National Academies Press: OpenBook

Selection and Application of Warning Lights on Roadway Operations Equipment (2008)

Chapter: Chapter 4 - Performance Experiment

« Previous: Chapter 3 - Static Screening Experiment
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
×
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
×
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
×
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
×
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
×
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Suggested Citation:"Chapter 4 - Performance Experiment." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
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22 presented at the same distances without the truck as a baseline measurement for pedestrian detection. Glare. The glare independent variable provided insight into the ability of the warning-light system to be visible when other (and particularly opposing) vehicles are present. The glare was simulated using the headlights of a parked vehicle close to the experimental truck. Weather. The weather independent variable provided insight on which type of warning light performs best in real- istic weather conditions. The three levels were dry, rain, and fog. Rain and fog were kept consistent among participants by using a weather-making system to control the levels. Ambient Lighting. The ambient lighting independent variable provided insight on which type of warning light performs best in daytime and nighttime conditions. Assignment of Treatments Treatments were balanced across age and gender to elimi- nate presentation bias among the groups. A balanced Latin Square was used to create the orders of presentation for each driving session. Eight orders were used for nighttime sessions, with each participant within an age and gender group receiv- ing a different order. Four orders were used for daytime ses- sions, with two participants within an age and gender group receiving the same condition. Dependent Variables Seven dependent variables were used in this investigation. The dependent variable of lane-change distance was tested only during the uninformed trial, which was at night in clear weather. The dependent variables of vehicle identification distance, pedestrian detection distance, and discomfort glare C H A P T E R 4 Performance Experiment From the results of the static screening experiment, four warning-light configurations were selected for further testing in dynamic conditions. Three measures considered the nighttime identifiability of a maintenance vehicle with warning lights; the detectability of a pedestrian standing close to the maintenance vehicle; and the ranking of the vehicle lighting in terms of discomfort glare, attention-getting, and urgency. The study in- cluded an uninformed trial where participants viewed the light- ing systems without knowing the nature of the study and also incorporated adverse weather in the testing conditions. Experimental Methods Experimental Design The choice of independent variables was driven by the results of the static screening experiment, and the need to provide realistic test scenarios that drivers are likely to encounter in everyday driving. The experiment included between-subjects variables (gender and age, each with two levels) and within- subject variables (warning light and pedestrian, each with four levels; glare and ambient lighting, each with two levels; and weather with three levels). Because of the large number of vari- ables, a mixed-factor partial factorial design was used to allow exploration of the most relevant main effects and interactions. The final experiment design resulted in a total of 116 conditions for each participant: 40 during the day, and 76 during the night. Within-Subject Variables Warning Light. Four warning light configurations were used: high-mounted rotating beacon, low-mounted rotating beacon, LED, and strobe. Pedestrian. The pedestrian was presented at two different distances from the experimental truck to see how each light affected the visibility of the pedestrian. A pedestrian was also

rating were tested only during nighttime conditions (night dry, night rain, and night fog). The dependent variables of attention- getting rating and confidence rating were tested only dur- ing daytime conditions (day dry and day fog). The dependent variable of urgency rating was tested during all nighttime conditions and during daytime fog conditions. Lane-Change Distance. During the uninformed trial, participants were forced to pass the slow-moving dump truck (experimental vehicle), unaware that the truck was involved in the study. The distance at which the participant initiated the lane change to pass the truck was marked as the lane-change distance. Vehicle Identification Distance. To establish each warn- ing light’s ability to alert a driver to the presence of a mainte- nance vehicle, the distance at which a participant could identify the light source as belonging to a vehicle was recorded. The dis- tance traveled between this point and when the participant vehicle passed the experimental truck was defined as the vehi- cle identification distance for the warning light on display. This measurement was taken during each nighttime condition. Pedestrian Detection Distance. To establish each warn- ing light’s ability to allow a driver to see maintenance workers near a maintenance vehicle, the distance at which a participant could detect a pedestrian standing near the experimental dump truck was recorded. The pedestrian detection distance for the warning light on display was defined as the distance traveled from this point to the point at which the participant vehicle passed the pedestrian. This measurement, along with a base- line measurement, was taken during each nighttime condition. Urgency. The urgency rating dependent variable mea- sured the level of urgency that subjects felt was conveyed by the warning lights. A five-point Likert-type rating scale with end points “not at all urgent” and “totally urgent” was used to capture their ratings. This scale was administered during each nighttime condition and the daytime fog condition. Discomfort Glare. The discomfort glare rating dependent variable measured the discomfort experienced by the subjects when presented with the warning lights. A nine-point rating scale with end points “not noticeable” and “unbearable” was used to capture their ratings. This scale was administered only at nighttime when discomfort glare is at its worst due to the high degree of contrast between the lights and their backgrounds. Confidence. The confidence rating dependent variable measured the confidence level of the subjects that they could see the warning light. The scale was a five-point Likert-type rating scale with end points “not at all confident” and “totally confident.” This scale was administered only during daytime conditions when there was a low degree of contrast between the lights and the background. Attention-Getting. To establish the effectiveness of the warning lights, a metric for conspicuity was established. This was done using a seven-point rating scale ranging from “not at all attention getting” to “extremely attention getting.” This scale was administered during daytime conditions. Participants Thirty-two subjects, 16 males and 16 females, were selected to participate in this study. The subjects were evenly selected from two age groups of 25 to 35 years of age and 65+ years of age. IRB approval was obtained prior to recruiting sub- jects. When subjects arrived for the first session, they signed an informed consent form before beginning any experimen- tal activities. Subjects were paid $20/h for each driving session, and a $30 bonus if they completed all four driving sessions. They were allowed to withdraw at any point in time, with compensation adjusted accordingly. Apparatus Test Road The experiment took place on the Smart Road—a 2.2-mile- long controlled-access, two-lane road. A 0.5-mile section of the Smart Road is equipped with an artificial weather-making system that was used to create the rain and fog conditions for this study. A degree of control was attained by not allowing pub- lic vehicles and pedestrians to enter the Smart Road and by con- trolling the level of the rain and fog so that it was consistent among participants. Test Vehicles Like the static experiment, two test vehicles were used in this experiment: a participant vehicle and an experimental vehicle. The experimental vehicle was the same VDOT dump truck that was used in the static experiment. However, the dump truck was outfitted with the four warning lights of interest (Figure 4). The high beacon lights were placed above the cab of the truck, one on each side. The low beacons were placed on small shelves on the back of the tailgate, one on each side. The strobes and LEDs were mounted on a rack on the tailgate, one on each side. All lights were manually controlled by an operator who sat in the cab of the truck. There was radio communication between the participant vehicle and the experimental vehicle 23

that allowed the in-vehicle experimenter to prompt the light operator for the next light. The participants drove a 2002 Cadillac Escalade; an in-vehicle experimenter rode along to provide directions and to record data. Vehicle identification distance and pedestrian detection distance were recorded using a data acquisition system (DAS) installed in the vehicle. Attention-getting, discomfort glare, urgency, and confidence ratings were recorded by hand on an order sheet and later entered into a spreadsheet. The DAS recorded vehicle network data such as accelera- tion and speed, as well as four camera angles and information entered by the in-vehicle experimenter such as the partici- pant number, participant age, experimental order, and button presses. Pedestrian The pedestrian was an on-road experimenter who wore denim surgical scrubs and a VDOT-issued reflective safety vest. Depending on the presentation order, the pedestrian would either stand 40 ft or 80 ft behind the dump truck in the center of the lane for each lap driven by the participant. When the participants verbally indicated that they could see the pedestrian, the in-vehicle experimenter would say “clear” over the radio. This was the pedestrian’s signal to clear the road. If for any reason the in-vehicle experimen- ter did not give the clear signal, the pedestrian would clear the roadway automatically when the vehicle reached a pre- determined proximity. Once the participant vehicle turned around and passed the dump truck on the way back to the top of the road, the pedestrian would get into position for the next lap. A baseline pedestrian was also presented several times for each participant. This pedestrian would follow the same pro- cedures, but stood on a section of road away from the dump truck. This allowed for a comparison of detection distances. Stimuli The stimuli used for this experiment were commercially available light sources acquired from manufacturers. The light sources were selected based on their performance in the static screening experiment, photometric characteristics, and the suitability for the experiment. High-Mounted Beacon. The high-mounted beacon used was a PSE Amber, model 550 FRAMH 12 V 100 W. A sum- mary of the rotating beacon light characteristics is provided in Appendix C1. Low-Mounted Beacon. The low-mounted beacon used was also a PSE Amber, model 550 FRAMH 12 V 100 W, also provided in Appendix C1. LED. The LED used was an amber Whelen 500 Linear LED Flash Light. One light on each side of the truck was used. The LEDs were displayed in a 1 Hz asynchronous pattern. A summary of the LED light characteristics is provided in Appendix C1. Strobe. The strobe used was an amber Whelen 500 Linear Strobe Light. One strobe light on each side of the truck was used. The strobes were displayed in a 1 Hz asynchronous pat- tern. A summary of the strobe light characteristics is provided in Appendix C1. 24 Figure 4. VDOT dump truck outfitted with warning lights.

Methods Upon arriving at VTTI for the first driving session, each participant was asked to read and sign the Information Sheet (Appendix E1), and fill out a health and vision screening ques- tionnaire. Each participant was also required to take an in- formal visual acuity test using a Snellen chart. The vision test was performed to ensure that all participants had at least 20/40 vision, which is the legal minimum to hold a driver’s li- cense in Virginia. Participants were tested for color blindness using pseudo isochromatic plates, but were not excluded based on results. At the beginning of each driving session, the participants were given an information sheet that explained that they would be expected to drive on Main Street in Blacksburg and on the Smart Road under various weather conditions. It also outlined the risks involved, and their rights and responsibilities as par- ticipants. Before the participants’ first driving session, they would sign and date the information sheet. Upon arriving at VTTI for each subsequent driving session, the participants were asked to review the same sheet, and initial and date it for each visit. First Driving Session: Surprise, Nighttime Dry, and Nighttime Rain The first driving session consisted of three parts: the sur- prise, nighttime dry, and nighttime rain trials. During the surprise trial, the participant was unaware that the focus of the study was on vehicle warning lights. Participants were instructed to drive on Main Street towards Blacksburg, where the experimental dump truck was waiting ahead. Instructions read to the participant were designed to force them into passing the truck. The lane-change distance was defined as the distance between the participant vehicle and the truck at the moment a lane change was initiated. This trial was followed up by a questionnaire, which was administered in the nearest parking lot. The participant was then debriefed on the true purpose of the research and signed an informed consent form for continued participation. During the nighttime dry trials, the participant drove on the Smart Road toward the experimental dump truck that was dis- playing one of the four warning lights. The participant would verbally indicate when he or she could identify the light source as a vehicle, and when he or she could detect a pedestrian in the roadway near the truck. These points were marked by the in-vehicle experimenter in the DAS data by use of a push button. The distance between the participant vehicle and the identified target (i.e., the dump truck or pedestrian) was defined as the vehicle identification and pedestrian detection distances for the warning light on display. This procedure was repeated twice for each warning light: one trial with a glare vehicle present and one without. A second pedestrian was oc- casionally presented on a section of road not near the truck in order to get a baseline pedestrian detection distance. In addition, participants were also asked to rate the warning lights in terms of discomfort glare and urgency at two distances (2400 ft and 1200 ft). This procedure was also done twice: once with a glare vehicle and once without. The first ratings were done after the first four laps, and the second ratings were done after the last four laps. Depending on the presentation order being used, a glare vehicle would either be present for the first two ratings or for the last two ratings. The rain towers were then turned on, and the same steps were repeated for the nighttime rain condition. The participant’s speed limit was reduced from 35 mph to 25 mph for safety because of the wet road surface and decreased visibility of the pedestrian. The distances at which the discomfort glare and urgency ratings were recorded were also reduced (to 1200 ft and 600 ft). Once all laps and ratings were complete, partici- pants were instructed to return to the VTTI building where they were compensated for their participation. Second Driving Session: Daytime Dry During the daytime sessions, only subjective ratings of the warning lights were collected. Ratings were taken at four dis- tances (4800 ft, 3600 ft, 2400 ft, and 1200 ft) and in two direc- tions (facing downhill and facing uphill). These directions were used because of the difference in contrast of the lights and their background. With the uphill view of the experimental truck, the warning lights on top of the vehicle were visible against the sky, whereas for the downhill view, the lights appeared against a foliage background as shown in Figure 5. For this session, the participants were first asked if they could see the warning light being displayed. If they answered “yes,” they were asked to rate how confident they were that they saw it using the confidence rating scale. Finally, the participants were asked to rate the attention-getting nature of the light using the attention-getting rating scale. The participants would an- swer these questions about each warning light at each distance. The downhill ratings were collected first, followed by the uphill ratings. Upon completion, the participants were instructed to return to the building where they were paid for their time. Third Driving Session: Nighttime Fog During the nighttime fog session, the participants followed the same protocol as the nighttime rain session. For safety, the participants were instructed to drive at 15 mph while in the fog. Each participant drove a total of eight laps on the Smart Road, indicating when they could identify the light source as a vehicle and when they could detect the pedestrians in the road. After the fourth and eighth laps, the participants were 25

asked to rate the discomfort glare and urgency for each warn- ing light at 600 ft and 300 ft. When the last set of ratings was complete, the participants were instructed to drive back to the building where they were paid for their participation. Fourth Driving Session: Daytime Fog During the fourth driving session, participants viewed each warning light at 600 ft and 300 ft. At each distance, subjects were first asked if they could see each warning light. If they answered “yes,” they were also asked how confident they were that they saw the light using the confidence rating scale, to rate the attention-getting nature of the light using the attention- getting rating scale, and to rate the urgency of the light. These questions were asked for each light and catch trial (no light pre- sented). Once both sets of ratings were done, the participants were instructed to drive back to the building where they were paid for their participation. Subjects who participated in all four driving sessions also received a $30 bonus. Data Analysis A statistical analysis was undertaken that compared the re- sults using analysis of variance (ANOVA) testing for each of the independent and dependent variables. Factors and inter- actions were considered to be significant at an α = 95% level. When possible, a Student-Newman-Keuls (SNK) pairwise post hoc test was used to further determine significant factors. The final step was the photometric analysis of the light sources based on the measurements from the earlier experi- ment. Trends and the correlation of the photometry and the dynamic testing were compared to provide further insight into the lighting system performance. Summary of Results The results of the dynamic conditions—lane-change, vehi- cle identification, and pedestrian detection distances—will be discussed first. These results are then followed by the analy- sis of the rankings of attention-getting, confidence, urgency, and glare. Dynamic Conditions • Lane Change Distance – The LED warning lights caused significantly longer lane- change distances than either the beacons (low- and high- mounted) or the strobes. This result may be related to the light’s effective intensity that caused participants to change lanes to avoid glare from the lights. – Results from all other warning lights were not significantly different from each other. • Vehicle Identification Distance – Vehicle identification distances were significantly shorter (worse) in rain and fog conditions than in dry conditions (Figure 6). – High-mounted beacons provided significantly longer (better) identification distances than the other warning lights (Figure 7). The light’s separation from the vehicle’s tail lights may have been a contributing factor because 26 Lights against foliage in downhill view Lights against sky in uphill view. Figure 5. Views of the experimental truck.

a majority of participants suggested that the tail lights were the main influence in determining that the light source was located on a vehicle. – LEDs provided significantly shorter identification dis- tances than the other warning lights (Figure 7). This phe- nomenon may be attributed to the lights washing out the vehicle’s tail lights because of their proximity and high intensity. • Pedestrian Detection Distance – Pairwise post hoc SNK analysis found that detection dis- tances were significantly shorter in rain and fog conditions than in dry conditions (Figure 8). – A pairwise SNK analysis also found that the LEDs had a significantly shorter pedestrian detection distance than all other warning lights (Figure 9). This phenomenon is likely attributed to a bloom effect caused by the light’s high effective intensity, which washed out the pedestrian’s reflective vest. – All other warning lights produced results that were not significantly different from having no warning lights at all. Analysis of Ratings • Attention-Getting Rating – On average, the LEDs were rated as the most attention- getting, and the high-mounted beacons were rated the lowest. This result may be due to the relatively high effective intensity of the LEDs and the low effective inten- sity of the high-mounted beacons. – Attention-getting ratings were significantly worse for all lights except the low-mounted beacons during the uphill 27 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Dry Rain Fog Ve hi cl e Id en tif ica tio n Di st an ce (ft ) Weather No Glare Glare 0 200 400 600 800 1000 1200 1400 High-Mounted Beacon Low-Mounted Beacon LED Strobe Ve hi cl e Id en tif ica tio n Di st an ce (ft ) Warning Light Figure 6. Mean vehicle identification distance for each glare and weather condition. Figure 8. Mean pedestrian detection distance for each weather condition. Figure 9. Mean pedestrian detection distance for each warning light. Figure 7. Mean vehicle identification distance for each warning light. 0 50 100 150 200 250 300 350 400 Dry Rain Fog Pe de st ria n De te ct io n Di st an ce (ft ) Weather 0 50 100 150 200 250 300 350 High- Mounted Beacon Low- Mounted Beacon LED Strobe No Lights Pe de st ria n De te ct io n Di st an ce (ft ) Warning Light

condition (Figure 10). The high-mounted beacons were most affected by the “daylighting” effect of viewing the lights against the bright sky. – Because the high- and low-mounted beacons are iden- tical lights, their different attention-getting results can be attributed to their placement on the experimental truck. The high-mounted beacons become much less attention- getting during the uphill conditions because the lights blend in with the sky behind them. Because the back- ground for the low-mounted beacons remains the same (i.e., the tailgate of the truck), there is no significant loss in attention-getting rating (Figure 11). • Confidence Rating – Post hoc SNK analysis shows confidence rating was sig- nificantly lower for the high-mounted beacons than all other lights (Figure 12). – LEDs and low-mounted beacons provided the highest confidence ratings. – Direction had a significant impact only for the high- mounted beacons and strobes (Figure 13). – Increasing distance caused significant drops in confidence rating for the high-mounted beacons, worsening the effect of viewing the light against the sky. • Discomfort Glare – For the surprise trial, average discomfort glare ratings for all warning lights were better than “satisfactory.” Possible explanations are (1) the participants did not look directly at the lights because they were unaware the lights were the focus of the study, or (2) the lit roadway provided light- ing on or near the roadway that made the warning lights seem less glaring by comparison. – On average, the LEDs had the highest discomfort glare ratings, and the high-mounted beacons had the lowest (Figure 14). – The LEDs were the only warning lights to get a lower discomfort glare rating in fog conditions, which may be because of increased light scatter. 28 1 2 3 4 5 6 7 Downhill Uphill At te nt io n- G et tin g Ra tin g Direction High-Mounted Beacon Low-Mounted Beacon LED Strobe 1 2 3 4 5 6 7 1200 2400 3600 4800 1200 2400 3600 4800 High-Mounted Beacon Low-Mounted Beacon At te nt io n- G et tin g Ra tin g Warning Light and Distance (ft) Downhill Uphill 0 20 40 60 80 100 High-Mounted Beacon Low-Mounted Beacon LED Strobe Co nf id en ce R at in g Warning Light 0 20 40 60 80 100 High-Mounted Beacon Strobe Co nf id en ce R at in g Warning Light Downhill Uphill Figure 10. Mean attention-getting rating for each warning light by direction (1–7 scale). Figure 12. Mean confidence rating for each warning light (0–100 scale). Figure 11. Mean attention-getting rating for each beacon warning light by distance and direction (1–7 scale). Figure 13. Mean confidence rating by direction (0–100 scale).

– All warning lights received lower discomfort glare ratings when a glare vehicle was present. This outcome may be because participants attributed more of the glare to the headlights, or because the addition of the headlights made the warning lights seem less glaring in comparison. – Distance was a significant factor in all weather conditions, with discomfort glare ratings decreasing with increased distance. • Urgency – For the surprise trial, the high-mounted beacons were rated significantly lower for urgency than the other warn- ing lights. The similar low-mounted beacons had the highest average rating, possibly because the low-mounted beacons were closer to the participant’s eye height and the light was reflected off the truck’s tailgate making the lights seem more intense (Figure 15). – Post hoc analysis shows the LEDs were rated signifi- cantly lower for urgency than the other warning lights (Figure 16), possibly because of the relatively long flash duration of LEDs compared to that of the beacons and strobes. – A significant difference in urgency rating for the LEDs was due to age. This result may indicate that younger partic- ipants judge urgency based on flash duration, while older participants judge based on intensity. – Distance was a significant factor for urgency, with ratings decreasing as distance increased. Photometric Comparison For the photometric comparison, results of the photo- metric measurement for each of the lighting systems were com- pared to the performance of each system in terms of the de- pendent variables. As before, the photometric measurements used for the comparison were derived using the Form Factor method. • Vehicle Identification Distance – For vehicle identification distance, the high- and low- mounted beacons have the same effective intensity but different performance levels. The distance of the beacons from the vehicle’s tail lights, which participants used to identify the vehicle, provided the higher performance for the high-mounted beacons. – The LEDs had the shortest vehicle identification distance despite having the highest effective intensity. The light’s long flash duration gave the appearance of a slow flash, which participants confused the most with other road- way markings such as flashing signs and construction markers. 29 1 2 3 4 5 6 7 8 9 High-Mounted Beacon Low-Mounted Beacon LED Strobe D is co m fo rt G la re R at in g Warning Light Dry Rain Fog 0 10 20 30 40 50 60 70 80 90 100 High-Mounted Beacon Low-Mounted Beacon LED Strobe Ur ge nc y Ra tin g Warning Light 0 10 20 30 40 50 60 70 80 90 100 High-Mounted Beacon Low-Mounted Beacon LED Strobe Ur ge nc y Ra tin g Warning Light Figure 15. Mean urgency rating for each warning light following the surprise trial (0–100 scale). Figure 14. Mean discomfort glare rating for each warning light by weather (1–9 scale). Figure 16. Mean urgency rating for each warning light (0–100 scale).

• Pedestrian Detection Distance – The LEDs also had the shortest pedestrian detection distance due to the high effective intensity of the light washing out the view of the pedestrian (Figure 17). – The other (non-LED) warning lights had similar per- formances to each other with regard to pedestrian detec- tion distance; however, the low-mounted beacons had the highest mean distance. This result may be because the lights and the illuminated tailgate provided a contrasting background for the pedestrian’s silhouette. • Confidence Rating – The LEDs provided the highest confidence ratings due to higher effective intensity; however, from an application standpoint, the performance of every light was very high. – The low-mounted beacons provided higher confidence ratings than the high-mounted beacons of the same effec- tive intensity. This result was because the high-mounted beacons were more affected by “daylighting” during the uphill conditions. • Attention-Getting Rating – The high effective intensity of the LEDs provided the highest attention-getting ratings (Figure 18). – The low-mounted beacons provided higher attention- getting ratings than the high-mounted beacons (of the same effective intensity) and the strobes (which had a higher effective intensity). The added light reflection from the tailgate may have increased the low-mounted beacons’ visibility over the strobes. For the high-mounted beacons, low contrast as a result of being viewed against the sky is still a significant factor in the results. • Discomfort Glare – As expected, the LEDs (which have the highest effec- tive intensity) resulted in the highest discomfort glare ratings (Figure 19). – The low-mounted beacons had a higher discomfort glare rating than the high-mounted beacons with the same effective intensity, probably because the light reflecting off the tailgate added an additional source of glare. • Urgency – Higher effective intensity did not provide an additional urgency benefit. – The rotating beacons and strobes yielded a higher rat- ing with a less intense light because the flash patterns appeared faster than the LEDs. Discussion The experiment has shown that many of the factors in- volved in the design and layout of a vehicle’s warning-light system influence the response of the driver to that vehicle. 30 High Beacon Low Beacon LEDStrobe 1 2 3 4 5 6 7 0 500 1000 1500 2000 2500 A tte nt io n- G et tin g Ra tin g Form Factor Method High Beacon Low Beacon LEDStrobe 1 2 3 4 5 6 7 8 9 0 500 1000 1500 2000 2500 D is co m fo rt G la re R at in g Form Factor Method Figure 18. Daytime attention-getting rating by the light source effective intensity (1–7 scale). Figure 19. Discomfort glare rating by the light source effective intensity (1–9 scale). High Beacon Low Beacon LED Strobe 0 50 100 150 200 250 300 350 0 500 1000 1500 2000 2500 Pe de st ria n De te ct io n Di st an ce (f t) Form Factor Method Figure 17. Pedestrian detection distance by the light source effective intensity (Form Factor method).

The purpose of the dynamic experiment was to further re- fine the requirements of the warning-light system in a driv- ing environment with the addition of adverse weather con- ditions. The aspects of the warning-light system that must be considered are the lighting layout, adverse weather in- fluences, further refinements of the lighting characteristics, and influence of other environmental factors such as ap- proaching vehicle glare. • Lighting System Layout – Separation of the warning-light system from the tail lights of the vehicle aided in the identification of the vehicle. Many participants indicated that tail lights were the important cue for the vehicle identification distance. – One of the difficulties with placing the warning-light system high on the vehicle is that the lights may appear against the sky. The lighting should be placed either such that the vehicle is behind the source or such that a back- ground is located behind the light in order to control the contrast. • Adverse Weather – The influence of the rain and fog conditions did not seem to significantly influence the participants’ subjective ratings of lighting system performance. – The rain and fog significantly reduced the vehicle identi- fication and pedestrian detection distances for all the light sources and seemed to moderate the differences between the systems except for the LEDs. – The LEDs resulted in much lower pedestrian detection distances than the other systems. The LED system had the highest effective intensity, which resulted in a larger amount of light scatter observed by the approaching driver in rain and fog conditions. – It is expected that the effective intensity of the light would have to be limited to avoid the impact of light scatter under adverse weather. • Lighting Characteristics – The effective intensity of the sources influenced the de- tection of the pedestrian and the assessment of the light source glare. The higher effective intensity reduced the ability of the driver to see the pedestrian and also resulted in a higher discomfort glare rating. – The vehicle detection distance was reduced for the LEDs because of the system’s longer flash duration. The double flash of the strobes and the effective intensity changes of the rotating beacon seem to have provided an additional clue to the nature of the lighting system. – The urgency rating was also reduced for the LEDs. The urgency of the lighting system also seems to be more closely related to the apparent speed of the flashing. Use of a double flash or a beacon seems to improve the driver’s response. – The use of a 360° light source close to the line of sight of the driver increased the experienced glare; it should be avoided. • Other Environmental Factors – The presence of the opposing vehicle on the roadway re- duced the pedestrian detection distance by increasing the disability glare. – The opposing vehicle also reduced the discomfort glare ratings, because the warning-light systems are not as sig- nificant a source of glare as the opposing headlamps. – The presence of the opposing vehicle did not affect the vehicle identification distance. – The presence of other lighting systems, such as the road- way lighting experienced during the surprise trial, greatly reduced the discomfort glare rating of the warning-light systems but did not change the urgency rating. – A higher effective-intensity light source may be required in the presence of roadway lighting, or in daylight con- ditions, as suggested by the lane-change distance results from the surprise trial. 31

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TRB's National Cooperative Highway Research Program (NCHRP) Report 624: Selection and Application of Warning Lights on Roadway Operations Equipment explores recommended guidelines for the selection and application of warning lights on roadway operations equipment.

Appendixes A through E to NCHRP Report 624 are available online. The appendixes contain detailed information on relevant literature, the experiments performed, and data analysis associated with NCHRP Report 624.

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