Color blindness

THE MINNESOTA DOT RECEIVED A COMPLAINT FROM A COLOR-BLIND PERSON THAT, UNDER DIRECT SUNLIGHT REFLECTION, HE COULD NOT TELL WHETHER THE RED OR GREEN LED INDICATION WAS ON. A FIELD TEST CONFIRMED THAT SOME LED GREEN INDICATIONS COULD BE PERCEIVED ERRONEOUSLY DURING DIRECT SUNLIGHT CONDITIONS BY RED-GREEN COLOR-BLIND TRAVELERS.

BACKGROUND

Light-emitting diode (LED) traffic signal indications have become increasingly popular in the United States in the last 10 years. The improved technology and significant energy savings give LEDs an advantage over traditional incandescent traffic signal indications.

In Minnesota as well, the use of LED indications is on the rise, primarily for red and green indications. Many jurisdictions are requiring LED indications in all new signal designs.

The Minnesota Department of Transportation (Mn/DOT) received a complaint from a color-blind person about a change in the appearance of traffic signals over the last few years. Specifically, green LED traffic signal indications in Washington County were mentioned. According to the complaint, in direct sunlight, the green indication appeared to be on when it was not, especially near the spring and fall equinox.

Mn/DOT contacted Washington County to discuss the issue. Washington County has been upgrading to LED signal indications since 1998. The county currently requires all new signal installations to be LED. In 2002, Mn/DOT and Washington County conducted a preliminary investigation, which indicated that the most common type of green LED in the county has a clear lens.

Although it is not green in color, the light reflection of the unlit green LED could appear brighter than either the lit red or amber indication under direct sunlight. For a person with a color vision deficiency, brightness-not color-is the most important cue.

Figure 1 shows the appearance of signals under direct sunlight and without color information. In both cases, the red (top) indication is on. However, this is difficult to discern without color information.

To further investigate the issue, Washington County installed several different LED signal head designs on Valley Creek Road. To understand the problem and better serve the general traveling public, the Mn/DOT Office of Traffic, Security and Operations (OTSO) initiated an internal study on color-blind vision issues related to LED traffic signal indications, utilizing the Valley Creek Road test site (see Figure 2).

LITERATURE REVIEW

There is literature regarding LED signal indications, but none of the literature directly addresses color blindness related to green LED traffic signal indications.

In a study by Bullough et al., it was found that green signals resulted in the longest reaction time for non-color-blind subjects, compared with red and yellow indications.1 Huang, Bullough, Boyce and Bierman also conducted a study of red and yellow traffic signals involving protans-people with a color deficiency involving long wavelengths of light.2

King found that signal color was the most important factor affecting perception of the signal indication and that green signal indications resulted in the poorest response during a daytime test.3 Several studies on traffic signals that do not apply directly to color-blind perception of LED signals also have been conducted.4-8

In his study, Whillans used 100-watt bulbs for signal recognition tests with both normal and color-blind subjects.9 He also conducted a survey through a formal questionnaire and informal interviews. He concluded that "color-blind drivers compensate well and drive as safely as drivers with normal vision."

In 2003, Andersen presented an overview to the Commission Internationale De L'Eclairage (CIE) on development of LED standards based on human performance in the United States.10 The current ITE requirement for LED traffic signal luminous intensity is at ratios of 1:2.5:1.3. "The LED signal provided a much more saturated color than the filtered incandescent signal."

Traffic signal color requirements in the United States are different from CIE recommendations. The ITE green traffic signal color requirement is "to provide a more uniform green signal light, with better detection and recognition (by color-blind drivers)."

In "The Use of LEDs in Road Transport," Jenkins summarized current research and pointed out that "the effects of sun-phantom on LED signals with colored covers or lenses is visually much the same as for incandescent signals."11

FIELD TEST

Goals and Objectives

The objectives of the field test were to answer the following questions:

* Does color blindness affect the perception of the LED green indication?

* Are any of the available designs better than other designs?

* Is the best solution for people with color blindness acceptable for people who do not have color blindness?

The goal of the study was to help the OTSO signal unit write the most appropriate specifications for purchasing LED traffic signals, which many agencies in Minnesota (including Washington County) utilize for traffic signal project development.

Test Site

The field test site consisted of seven intersections on Valley Creek Road in Woodbury, MN, USA. The intersections are in the westbound direction from Interlachen Parkway to Valley Creek Plaza on a two-way, four-lane divided highway. The road is oriented directly east and west except for the intersection at Bielenberg Drive, at which the road angles in a northwest-southeast direction. At Bielenberg Drive, both indications remained as the LED types that were in place prior to the test. At the Valley Creek Plaza intersection, the right lane signal was partially shaded by a tree.

Test Subjects

All test subjects were volunteers recruited from Mn/DOT. Four colorblind volunteers and four non-colorblind volunteers participated in the test. The age of the volunteers ranged from 22 to 54 (two were 20-29, one was 30-39, three were 40-49, one was 54 and one did not report). All four color-blind participants were male; three non-colorblind participants were male. The female participant was not color blind.

Tested Traffic Signal Designs

Six different designs of green LED indications and one green incandescent traffic signal indication were tested at the seven intersections. All the signals were mounted overhead, directly over the center of the left and right through lanes. The differences among the tested green LED indications can be summarized into the following categories:

* Green tinted lens versus clear lens

* Old technology with high LED count versus new lens designs (new "incandescent look" technology with lower LED count)

* Two different brands

Test Questions

The Mn/DOT market research group assisted in developing the test questions for the study. At each intersection, an assistant asked each volunteer the following three questions:

* For the signal over the left lane, does the green light on the bottom appear to be lit?

* For the signal over the right lane, does the green light on the bottom appear to be lit?

* Do you have any comments about the appearance of these two green lights?

Answers to the first two questions were either "yes," "no," or "cannot tell." The questions were asked concerning the green light when the red light was lit, when the green light was unlit and when the signal head was in direct sunlight.

Test Conditions

The test was performed shordy after die spring equinox early in die morning, so that the sun was low in the sky direcdy behind the observer and was shining directly on the signal in the westbound direction.

Test Procedures

The tests were performed over three days from April 8 to April 10, 2003. There were three people in each test vehicle. The test participant sat in the passenger seat, a driver sat in front and an interviewer sat in the back. Two test vehicles traveled one behind the other as one fleet in the right lane. Washington County provided a shadow truck with an arrow board and a traffic impact attenuator to follow the test vehicles.

The test vehicle stopped near each intersection where a good sun reflection could be seen. The test vehicles remained in place while the interviewer asked the questions and recorded each volunteer's responses. If the light changed before the volunteer finished answering, the vehicles remained in place until the next cycle.

TEST RESULTS

Eight volunteers observed two signals at each of seven intersections, for a total of 112 observations. Table 1 provides the total summaries of the first two questions for each tested intersection and for both right and left traffic signal indications.

Less than 4 percent of non-colorblind participants saw a green light on when it was not; 25 percent of colorblind participants saw a green light on when it was not. Table 2 presents the totals for each individual signal head.

Table 3 presents the observations of misperceived green signals (appeared on) grouped by signal design feature. The left and right lane observations are shown to verify that observation angle differences in the two lanes did not influence the outcome.

About 10 percent of males have redgreen color blindness. Therefore, assuming a 50-50 split between males and females in the total population, about 5 percent of the total population has redgreen color blindness. A very coarse approach to looking at the overall result is to provide a weighted total, to which color-blind observations make a 5-percent contribution and non-color-blind observations make a 95-percent contribution. The weighted total is calculated as shown in equation 1. This weighted total is included in Table 3.

The participants also provided comments about the appearance of the two signals at each intersection (question 3). In general, they provided comments if they observed any differences between the signals. Some provided additional comments regarding the left signal; some provided comments regarding the right signal.

Table 4 summarizes the comments the participants provided for each intersection. The table reports what the participants said. The participants were not always precise as to which signal (left or right) at the intersection their comments applied to.

DATA ANALYSIS

There were differences between color-blind and non-color-blind participants perceiving the green LED as on when it was not lit. For each factor in the LED signal design, the type that had the highest percentage of wrong perception was opposite for the color-blind participants compared with the non-colorblind participants.

There were no misperceptions regarding the incandescent green signal. However, there was only one incandescent signal among the 14 tested signals. One of the tinted, new technology, brand A signals and one of the clear, old technology, brand B signals also had no misperceptions. All of the clear, old technology, brand A signals were misperceived at least once.

The participants were told that the light would be red and could see the light turning red. Their perceptions of the green being lit or not lit may have been influenced by this knowledge. Therefore, this may not reflect real world driving experience.

SUMMARY OF RESULTS

The test did confirm that people with red-green color blindness could perceive some common LED green signals erroneously during direct sunlight conditions. Color-blind and non-color-blind travelers perceived LED green indications differently.

The test also indicated that design makes a difference in perception. One design was better for color-blind participants and another was better for noncolor-blind participants.

The best solution for color-blind travelers was not the best solution for noncolor-blind travelers. More study would be required to determine conclusively whether any of the designs are acceptable.

The green LED traffic signal test was conducted under direct sunlight in the field. There were four color-blind and four non-color-blind participants. The following was observed:

* The misperception of the green LED was different between color-blind and non-color-blind participants.

* Color-blind and non-color-blind participants were opposite in terms of how three factors affected their perception of the green signal: clear lens versus tinted lens, old technology versus new technology and brand A versus brand B.

The above observations were not statistically analyzed based on the following considerations:

* The small sample size of participants.

* All volunteers were from Mn/DOT, rather than a random representation.

* The participants knew the light was not lit when they were asked if it appeared lit.

* The various signal configurations were not equally represented at the test site.

This study will contribute to Mn/ DOT's considerations in specifying the design of green LED traffic signal indications for procurement. Mn/DOT specifies the use of green tinted lenses. It allows the use of both old and new technology in products as supplied as brand A and brand B, as well as other brands of LED traffic signal indications.

References

1. Bullough, J. et al. "Response to Simulated Traffic Signals Using Light-Emitting Diode and Incandescent Sources." Transportation Research Record, No. 1724 (1998): 39-46.

2. Huang, M.K., J. Bullough, ER. Boyce and A. Bierman. "Detection and Identification of LED Traffic Signals by Protan Observers." Transportation Research Record, No. 1844 (2003): 52-58.

3. King, G. "Visibility of Circular TrafficSignal Indications." Transportation Research Record, No. 811 (1981): 7-13.

4. Bullough, J. et al. "Traffic Signal Luminance and Visual Discomfort at Night." Transportation Research Record, No. 1754 (2001): 42-47.

5. Bullough, J. "Are Yellow Traffic Signals Brighter Than They Need To Be?" Lighting Design and Application, Vol. 32, No. 12 (2002): 18-21

6. New York State Energy Research and Development Authority. "Evaluation of NYSDOT LED Traffic Installation" Accessible via www.nyserda.org/.

7. International Commission on Illumination. "Light Signals for Road Traffic Control." Commission Internationale De L'Eclairage (CIE) (Publication No. 48 (TC-1.6)), 1980

8. Bullough et al., note 1 above.

9. Whillans, M.G. "Colour-Blind Drivers' Perception of Traffic Signals." Canadian Medical Association Journal, Vol. 128, No. 10 (May 1983): 1187-1189.

10. Andersen, C.K. "Development of a U.S. Standard for Direct Emitting Traffic Signal Lights." Paper presented at CIE, 2003.

11. Jenkins, S. "The Use of LEDs in Road Transport." Report presented at CIE, 2003.

BY RAY A. STARR, P.E., PTOE, WAYNE H. SANDBERG, P.E. AND YUZHE (RACHEL) GUAN, P.E., PTOE

RAY A. STARR,

P.E., PTOE, is the traffic electrical systems engineer at the Minnesota Department of Transportation. He provides technical support and standards development for traffic signals, roadway lighting and intelligent transportation systems for the state of Minnesota. He holds a bachelor of electrical engineering from the University of Minnesota. He is a member of ITE.

WAYNE H. SANDBERG,

P.E., is the transportation engineer for Washington County, MN, USA. He currently is on the Board of Directors of the North Central Section of ITE and has served on numerous traffic engineering related committees. He has a B. S. in civil engineering from the University of Minnesota. He is an associate member of ITE.

YUZHE (RACHEL) GUAN,

PE, PTOE, is the traffic electrical standards engineer at the Minnesota Department of Transportation. She graduated from the University of Minnesota with a B. S. and an M.S. in civil engineering. She is an associate member of ITE.

Copyright Institute of Transportation Engineers Aug 2004
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