NCHRP National Cooperative Highway Research Pro~ram RESEARCH REsULTS DIGEST April 1995 Number 205 These Digests are issued in the interest of providing an early awareness of the research resulis emanating from projects in the NCHRP. By making these results known as they are developed, it is hoped tha1 the potential user.; of the research findings will be encouraged loward their early implementation in operating practices. Persons wanting co pursue the project subject maner in grealcr depth may do so through contact with the Cooperative Research ProgrJms Scaf!, Transponacion Research Board, 2101 Constirution Al'e., N.W, Washingion, D.C, 20418. Subject Area: IV A Highway Operations, Capacity, and Responsible Senior Program Officer: B. Ray Derr Traffic Control Requirements for Application of Light Emitting Diodes (LEDs) to Traffic Control Signals "-..,.1· ' ) An NCHRP digest of the findings from the final report on NCHRP Project 5-12, "Requirements for Application of Light Emitting Diodes (LEDs) to Traffic Control Signals," conducted by Lighting Sciences, Inc. Mr. John Corbin, Mr. Michael Janoff. and Dr. Ian Lewin served as Principal Investigators. INTRODUCTION LED technology is rapidly evolving, and the research project evaluated the technology available Incandescent lamps are presently the major at the time. Some of the findings and recommend­ illumination source for traffic signals. The incan­ ations will undoubtedly become obsolete as the descent lamps used for traffic signals are, however, technology progresses. inefficient compared with other light sources. Maintenance costs are high for these lamps, and the light output degrades as the lamp ages. These ADHERENCE TO COLOR STANDARDS problems-plus the ever-increasing cost of energy­ justify considering other light sources. One The Institute of Transportation Engineers (ITE) alternative is light emitting diodes (LEDs). standard, "Vehicle Traffic Control Signal Heads," is Before existing incandescent lamps are replaced commonly used throughout the United States. This by LEDs, it is necessary to show that LED signals standard defines the required color for new traffic will meet applicable standards for color and signals. intensity, will not adversely affect the safety or An LED's color is usually referred to by the operation of the roadway, and will be economically peak wavelength of the spectral distribution. This advantageous. NCHRP Project 5-12 was initiated in can be misleading. The other colors in the response to these needs. The objective of the distribution cause the dominant wavelength (the research was to determine the feasibility and apparent color) to differ from the peak wavelength. implementation potential of LEDs. The project For this reason, LEDs should be plotted on a found that red and Portland Orange (pedestrian) chromaticity diagram (Figure 1) to verify whether LED signals are currently feasible. they meet the color requirements. This digest summarizes the project findings and Red. At this time, the most common red LEDs includes testing results, a discussion of economic consist of Gallium-Aluminum-Arsenide (known as analysis, specification guidelines, recommendations GaAlAs or AlGaAs) and emit a 660-nm peak for LED use, and suggested future research. The wavelength. These fall around 640-nm dominant digest will be of interest to those involved in wavelength on the chromaticity diagram. Generally designing and specifying traffic signals. all of the 660-nm LEDs tested will fall within the TRANSPORTATION Rli5EARCH BoARD NA"nONAL RF.SEARCH COUNCIL 6 -.. 0-+-----1-------1--------t----;----------t------t -30 -20 -10 0 10 20 30 Horizontal Angle Type of Signal Head • ITE Req. u 30° LEDs + 8° LEDs * 8° LEDs w/ Prism. Lens Figure 4. -12.5° vertical viewing angle. 140 --·--· ------· 120 ------1--- 'ii, lil 100 ----- ···-··- "U !a -9. >, 1i m -C 04--- ---+-----+-----+------,,-----+----I -30 -20 -10 0 10 20 30 Horizontal Angle Type of Signal Head • ITE Req. a 30° LEDs + 8° LEDs * 8° LEDs w/ Prism. Lens Figure 5. -17.5° vertical viewing angle. 7 • 30/30 1.00 - Ill D 40/20 <o 0.95 .n • 50/10 ,_ OD .~. o 75/15 0.90 --p-0•-• ·-- t Do •• • 0 D • 0.85 0 ~ D ~-. •••• •• • D •••• ~ 0.80 - C/1 " Do •••••••••• • C •0 D • GI 0 • ... an • • • •i ~ .5 0.75 • GI -o - c.t. Dao ~ ooDoooooo ai C. n D 0::: 0.70 u ... o t•~t.o - - 0 a ·~·<f t·..t~ •• 0.65 VQ " 0 • . • o• ~ 0 • 0 • 0.60 0.55 I I I I I I I 0.50 I I I I 'I I I I I 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 minutes Figure 6. Output vs. time for 12" red signal-three duty cycles. 8 5. The use of visors on LED signals normally an orange pedestrian-hand symbol. Typical costs for decreases their visibility while the opposite is true some types of heads are 12" red ball-$230; 8" red for incandescent signals. ball-$190; 12" red arrow-$115; and orange 6. Visibility and conspicuity are directly related pedestrian-hand symbol-$250. On the other hand, to the signal intensity.. The required intensity for a an incandescent krypton bulb costs about $2. As a red ball LED signal seems to be in the range of 300 market is established for LED signals, the initial cost to 500 candelas. Red arrows appear to require about will undoubtedly decrease. 44 additional candelas of intensity. Yell ow ball LED signals probably require about twice the Power Savings. LED signals consume much .. intensity as a red ball LED signal and green ball less power than incandescent signals. The reduced LED signals require about 2 1h times the red ball cost of power can be estimated using the following signal intensity. equation: 7. Improving the light distribution of an LED signal can significantly reduce the intensity A = 8760*D*(PrPL)*C, where requirements. The intensity required for red ball A is the annual power cost savings ($/yr), LED signals can be reduced by about 26 percent to 8760 is the number of hours in a year (hr/yr), 52 percent if the distribution is doubled (from a D is the duty cycle (percent time on), viewing angle of 7° to 15°). Yellow and green LED P1 is the incandescent power consumed (kW), signals do not show as dramatic an improvement. PL is the LED power consumed (kW), and 8. The results for red signals are believed to be C is the cost per kilowatt-hour ($/kWhr). much more accurate and valid than for the other two colors. Similarly, the results for ball signals are Some typical examples of annual power savings are believed to be more accurate and more valid than for shown in Table 1. arrow signals. This is partly attributed to the larger amount of data available for the red ball signals. Life. LED signals will last longer than 9. Color-vision deficient drivers and normal incandescent signals though good estimates of vision drivers seem to have similar error patterns expected life are not yet available. Incandescent and reaction times, but the small number of color­ bulbs are replaced every 6 months to 1 year. The vision deficient drivers makes generalizations life of the LED signal is a critical factor because somewhat tenuous. calculations using a 5-year life can justify LED 10. Performance at night appears to be similar to signals not justified using a 3-year life. daytime performance. Overall, nighttime If an entire intersection could be converted to performance was better than daytime performance. LED signals, there would be significant savings 11. The red ball, red arrow, and Portland because semiannual relampings would not need to be Orange pedestrian signals appear to perform as well done. This would reduce the amount of preventive as incandescent signals, both during the day and at maintenance required and the number of trips. The night. lack of a satisfactory green LED makes this unlikely in the near future. Analysis. One method of analysis is to ECONOMIC ANALYSIS determine the payback period. This is the amount of time over which the savings in operating costs will Justification for use of LED signals must depend equal the increased initial cost. Figure 7 shows a on an economic analysis that demonstrates a cost payback-period analysis for the four most promising savings. This analysis should consider the following LED applications. factors. While the payback-period analysis method is useful, it fails to consider the time value of money. Initial Cost. LED signals are very expensive Another approach is to convert the annual cost relative to incandescent signals. The cost of a single savings over the life of the signal into present day lens can range from $115 for an arrow to $430 for dollars and then compare that to the initial cost. 9 TABLE 1 Annual power cost savings of LED signals Duty Cycle 12" Red Ball 8" Red Ball 12" Red Arrow Orange Ped Signal 50% $54.3 $24.8 $57.5 $42.8 60% $65.1 $29.8 $69.0 $51.3 70% $76.0 $34.8 $80.5 $59.9 80% $86.8 $39.7 $92.0 $68.4 LED Power 17 W 15 W lOW 23 W Consumption Incandescent Power 135 W 69W 135 W 116 W Consumption Power is assumed to cost $0.105/kWhr. 7 en <ii Q) >- 2 0.45 0.5 0.6 0.65 0.75 0.8 0.85 Duty Cycle Type of Signal Head • 12" red ball o 8" red ball + 12" red arrow * orange ped. signal Figure 7. Payback period for LED traffic signal heads. 10 SPECIFICATION GUIDELINES specified (e.g., "signal output at nominal voltage and room temperature conditions shall not fall below If a jurisdiction chooses to use LED signals, it 80 % of intensity requirements before 1000 hours of should consider the following points in developing field operation").