST OF FIGURES Figure 1. Luminous efficiency functions for color-normal and protan observers, and spectral distribution from red incandescent and LED traffic signal modules (Andersen, 2002). Figure 2. a) Average reaction times and interquartile ranges for color-normal subjects; b) average reaction times and interquartile ranges for protan subjects (Huang et al., 2003). Figure 3. Block diagram of a typical LED signal module design. Figure 4. Arrays of 2 × 20 and 4 × 20 LEDs in parallel. Figure 5. Power resistor and smoke residue on nearby LEDs. Figure 6. Defective Schottky diode. Figure 7. Locations of failed LEDs.
LIST OF TABLES Table 1. Measured illuminance and calculated luminous intensity values for different measurement distances (red signal module). Table 2. Measured illuminance and calculated luminous intensity values for different measurement distances (green signal module). Table 3. Luminous intensity values estimated from luminance values using luminance test method 1 (red signal module). Table 4. Luminous intensity values estimated from luminance values using luminance test method 1 (green signal module). Table 5. Luminous intensity values estimated from luminance values using luminance test method 3 in an exterior, daytime environment (green signal module). Table 6. Long term annual costs for spot and group replacement strategies for an agency responsible for maintaining 100 signalized intersections, assuming a 7-year expected life. Shaded cells indicate when group replacement is estimated to be more costly than spot replacement. Table 7. Long term annual costs for spot and group replacement strategies for an agency responsible for maintaining 100 signalized intersections, assuming a 10-year expected life. Shaded cells indicate when group replacement is estimated to be more costly than spot replacement.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
ACKNOWLEDGMENTS The authors acknowledge sponsorship of this research from the National Cooperative Highway Research Program (NCHRP) of the Transportation Research Board. The authors would also like to gratefully acknowledge the City of Los Angeles, the Nebraska Department of Roads, the New Jersey Department of Transportation, the New York State Department of Transportation, and the Wisconsin Department of Transportation for providing traffic signal modules to the project team for evaluation.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
ABSTRACT This report documents and presents the results from a study of the photometric requirements, measurement and maintenance of traffic signal modules using light emitting diodes (LEDs). Differences between LED technology and the incandescent lamps used in previous traffic signal modules in terms of photometric performance, color, and failure modes require new approaches to traffic signal maintenance. Findings from a review of literature on human factors and maintenance practices, from a series of laboratory and field measurements of LED traffic signal modules, and from an analysis of the failure mechanisms of traffic signal modules provided by several different transportation agencies, are provided. Based upon these findings and upon an economic analysis with different assumptions regarding LED traffic signal module failure rates, some preliminary guidance for identifying when group replacement of LED signal modules is feasible is provided, and some possible avenues for future research are recommended.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
CHAPTER 1 BACKGROUND Effective January 2006, the Department of Energy legislated that signal manufacturers may only manufacture traffic signals that meet ENERGY STAR (2003) power requirements, effectively requiring the use of light-emitting diodes (LEDs) in traffic signal heads. In the absence of standards for maintenance of LED traffic signals, transportation agencies face a huge challenge in defining the life expectancy and creating their operational budgets for maintenance of LED traffic signals. As an initial step in developing maintenance guidelines for LED traffic signals, the National Cooperative Highway Research Program (NCHRP) established Project 207/246, "Replacement Processes for Light Emitting Diode Traffic Signals," to investigate methods for determining when LED traffic signal heads should be replaced, and to discuss possible specifications for LED traffic signals to maximize reliability and minimize maintenance costs. A recently-published NCHRP synthesis report, "LED Traffic Signal Monitoring, Maintenance, and Replacement Issues" (Urbanik, 2008), provides very useful background information to the reader. Urbanik (2008) reviewed the results of a survey conducted by the Institute of Transportation Engineers (ITE) in 2006, of transportation agencies regarding their experienced and practices with LED traffic signals. In general it was found that most agencies did not have a systematic replacement program for LED signal modules, nor was funding in place to monitor and (when necessary) replace modules not performing adequately. Some agencies have established replacement programs whereby modules are to be replaced on a group basis after a number of years in service, but the replacement periods can range from five years to longer periods depending upon the assumptions made by each agency. Urbanik (2008) also reported that one state agency (Louisiana Department of Transportation and Development) specified the use of light output monitoring in LED traffic signal modules, but that there were no modules available meeting such specifications. Because LED traffic signal module failure modes are very different from those of incandescent traffic signal modules, and because the expected operational lifetime of LED modules is so much longer than the operating life of incandescent lamps used in traffic signals, there is a lack of national consensus regarding the best practices for LED traffic signal replacement and for monitoring of signals. The objective of the present project is to identify some of the factors that are related to LED traffic signal module failure, and to discuss some of the steps that might be taken by transportation agencies in the maintenance of LED traffic signal systems.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
CHAPTER 2 RESEARCH APPROACH The project activities, documented in the present report, consisted of the following: •
A review and synthesis of recent human factors research relevant to the visibility and photometric performance of LED traffic signals.
•
Evaluation of methods for laboratory and field measurement of LED traffic signal photometric performance.
•
Investigation of failure modes of LED traffic signal modules.
•
Summary of considerations for departments of transportation (DOTs) in specifying, deploying and maintaining LED traffic signals.
The results of the aforementioned tasks are documented in Chapter 3 of this report, "Findings and Applications." Chapter 4, "Conclusions, Recommendations, and Suggested Research," provides some preliminary guidelines for DOTs regarding practices for maintenance and replacement of LED traffic signal heads.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
CHAPTER 3 FINDINGS AND APPLICATIONS RESEARCH REVIEW AND SYNTHESIS In the present review and synthesis of human factors research associated with the perception of colored signal lights such as traffic signals, emphasis is placed on recent research on reaction times and missed signals, discomfort glare, and brightness perception. An introduction briefly summarizes current maintenance practices. The present review focuses on research published after 1998, when the first interim specification for LED traffic signals was published by the Institute of Transportation Engineers (ITE, 1998). LED traffic signal modules have gained popularity in the U.S. primarily because they consume far less energy than incandescent lamps - 85% less, on average (Iwasaki, 2003). Because of these energy savings, the U.S Environmental Protection Agency (EPA) recognized LED traffic signal modules as an ENERGY STAR product in 2000, and then Congress mandated that as of January 2006, all red and green traffic signal modules must meet the energy consumption specifications stipulated by the ENERGY STAR requirements (ENERGY STAR, 2003). In addition to energy savings, the use of LED traffic signals has the potential to increase safety at intersections. The reduced power required to operate LED traffic modules has allowed the use of low cost battery backup systems at intersections, increasing safety in the case of blackouts; a $3,000 backup system can power an intersection for two to four hours (Iwasaki et al., 2003). Also, since the late 1990s, LED modules typically reach end of life by reduced light output, rather than complete failure, so even “failed” modules usually give some signal information to drivers (Behura, 2007). Maintenance Behura (2007) reported on a survey of LED traffic signal maintenance practices. He observed that the use of LED traffic signals had been expected to reduce the lifetime cost per module compared with incandescent modules. In addition to reduced energy expenditures, the extended lifetime of the LED modules had been expected to reduce relamping material and labor costs by reducing the frequency these costs will be incurred. Behura (2007) stated that many agencies had not implemented appropriate maintenance programs for LED signal modules. The survey showed that: •
35% have no replacement program
•
35% are complaint driven (despite the fact that LED modules typically reach end of life due to dimming rather than complete failure)
•
24% implement routine, scheduled replacement
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
•
3% replace on vendor product life cycle
•
3% replace based on in-service test results
Rather than relying on a passive maintenance scheme, Behura (2007) suggested several maintenance schemes from most to least precise: •
Remove modules from service and test light output in a laboratory
•
Field measurements might provide good results, if they could be done properly
•
Statistical analysis based on time since installation
•
Replace based on the warranty period
Regardless of the maintenance practice, Behura (2007) recommended keeping a database of modules in the field including location, color, type, manufacturer and model, serial number, date of purchase, date of installation, and warranty end date. Behura also recommended that agencies clean lenses at intervals of one to two years (which would provide an opportunity for field measurements too). Visibility Perception and Reaction Time A study by Bullough et al. (2000) showed that under simulated daylight viewing conditions, LED and incandescent modules of the same nominal color, luminance, and onset time resulted in no statistically significant differences in mean reaction times, percentages of missed signals, color identification accuracy, and subjective brightness ratings. That study (Bullough et al., 2000) did find that reaction times and the percentage of missed signals decreased as luminous intensities (or luminances) increased, and that to obtain the same performance as a red traffic signal meeting then-current photometric requirements for luminous intensity for a 200-mm diameter module, yellow signals had to have a luminous intensity between 1.4 and 2.4 times higher than the red signal, and green signals had to have a luminous intensity between 2.4 and 2.8 times higher than the red signal. Freedman (2001) reported that yellow signals required a luminous intensity for the yellow about twice that of the red to obtain equal visual response, and that green signals required a luminous intensity of 1.3 times higher than red to obtain equal visual response. (Fisher and Cole [1975] recommended a 3:1 luminous intensity ratio for yellow:red and a 1.3:1 ratio for green:red). Taking these studies into account, the ITE recommended a luminous intensity ratio of 2.5:1 for yellow:red and 1.3:1 for green:red in its later specification for LED traffic signal performance (ITE, 2005). The only discrepancy among these studies is the higher ratio for green signals obtained by Bullough et al. (2000), which might be explained by the background light source used in their study, which had a correlated color temperature of about 3850 K, slightly lower than typical daylight illumination between 5500 and 6500 K (Wyszecki and Stiles, 1982). Regardless, since
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
the fundamental meaning of the green signal (i.e., "go") is different from that of the red and yellow signals (i.e., "stop"), it could be argued (Bullough, 2002) that equivalent reaction time and brightness for the green signal (relative to red) is not critical for driving safety. In the study by Bullough et al. (2000) lamp onset time was held constant (by using an electromechanical shutter). However, in the field, the onset time of incandescent traffic control signals is longer than that of LED sources (100 to 200 ms for incandescent, versus 13 to 32 ms for LEDs). Bullough (2005) reported the effect of this variation in onset time, and found that the differences in response time were very short. For example, the difference in response time for red traffic signals meeting the ITE (1998) specifications for luminous intensity, and having rise times of either 17 or 87 ms, was about 30 ms. The same comparison for yellow signals meeting ITE (1998) specifications yielded a response time difference of about 25 ms. Nor did rise time affect the consistency with which a signal was detected, so it was concluded by Bullough (2005) that onset time has little practical consequence for traffic signals. Cohn et al. (1998) confirmed that the visibility of red LED modules and red incandescent signal modules was about equal under daylight conditions, and concluded that the pixilated appearance of some LED signals might actually provide a visual benefit. This conclusion was confirmed in a subsequent study by Bullough et al. (2002) who found response times to a signal light consisting of an array of point sources, but with equivalent far-field luminous intensity as a diffuse signal light, were shorter than to the diffuse signal light. While it could be argued that performance metrics such as reaction time and missed signals are most important when considering traffic signals, some studies examined subjective metrics such as brightness and visibility. As described above, Bullough et al. (2000) found no statistically significant difference in perceived brightness between incandescent and LED lamps of the same luminance (for red, yellow, and green) under simulated daylight conditions, although the number of brightness judgments made in that study was relatively small. A study by Bullough et al. (2007) of green LED versus green incandescent signals viewed under nighttime conditions found that the LEDs appeared to be 1.4 to 1.7 times brighter, which is attributed to their saturation.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
Figure 1. Luminous efficiency functions for color-normal and protan observers, and spectral distribution from red incandescent and LED traffic signal modules (Andersen, 2002). Color Vision Deficiency According to a review by Cole (2004) of 124 journal articles, color-deficient drivers: •
Have longer reaction times to signals
•
May confuse signal lights with street lights
•
Have shorter recognition distances, especially against a bright sky
•
Can mistake red lights for yellow (with greater luminance correlated with a greater error rate; this is because color-deficient drivers tend to rely on relative luminance to distinguish between red and yellow signals, so increasing the luminance of a red signal makes it more likely to be interpreted as being yellow)
Because approximately 4% of the population has color deficient vision, significant attention has been paid to this issue when specifying signal properties. Andersen (2002) noted that the specifications were particularly important for the long wavelength cutoff for red signals because the long wavelength end of the luminous efficiency function for protan observers overlaps only with the short wavelength end of the red LED spectrum as shown in Figure 1. He found that shifting the red LED loci by only 8 nm toward the longer wavelengths can result in a 21% difference in the visual signal provided to protan observers.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
a.
b.
Figure 2. a) Average reaction times and interquartile ranges for color-normal subjects; b) average reaction times and interquartile ranges for protan subjects (Huang et al., 2003). Huang et al. (2003) found in a series of experiments that for red and yellow traffic signals using LED and incandescent sources, protan observers had longer reaction times (Figure 2), a greater number of missed signals, and a greater number of color misidentifications than colornormal observers. Huang et al. (2003) tested red LED signals with different dominant wavelengths and concluded that the shorter dominant wavelengths improved detection among protan observers, but decreased the rate of correct color identification. The results suggested that the color boundaries specified by the ITE (1998) for each signal color might be improved if boundaries more consistent with recommendations from the Commission Internationale de l'Éclairage (CIE) were used; and this is the case for the current ITE (2005) specification for LED traffic signal colors. Starr et al. (2004) conducted a field study of green LED traffic signals when viewed under direct sunlight. When traffic signals are viewed under these conditions, they can appear to be lighted even when they are not (the sun phantom effect). Both color-normal and colordeficient observers can misread a signal under these conditions, but it is more common among the color-deficient group. Starr et al. installed fourteen green signals along a route in Minnesota. One of the signals was incandescent, while the rest were LED modules that varied by brand, lens type (tinted or clear), and LED technology (old technology with high LED count versus new technology with lower LED count). Subjects observed the modules while direct sunlight fell on them. The results showed that few (< 4%) color-normal reported that a green signal was on when it was not, but that many more (~25%) of the color-deficient participants falsely reported that a green signal indication was on. While there were variations in results between modules, no clear-cut advantages were identified among the signal modules tested by Starr et al. (2004). Fog Kurniawan et al. (2008) conducted a laboratory study of apparent brightness when subjects viewed LED lamps through a fog of water droplets in the laboratory. Subjects viewed LEDs of various colors through fogs of various water droplet sizes and reported the observed brightness level. The authors found that apparent brightness decreased as the fog droplet size increased, and that all colors were affected about equally.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
In their study of signal light brightness, Bullough et al. (2007) found that viewing signals through fog reduced the brightness enhancement of LED signals relative to incandescent signals under nighttime viewing conditions, primarily because scattered light from different light sources is superimposed on the signal images, reducing differences among different signal lights. Discomfort Bullough et al. (2001) studied the visual discomfort that results from viewing LED traffic modules at night. Based on their results and the 1998 interim LED traffic signal specifications (ITE, 1998), about 40% of the population would be expected to experience discomfort when viewing yellow and green LED signals at night, while red signals would not be expected to produce discomfort. Using the current specifications (ITE, 2005), the percentages for yellow and green would be reduced to about 20%, and for red would remain 0%. Reductions in luminous intensity for yellow and green signals by about 30% at night would be expected to reduce discomfort glare almost completely, while having little impact on the visibility of the signals (Freedman et al., 1985). Potential Future Research The research summarized above point to a number of areas where traffic signals could be improved through additional research and development. Several of these concepts have been patented, but are not in widespread practice. Behura (2007) indicated that LED signal modules should be replaced based on when they become too dim. To streamline the module testing process, a low cost photosensor could be built into each module. A signal indicating the luminous intensity could be transmitted, such as over a dedicated signal line to the control box or using radio frequency transmissions, to the maintaining agency. Behura (2007) also indicated that the power circuitry is now more likely to fail than the LED light engine itself. This indicates that research on methods to construct low cost, durable power circuitry and devices would be a fruitful way of decreasing the lifetime operating costs of LED modules. Extensive research has shown that color-deficient drivers have some difficulty detecting and correctly identifying traffic signals under all conditions and that color-normal drivers have difficulty when modules fall under direct sunlight (sun phantom effect). Shape-coding of traffic signal modules has been suggested as a countermeasure for helping overcoming difficulty in viewing by color-deficient observers, and the use of a flashing display is another (Whillans, 1983). Visibility during sun-phantom conditions could be improved by simply increasing (on a temporary basis) the luminous intensity during such conditions and poor ambient weather. Dynamic control of LED intensity results in smaller chromaticity shifts than can be achieved with incandescent lamps, and of course, reductions in intensity can be performed at night to reduce viewer discomfort in accordance with ITE (2005) specifications (provided this does not increase the potential for conflict monitors to have difficulty identifying when the dimmed signals are switched on).
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
Finally, the research by Starr et al. (2004) shows it would be useful to develop LED modules that do not permit (or at least reduce) the sun phantom effect. In addition to visors, this may be possible through controlling lens properties or the albedo of the back surface of the module. PHOTOMETRIC MEASUREMENT TECHNIQUES As described by Behura (2007), luminous intensity of LED traffic signal modules can degrade over time. In accordance with the ITE (2005) specifications for the photometric performance of LED signal modules, the ITE recommends that LED traffic signals be replaced when the intensity of the fixture no longer produces the minimum specified luminous intensity. In order to determine the luminous intensity, the ITE suggests monitoring signals over time using a calibrated light meter. In this manner, light measurements from the same signal could be compared over time to determine a percentage of degradation. The ITE (1998) points out that these relative measurements may not provide an accurate measure of absolute intensity. It can be difficult for transportation agencies to determine the performance of a traffic signal both in the field and in the shop, because most agencies do not have photometric measurement equipment and sending modules to a laboratory for testing can be expensive and time consuming. For this reason, the project team investigated several simple methods for measuring LED traffic signal luminous intensity. Two different types of light measuring instruments, an illuminance meter and a luminance meter, were used to estimate the luminous intensity of red and green traffic signal modules under laboratory conditions and under field conditions. Note that the same signals were used throughout the study so that the various methods could be readily compared. Illuminance Test Method Illuminance is a measure of the density of light falling on a surface (Rea, 2000). The luminous intensity of a traffic signal module can be determined by measuring the illuminance falling on a light meter (at a sufficient distance from the module) and then applying the inverse square law to determine the luminous intensity needed to produce the measured illuminance. This is the basis for the following test method. Materials Red and green 300-mm traffic signal modules, an illuminance meter calibrated to measure narrow-bandwidth spectra, a black-painted room capable of being completely dark, a tape measure, a tripod, and a level were used.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
Procedure The traffic signal module was secured to a table approximately 1.5 m above the floor and leveled such that the face of the signal module was perpendicular to the floor. The signal module was turned on for at least 30 minutes in order for the signal to thermally stabilize to the room temperature environment. The tape measure was extended along the floor from a point directly below the face of the signal to the furthest measuring distance used (approximately 15 m). The illuminance meter was attached to the tripod and the face of the illuminance meter was adjusted so that it was level with the center of the traffic signal. The tripod was then moved to each desired measuring distance. The lights of the room were turned off so that the only light in the room was the light being produced by the signal. A measurement of illuminance was then recorded, and then the lights were turned back on. This process of turning off the lights and recording the illuminance was repeated for every test distance listed in Tables 1 and 2. Using the measured illuminance from the signal modules at each distance, the luminous intensity of the signal could be determined by applying the inverse square law: Luminous Intensity (cd) = Illuminance (lx) × Distance² (m) Results Tables 1 and 2 summarize the results of the illuminance test method. Discussion The luminous intensity of a signal light in a particular direction is invariant as a function of distance from the signal light. Therefore after applying the inverse square law for every illuminance and distance combination in one direction, the luminous intensity value should be the same. The results show that this is the case. The reason this is important is that in order for the inverse square law to be applied correctly the light source must approximate a point source with light diverging from the source. A photometric rule of thumb (Rea, 2000) is that in order for the inverse square law to be applied with low error (
Replacement Processes for Light Emitting Diode (LED) Traffic Signals John D. Bullough Jeremy D. Snyder Aaron M. Smith Terence R. Klein Lighting Research Center, Rensselaer Polytechnic Institute Troy, NY
Contractor’s Final Report for NCHRP Project 20-07/Task 246 Submitted August 2009 National Cooperative Highway Research Program
ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies.
COPYRIGHT PERMISSION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FTA, Transit Development Corporation, or AOC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP.
DISCLAIMER The opinion and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the TRB, the National Research Council, AASHTO, or the U.S. Government. This report has not been edited by TRB.
NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
CONTENTS LIST OF FIGURES .................................................................................................................... iv LIST OF TABLES ....................................................................................................................... iv ACKNOWLEDGMENTS .............................................................................................................v ABSTRACT .................................................................................................................................. vi CHAPTER 1 Background ............................................................................................................1 CHAPTER 2 Research Approach ...............................................................................................2 CHAPTER 3 Findings and Applications ....................................................................................3 Research Review and Synthesis...........................................................................................3 Photometric Measurement Techniques ................................................................................9 LED Traffic Signal Failure Modes ....................................................................................17 CHAPTER 4 Conclusions, Recommendations, and Suggested Research .............................26 Conclusions ........................................................................................................................26 Recommendations for Replacement Strategies .................................................................27 Suggested Research ...........................................................................................................31 REFERENCES .............................................................................................................................33
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
LIST OF FIGURES Figure 1. Luminous efficiency functions for color-normal and protan observers, and spectral distribution from red incandescent and LED traffic signal modules (Andersen, 2002). Figure 2. a) Average reaction times and interquartile ranges for color-normal subjects; b) average reaction times and interquartile ranges for protan subjects (Huang et al., 2003). Figure 3. Block diagram of a typical LED signal module design. Figure 4. Arrays of 2 × 20 and 4 × 20 LEDs in parallel. Figure 5. Power resistor and smoke residue on nearby LEDs. Figure 6. Defective Schottky diode. Figure 7. Locations of failed LEDs.
LIST OF TABLES Table 1. Measured illuminance and calculated luminous intensity values for different measurement distances (red signal module). Table 2. Measured illuminance and calculated luminous intensity values for different measurement distances (green signal module). Table 3. Luminous intensity values estimated from luminance values using luminance test method 1 (red signal module). Table 4. Luminous intensity values estimated from luminance values using luminance test method 1 (green signal module). Table 5. Luminous intensity values estimated from luminance values using luminance test method 3 in an exterior, daytime environment (green signal module). Table 6. Long term annual costs for spot and group replacement strategies for an agency responsible for maintaining 100 signalized intersections, assuming a 7-year expected life. Shaded cells indicate when group replacement is estimated to be more costly than spot replacement. Table 7. Long term annual costs for spot and group replacement strategies for an agency responsible for maintaining 100 signalized intersections, assuming a 10-year expected life. Shaded cells indicate when group replacement is estimated to be more costly than spot replacement.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
ACKNOWLEDGMENTS The authors acknowledge sponsorship of this research from the National Cooperative Highway Research Program (NCHRP) of the Transportation Research Board. The authors would also like to gratefully acknowledge the City of Los Angeles, the Nebraska Department of Roads, the New Jersey Department of Transportation, the New York State Department of Transportation, and the Wisconsin Department of Transportation for providing traffic signal modules to the project team for evaluation.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
ABSTRACT This report documents and presents the results from a study of the photometric requirements, measurement and maintenance of traffic signal modules using light emitting diodes (LEDs). Differences between LED technology and the incandescent lamps used in previous traffic signal modules in terms of photometric performance, color, and failure modes require new approaches to traffic signal maintenance. Findings from a review of literature on human factors and maintenance practices, from a series of laboratory and field measurements of LED traffic signal modules, and from an analysis of the failure mechanisms of traffic signal modules provided by several different transportation agencies, are provided. Based upon these findings and upon an economic analysis with different assumptions regarding LED traffic signal module failure rates, some preliminary guidance for identifying when group replacement of LED signal modules is feasible is provided, and some possible avenues for future research are recommended.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
CHAPTER 1 BACKGROUND Effective January 2006, the Department of Energy legislated that signal manufacturers may only manufacture traffic signals that meet ENERGY STAR (2003) power requirements, effectively requiring the use of light-emitting diodes (LEDs) in traffic signal heads. In the absence of standards for maintenance of LED traffic signals, transportation agencies face a huge challenge in defining the life expectancy and creating their operational budgets for maintenance of LED traffic signals. As an initial step in developing maintenance guidelines for LED traffic signals, the National Cooperative Highway Research Program (NCHRP) established Project 207/246, "Replacement Processes for Light Emitting Diode Traffic Signals," to investigate methods for determining when LED traffic signal heads should be replaced, and to discuss possible specifications for LED traffic signals to maximize reliability and minimize maintenance costs. A recently-published NCHRP synthesis report, "LED Traffic Signal Monitoring, Maintenance, and Replacement Issues" (Urbanik, 2008), provides very useful background information to the reader. Urbanik (2008) reviewed the results of a survey conducted by the Institute of Transportation Engineers (ITE) in 2006, of transportation agencies regarding their experienced and practices with LED traffic signals. In general it was found that most agencies did not have a systematic replacement program for LED signal modules, nor was funding in place to monitor and (when necessary) replace modules not performing adequately. Some agencies have established replacement programs whereby modules are to be replaced on a group basis after a number of years in service, but the replacement periods can range from five years to longer periods depending upon the assumptions made by each agency. Urbanik (2008) also reported that one state agency (Louisiana Department of Transportation and Development) specified the use of light output monitoring in LED traffic signal modules, but that there were no modules available meeting such specifications. Because LED traffic signal module failure modes are very different from those of incandescent traffic signal modules, and because the expected operational lifetime of LED modules is so much longer than the operating life of incandescent lamps used in traffic signals, there is a lack of national consensus regarding the best practices for LED traffic signal replacement and for monitoring of signals. The objective of the present project is to identify some of the factors that are related to LED traffic signal module failure, and to discuss some of the steps that might be taken by transportation agencies in the maintenance of LED traffic signal systems.
1
NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
CHAPTER 2 RESEARCH APPROACH The project activities, documented in the present report, consisted of the following: •
A review and synthesis of recent human factors research relevant to the visibility and photometric performance of LED traffic signals.
•
Evaluation of methods for laboratory and field measurement of LED traffic signal photometric performance.
•
Investigation of failure modes of LED traffic signal modules.
•
Summary of considerations for departments of transportation (DOTs) in specifying, deploying and maintaining LED traffic signals.
The results of the aforementioned tasks are documented in Chapter 3 of this report, "Findings and Applications." Chapter 4, "Conclusions, Recommendations, and Suggested Research," provides some preliminary guidelines for DOTs regarding practices for maintenance and replacement of LED traffic signal heads.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
CHAPTER 3 FINDINGS AND APPLICATIONS RESEARCH REVIEW AND SYNTHESIS In the present review and synthesis of human factors research associated with the perception of colored signal lights such as traffic signals, emphasis is placed on recent research on reaction times and missed signals, discomfort glare, and brightness perception. An introduction briefly summarizes current maintenance practices. The present review focuses on research published after 1998, when the first interim specification for LED traffic signals was published by the Institute of Transportation Engineers (ITE, 1998). LED traffic signal modules have gained popularity in the U.S. primarily because they consume far less energy than incandescent lamps - 85% less, on average (Iwasaki, 2003). Because of these energy savings, the U.S Environmental Protection Agency (EPA) recognized LED traffic signal modules as an ENERGY STAR product in 2000, and then Congress mandated that as of January 2006, all red and green traffic signal modules must meet the energy consumption specifications stipulated by the ENERGY STAR requirements (ENERGY STAR, 2003). In addition to energy savings, the use of LED traffic signals has the potential to increase safety at intersections. The reduced power required to operate LED traffic modules has allowed the use of low cost battery backup systems at intersections, increasing safety in the case of blackouts; a $3,000 backup system can power an intersection for two to four hours (Iwasaki et al., 2003). Also, since the late 1990s, LED modules typically reach end of life by reduced light output, rather than complete failure, so even “failed” modules usually give some signal information to drivers (Behura, 2007). Maintenance Behura (2007) reported on a survey of LED traffic signal maintenance practices. He observed that the use of LED traffic signals had been expected to reduce the lifetime cost per module compared with incandescent modules. In addition to reduced energy expenditures, the extended lifetime of the LED modules had been expected to reduce relamping material and labor costs by reducing the frequency these costs will be incurred. Behura (2007) stated that many agencies had not implemented appropriate maintenance programs for LED signal modules. The survey showed that: •
35% have no replacement program
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35% are complaint driven (despite the fact that LED modules typically reach end of life due to dimming rather than complete failure)
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24% implement routine, scheduled replacement
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
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3% replace on vendor product life cycle
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3% replace based on in-service test results
Rather than relying on a passive maintenance scheme, Behura (2007) suggested several maintenance schemes from most to least precise: •
Remove modules from service and test light output in a laboratory
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Field measurements might provide good results, if they could be done properly
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Statistical analysis based on time since installation
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Replace based on the warranty period
Regardless of the maintenance practice, Behura (2007) recommended keeping a database of modules in the field including location, color, type, manufacturer and model, serial number, date of purchase, date of installation, and warranty end date. Behura also recommended that agencies clean lenses at intervals of one to two years (which would provide an opportunity for field measurements too). Visibility Perception and Reaction Time A study by Bullough et al. (2000) showed that under simulated daylight viewing conditions, LED and incandescent modules of the same nominal color, luminance, and onset time resulted in no statistically significant differences in mean reaction times, percentages of missed signals, color identification accuracy, and subjective brightness ratings. That study (Bullough et al., 2000) did find that reaction times and the percentage of missed signals decreased as luminous intensities (or luminances) increased, and that to obtain the same performance as a red traffic signal meeting then-current photometric requirements for luminous intensity for a 200-mm diameter module, yellow signals had to have a luminous intensity between 1.4 and 2.4 times higher than the red signal, and green signals had to have a luminous intensity between 2.4 and 2.8 times higher than the red signal. Freedman (2001) reported that yellow signals required a luminous intensity for the yellow about twice that of the red to obtain equal visual response, and that green signals required a luminous intensity of 1.3 times higher than red to obtain equal visual response. (Fisher and Cole [1975] recommended a 3:1 luminous intensity ratio for yellow:red and a 1.3:1 ratio for green:red). Taking these studies into account, the ITE recommended a luminous intensity ratio of 2.5:1 for yellow:red and 1.3:1 for green:red in its later specification for LED traffic signal performance (ITE, 2005). The only discrepancy among these studies is the higher ratio for green signals obtained by Bullough et al. (2000), which might be explained by the background light source used in their study, which had a correlated color temperature of about 3850 K, slightly lower than typical daylight illumination between 5500 and 6500 K (Wyszecki and Stiles, 1982). Regardless, since
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
the fundamental meaning of the green signal (i.e., "go") is different from that of the red and yellow signals (i.e., "stop"), it could be argued (Bullough, 2002) that equivalent reaction time and brightness for the green signal (relative to red) is not critical for driving safety. In the study by Bullough et al. (2000) lamp onset time was held constant (by using an electromechanical shutter). However, in the field, the onset time of incandescent traffic control signals is longer than that of LED sources (100 to 200 ms for incandescent, versus 13 to 32 ms for LEDs). Bullough (2005) reported the effect of this variation in onset time, and found that the differences in response time were very short. For example, the difference in response time for red traffic signals meeting the ITE (1998) specifications for luminous intensity, and having rise times of either 17 or 87 ms, was about 30 ms. The same comparison for yellow signals meeting ITE (1998) specifications yielded a response time difference of about 25 ms. Nor did rise time affect the consistency with which a signal was detected, so it was concluded by Bullough (2005) that onset time has little practical consequence for traffic signals. Cohn et al. (1998) confirmed that the visibility of red LED modules and red incandescent signal modules was about equal under daylight conditions, and concluded that the pixilated appearance of some LED signals might actually provide a visual benefit. This conclusion was confirmed in a subsequent study by Bullough et al. (2002) who found response times to a signal light consisting of an array of point sources, but with equivalent far-field luminous intensity as a diffuse signal light, were shorter than to the diffuse signal light. While it could be argued that performance metrics such as reaction time and missed signals are most important when considering traffic signals, some studies examined subjective metrics such as brightness and visibility. As described above, Bullough et al. (2000) found no statistically significant difference in perceived brightness between incandescent and LED lamps of the same luminance (for red, yellow, and green) under simulated daylight conditions, although the number of brightness judgments made in that study was relatively small. A study by Bullough et al. (2007) of green LED versus green incandescent signals viewed under nighttime conditions found that the LEDs appeared to be 1.4 to 1.7 times brighter, which is attributed to their saturation.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
Figure 1. Luminous efficiency functions for color-normal and protan observers, and spectral distribution from red incandescent and LED traffic signal modules (Andersen, 2002). Color Vision Deficiency According to a review by Cole (2004) of 124 journal articles, color-deficient drivers: •
Have longer reaction times to signals
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May confuse signal lights with street lights
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Have shorter recognition distances, especially against a bright sky
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Can mistake red lights for yellow (with greater luminance correlated with a greater error rate; this is because color-deficient drivers tend to rely on relative luminance to distinguish between red and yellow signals, so increasing the luminance of a red signal makes it more likely to be interpreted as being yellow)
Because approximately 4% of the population has color deficient vision, significant attention has been paid to this issue when specifying signal properties. Andersen (2002) noted that the specifications were particularly important for the long wavelength cutoff for red signals because the long wavelength end of the luminous efficiency function for protan observers overlaps only with the short wavelength end of the red LED spectrum as shown in Figure 1. He found that shifting the red LED loci by only 8 nm toward the longer wavelengths can result in a 21% difference in the visual signal provided to protan observers.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
a.
b.
Figure 2. a) Average reaction times and interquartile ranges for color-normal subjects; b) average reaction times and interquartile ranges for protan subjects (Huang et al., 2003). Huang et al. (2003) found in a series of experiments that for red and yellow traffic signals using LED and incandescent sources, protan observers had longer reaction times (Figure 2), a greater number of missed signals, and a greater number of color misidentifications than colornormal observers. Huang et al. (2003) tested red LED signals with different dominant wavelengths and concluded that the shorter dominant wavelengths improved detection among protan observers, but decreased the rate of correct color identification. The results suggested that the color boundaries specified by the ITE (1998) for each signal color might be improved if boundaries more consistent with recommendations from the Commission Internationale de l'Éclairage (CIE) were used; and this is the case for the current ITE (2005) specification for LED traffic signal colors. Starr et al. (2004) conducted a field study of green LED traffic signals when viewed under direct sunlight. When traffic signals are viewed under these conditions, they can appear to be lighted even when they are not (the sun phantom effect). Both color-normal and colordeficient observers can misread a signal under these conditions, but it is more common among the color-deficient group. Starr et al. installed fourteen green signals along a route in Minnesota. One of the signals was incandescent, while the rest were LED modules that varied by brand, lens type (tinted or clear), and LED technology (old technology with high LED count versus new technology with lower LED count). Subjects observed the modules while direct sunlight fell on them. The results showed that few (< 4%) color-normal reported that a green signal was on when it was not, but that many more (~25%) of the color-deficient participants falsely reported that a green signal indication was on. While there were variations in results between modules, no clear-cut advantages were identified among the signal modules tested by Starr et al. (2004). Fog Kurniawan et al. (2008) conducted a laboratory study of apparent brightness when subjects viewed LED lamps through a fog of water droplets in the laboratory. Subjects viewed LEDs of various colors through fogs of various water droplet sizes and reported the observed brightness level. The authors found that apparent brightness decreased as the fog droplet size increased, and that all colors were affected about equally.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
In their study of signal light brightness, Bullough et al. (2007) found that viewing signals through fog reduced the brightness enhancement of LED signals relative to incandescent signals under nighttime viewing conditions, primarily because scattered light from different light sources is superimposed on the signal images, reducing differences among different signal lights. Discomfort Bullough et al. (2001) studied the visual discomfort that results from viewing LED traffic modules at night. Based on their results and the 1998 interim LED traffic signal specifications (ITE, 1998), about 40% of the population would be expected to experience discomfort when viewing yellow and green LED signals at night, while red signals would not be expected to produce discomfort. Using the current specifications (ITE, 2005), the percentages for yellow and green would be reduced to about 20%, and for red would remain 0%. Reductions in luminous intensity for yellow and green signals by about 30% at night would be expected to reduce discomfort glare almost completely, while having little impact on the visibility of the signals (Freedman et al., 1985). Potential Future Research The research summarized above point to a number of areas where traffic signals could be improved through additional research and development. Several of these concepts have been patented, but are not in widespread practice. Behura (2007) indicated that LED signal modules should be replaced based on when they become too dim. To streamline the module testing process, a low cost photosensor could be built into each module. A signal indicating the luminous intensity could be transmitted, such as over a dedicated signal line to the control box or using radio frequency transmissions, to the maintaining agency. Behura (2007) also indicated that the power circuitry is now more likely to fail than the LED light engine itself. This indicates that research on methods to construct low cost, durable power circuitry and devices would be a fruitful way of decreasing the lifetime operating costs of LED modules. Extensive research has shown that color-deficient drivers have some difficulty detecting and correctly identifying traffic signals under all conditions and that color-normal drivers have difficulty when modules fall under direct sunlight (sun phantom effect). Shape-coding of traffic signal modules has been suggested as a countermeasure for helping overcoming difficulty in viewing by color-deficient observers, and the use of a flashing display is another (Whillans, 1983). Visibility during sun-phantom conditions could be improved by simply increasing (on a temporary basis) the luminous intensity during such conditions and poor ambient weather. Dynamic control of LED intensity results in smaller chromaticity shifts than can be achieved with incandescent lamps, and of course, reductions in intensity can be performed at night to reduce viewer discomfort in accordance with ITE (2005) specifications (provided this does not increase the potential for conflict monitors to have difficulty identifying when the dimmed signals are switched on).
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
Finally, the research by Starr et al. (2004) shows it would be useful to develop LED modules that do not permit (or at least reduce) the sun phantom effect. In addition to visors, this may be possible through controlling lens properties or the albedo of the back surface of the module. PHOTOMETRIC MEASUREMENT TECHNIQUES As described by Behura (2007), luminous intensity of LED traffic signal modules can degrade over time. In accordance with the ITE (2005) specifications for the photometric performance of LED signal modules, the ITE recommends that LED traffic signals be replaced when the intensity of the fixture no longer produces the minimum specified luminous intensity. In order to determine the luminous intensity, the ITE suggests monitoring signals over time using a calibrated light meter. In this manner, light measurements from the same signal could be compared over time to determine a percentage of degradation. The ITE (1998) points out that these relative measurements may not provide an accurate measure of absolute intensity. It can be difficult for transportation agencies to determine the performance of a traffic signal both in the field and in the shop, because most agencies do not have photometric measurement equipment and sending modules to a laboratory for testing can be expensive and time consuming. For this reason, the project team investigated several simple methods for measuring LED traffic signal luminous intensity. Two different types of light measuring instruments, an illuminance meter and a luminance meter, were used to estimate the luminous intensity of red and green traffic signal modules under laboratory conditions and under field conditions. Note that the same signals were used throughout the study so that the various methods could be readily compared. Illuminance Test Method Illuminance is a measure of the density of light falling on a surface (Rea, 2000). The luminous intensity of a traffic signal module can be determined by measuring the illuminance falling on a light meter (at a sufficient distance from the module) and then applying the inverse square law to determine the luminous intensity needed to produce the measured illuminance. This is the basis for the following test method. Materials Red and green 300-mm traffic signal modules, an illuminance meter calibrated to measure narrow-bandwidth spectra, a black-painted room capable of being completely dark, a tape measure, a tripod, and a level were used.
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NCHRP Web-Only Document 146: Replacement Processes for Light Emitting Diode (LED) Traffic Signals
Procedure The traffic signal module was secured to a table approximately 1.5 m above the floor and leveled such that the face of the signal module was perpendicular to the floor. The signal module was turned on for at least 30 minutes in order for the signal to thermally stabilize to the room temperature environment. The tape measure was extended along the floor from a point directly below the face of the signal to the furthest measuring distance used (approximately 15 m). The illuminance meter was attached to the tripod and the face of the illuminance meter was adjusted so that it was level with the center of the traffic signal. The tripod was then moved to each desired measuring distance. The lights of the room were turned off so that the only light in the room was the light being produced by the signal. A measurement of illuminance was then recorded, and then the lights were turned back on. This process of turning off the lights and recording the illuminance was repeated for every test distance listed in Tables 1 and 2. Using the measured illuminance from the signal modules at each distance, the luminous intensity of the signal could be determined by applying the inverse square law: Luminous Intensity (cd) = Illuminance (lx) × Distance² (m) Results Tables 1 and 2 summarize the results of the illuminance test method. Discussion The luminous intensity of a signal light in a particular direction is invariant as a function of distance from the signal light. Therefore after applying the inverse square law for every illuminance and distance combination in one direction, the luminous intensity value should be the same. The results show that this is the case. The reason this is important is that in order for the inverse square law to be applied correctly the light source must approximate a point source with light diverging from the source. A photometric rule of thumb (Rea, 2000) is that in order for the inverse square law to be applied with low error (
