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Publication Number: FHWA-HRT-04-136
Date: December 2005
Enhanced Night Visibility, Volume V: Phase II—Study 3: Visual Performance During Nighttime Driving in Snow
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Twenty individuals participated in this study. Participants were divided into two age categories. The first group, referred to as "younger drivers," was made up of 10 participants who were between the ages of 18 and 25 years old. The second group, referred to as "middle-aged drivers," was made up of 10 participants who were between the ages of 40 and 50. Each age category had five males and five females. Candidates were allowed to participate only after they met the selection conditions of a screening questionnaire (appendix A). Candidates also had to sign an informed consent form (appendix B), present a valid driver's license, pass the visual acuity test (appendix C) with a score of 20/40 or better (as required by Virginia State law), and have no health conditions that made operating the research vehicles a risk.
Participants were instructed about their right to withdraw freely from the research program at any time without penalty, and they were told that no one would try to make them participate if they did not want to continue. If at any time they chose not to participate further, they were instructed that they would be paid for the amount of actual participation time. Participants received 20 dollars per hour for their participation. All data gathered as part of this experiment were treated with complete anonymity.
A mixed-factor design was used for the data collection of the onroad portion of the study (i.e., detection and recognition tasks). There were three independent variables:
The age factor had two levels: younger participants (18 to 25 years old) and middle-aged participants (40 to 50 years old). These age groups were created based on literature-review findings (listed in ENV Volume II) that suggest changes in vision during certain ages. (See references 1, 2, 3, 4, and 5). Each age group comprised five males and five females. Gender was used as a control, although it was not a factor of interest. Because of safety concerns, the older age group that was included in the studies on clear and rain conditions was not used in this study. It became apparent during pilot testing that the snow condition presented a risk of older participants slipping in the turnarounds, even though salt was spread throughout the areas where the participants would walk.
Following is a list of the VES configurations used in this study:
For a more indepth look at the technical specifications of each headlamp, refer to ENV Volume XVII, Characterization of Experimental Vision Enhancement Systems.
The order of presentation for each VES and object combination was counterbalanced. Table 3 provides an example of the VES configuration order for a pair of participants. The first column, "Order," indicates the order in which the VESs were presented. The second column, "VES," presents the configuration used. The third column, "Vehicle," describes the sport utility vehicle (SUV) used as a platform for the VESs.
The four VES configurations tested were selected based on several considerations. The HLB and the HID headlamps currently are available on the market, and they reflect the most commonly used headlamp type (HLB) and the headlamp type with a growing section of the market (HID). Therefore, these two headlamp types were added as two of the configurations to allow the comparison of new VES alternatives with what is readily available.
Both of the configurations that use the UV–A headlamps were paired with typical headlamps (e.g., HLB) because UV–A headlamps provide minimal visible light. The UV–A headlamps stimulate the fluorescent properties of objects contacted by the UV radiation, producing visible light. Their purpose is to supplement regular headlamps, not to eliminate them. These UV–A and HLB pairings resulted in two different VES configurations: five UV–A + HLB and hybrid UV–A + HLB. The hybrid UV–A headlamp is an experimental prototype that produces a significant amount of visible light, although not enough light to allow nighttime driving without low-beam headlamps. The spotlight UV–A headlamps used for the five UV–A configuration produce less visible light.
Several factors caused a decrease in the number of VES configurations used in the clear and rain condition studies (ENV Volumes III and IV). Potential changes in the snowmaking environment from night to night and the excessive time required for snowplowing between VES configuration changes resulted in the need to limit the experiment to one night. In addition, pilot testing included the infrared thermal imaging system (IR-TIS) configuration, but the IR camera became packed with snow, so it was not available for use in this study. Because the halogen low beam at a lower profile (HLB-LP) configuration had been used primarily as a comparison to the infrared thermal imaging system (IR-TIS), it also was not included in this study. Based on the results of the studies in clear and rain conditions, the high output halogen (HOH) and halogen high beam (HHB) headlamps tested were either no different or worse than the HLB, so it was unnecessary to further test those two configurations. Similarly, the UV–A and HID pairings also were found not to provide much improvement over HID alone in clear and rain conditions, so they were not included in this study.
Pedestrians were the three objects selected for this study, as noted in table 4 and figure 1 through figure 3. The main reason for using pedestrians was because of the high crash-fatality rates for these nonmotorists.(6,7) This study used real pedestrians to evaluate the effects of object motion on detection and recognition distances; previous research of this type used pedestrian mockups.(8)
Pedestrians were presented to the drivers at two different contrast levels: black clothing against the snow background at night and white clothing against the snow background at night. The pedestrians walked in two different directions: perpendicular to the vehicle path, representing a pedestrian crossing the road; and parallel to the vehicle path, representing a pedestrian walking along the shoulder. The perpendicular pedestrians wore either white (figure 1) or black (figure 3) clothing. The parallel pedestrians wore white clothing only (figure 2). Stations with no objects (blank) were included to keep the study's participants searching for objects and prevent the expectation that they would see two pedestrians while driving downhill and two pedestrians while driving uphill. Table 4 shows the reflectance of each object. Detailed information about the characterization of the different objects is provided in ENV Volume IX.
OBJECTIVE DEPENDENT VARIABLES
Detection and recognition distances were obtained to analyze the degree to which the different VES configurations enhanced night visibility while driving during the snow condition. These two variables, detection and recognition, were selected because of their common use and acceptance in the human factors transportation literature. (See references 9, 10, 11, 12, and 13.) Both terms, "detection" and "recognition," were explained to participants during the training session. Detection was explained as follows: "Detection is when you can just tell that something is on the road in front of you. You cannot tell what the object is, but you know something is there." Recognition was explained as follows: "Recognition is when you not only know something is there, but you also know what it is."
During training and practice, participants were instructed on the use of a hand-held wand to indicate when they detected and recognized objects. The participant pressed a button on the wand when he or she detected an object on the road, and then pressed the button again when he or she recognized the object. The in-vehicle experimenter pressed the laptop computer keyboard spacebar when the object of interest was aligned with the driver (i.e., the participant drove past the object). Detection and recognition distances were calculated from distance data collected at these three points in time.
Subjective ratings were also collected. Participants were asked to evaluate a series of seven statements for each VES using a seven-point Likert-type scale. The two anchor points of the scale were "1" (indicating "Strongly Agree") and "7" (indicating "Strongly Disagree"). The statements addressed each participant's perception of improved vision, safety, and comfort after experiencing a particular VES. Each participant was asked to compare the VES he or she was evaluating with his or her own "regular headlights" (i.e., the headlights on the participant's own vehicle). The assumption was made that a participant's own vehicle represented what the participant knew best, and therefore, was most comfortable using. Following is a list of statements used in the questionnaire. The statements on the questionnaire follow. Note that while the word "headlamp" is used throughout the ENV series, the subjective questions posed to the participants used the synonymous word "headlight," as reflected below.
Safety procedures were implemented as part of the experiment. These procedures were used to minimize possible risks to participants during the experiment. The safety measures required that:
The pedestrians were trained on when to move away from the road based on a preset safety-envelope mark. In addition, they were provided with radios in case the in-vehicle experimenter needed to communicate with them.
APPARATUS AND MATERIALS
Onroad driving was conducted using three SUVs. All vehicles were equipped with laptops for data collection. Software was developed to link the data collection system, such as an electronic odometer, to the vehicle to obtain distances traveled and speed. The distance to the object of interest was obtained from the electronic odometer data. The software program on the laptop allowed the in-vehicle experimenter to change between VES configurations and object orders. After all the objects had been presented for a VES configuration, the program switched to the subjective questions to be asked by the experimenter. In addition, the software gathered information such as the participant's age, gender, and assigned identification number.
The VESs were distributed among the three vehicles (figure 4 through figure 6). Note that either one of the two SUVs equipped with the UV–A headlamps (figure 4 and figure 5) also could be used for the HLB–only configuration.
This study took place at the all-weather testing facility on the Smart Road in Blacksburg, VA, shown in figure 7 and described in appendix G. Four different locations on the Smart Road were used to present the different objects, as shown in the diagram in figure 8. One onroad experimenter was assigned to each of the two turnarounds used for this study. At the turnaround, where the participant switched vehicles when changing VES configurations, the onroad experimenter was responsible for escorting the participant to the next vehicle, showing him or her where the different controls were, verifying that the correct VES configuration was being tested, and cleaning the windshields and headlamps. At the second turnaround, the other onroad experimenter was responsible for cleaning the windshields and headlamps halfway through the VES run. Appendix K gives a detailed protocol. Four other onroad experimenters were positioned at the predetermined object locations along the road, with two onroad experimenters assigned to cover locations 1 and 5 and two onroad experimenters for locations 2 and 4, as illustrated in figure 8. Appendix I gives details on the protocol for the onroad experimenters. Six onroad experimenters and two in-vehicle experimenters were involved in the study each night.
Figure 7. Photo. Snowmaking on the Virginia Smart Road.
The all-weather testing facility can generate snow by using controlled precipitation to ensure a constant amount of snowfall during the data collection effort. Data were not collected during heavy wind conditions. The selected rate of snowfall, which varied according to wind conditions, was between 5.1 cm/h (2 inches/h) and 12.7 cm/h (5 inches/h), which required most participants to use the vehicle windshield wipers at a high speed. The reason for the range in snowfall resulted from adjustments needed to maintain a similar amount of snowfall throughout the nights the experiment was performed because the environmental conditions for snow development were not consistent across nights. Appendix L gives an indepth description of snowmaking equipment).
Figure 8. Diagram. Locations where the objects were presented for the adverse weather condition
The headlamps used for several of the VES configurations were on externally mounted light bars. These light bars were used for the HLB, HID, and UV–A configurations. Each light assembly movement required a re-aiming process, which took place each night before the study started. The protocol used for aiming was developed with the help of experts in the field. (See references 14, 15, 16, and 17.) Appendix J gives the details of the aiming protocol used for this specific study. During the photometric characterization of the headlamps, it was discovered that the position of the maximum intensity location of the HLB configuration was aimed higher and more toward the left than typically specified. The effect of this aiming deviation on detection and recognition distances is indeterminate. The aiming could have resulted in more illumination on pedestrians, therefore leading to increased detection and recognition distances. The aiming also could have led to increased backscatter from the snow, resulting in decreased detection and recognition distances. Details about the aiming procedure and the maximum intensity location are discussed in ENV Volume XVII, Characterization of Experimental Vision Enhancement Systems.
The experiment was performed in one night, which included the laboratory training and the Smart Road experiment. The entire session lasted approximately 3.5 hours, and two participants performed the experiment simultaneously. The participants were familiarized with the Smart Road and the experimental objects before starting the experiment. During the onroad portion, four VES configurations were presented to the participants; the order of VES presentation was counterbalanced. Details of the procedures are discussed next.
Initially, candidates were screened over the telephone (appendix A), and if a candidate qualified for the study, a time was scheduled for testing. Qualified candidates were instructed to meet the experimenter at the contractor facility in Blacksburg, VA. After arriving, each candidate was given an overview of the study, then asked to complete the informed consent form (appendix B) and take an informal visual acuity test using a Snellen chart, a contrast sensitivity test, and a color vision test (appendix C). Appendix D describes a detailed experimenter protocol for vision testing. After these steps, and if no problems were identified, the candidate was accepted as a participant and then trained on the experimental tasks to be performed during the drive.
Each participant was instructed on how to perform the tasks associated with object detection and recognition and how the questionnaires would be used. The study protocol and pictures of the objects were presented at this point (appendixes D and E). The detection and recognition definitions, the use of the pushbutton wand, and the Likert-type scales for the questionnaire were also explained. The training presentation outlined the onroad procedures, showed pictures of the objects, and allowed for questions. The purpose of this lab training was to allow all participants to begin the experiment with a standard knowledge base.
Because participants changed vehicles as part of the study, each participant was familiarized as soon as he or she reached an experimental vehicle. While the vehicle was parked, the onroad experimenter reviewed general information concerning its operation (appendix K). Each participant was asked to adjust the seat and steering wheel position for his or her driving comfort. When the participant felt comfortable with the controls of the vehicle, the experiment was ready to start.
Participants received instructions to remain in the center of the roadway while driving, which ensured an even snow coverage over the experimental vehicle because the snowmaking towers were extended to be directly centered over the roadway. Participants were also instructed to place the vehicle in park after reaching each of the turnarounds, allowing time for the onroad objects to be changed. Participants were instructed to drive at 16 km/h (10 mi/h) or below on the road where the snow was falling. Participants were required to follow instructions from the in-vehicle experimenters at all times.
Driving and Practice Lap
Each participant drove down the road to become familiar with the road and the vehicle; no objects were presented during this test run. At the bottom turnaround, the experimenter gave the pushbutton wand to the participant and instructed the participant that this portion of the session was a practice to familiarize him or her with the objects. The participant then drove back up the road for a practice run of detection and recognition tasks, obtaining feedback from the experimenter as needed. After the practice tasks, the participant began the experimental tasks, driving with the four VESs corresponding to their assigned order.
General Onroad Procedure
Distance data were collected while each VES was evaluated. The in-vehicle experimenter provided the participant with a pushbutton wand to flag the data collection program when detection and recognition were performed. Other than detection, recognition, and maintaining 16 km/h (10 mi/h) or below, participants performed no other tasks while driving. The experimenter, seated in the passenger seat, let the participant know when to start driving and where to park. The in-vehicle experimenter also administered the questionnaires after each VES configuration and controlled the data collection program. For more details on the in-vehicle experimenter protocol, refer to appendix F.
A snowplow was used between VES reconfigurations to maintain consistent snow coverage on the road. The snowplow usually required about four passes of the road to clear the accumulation, and the in-vehicle experimenter used this time to administer the subjective questions.
Sequence of Data Collection
Each participant followed the same sequence of events for data collection for each of the VES configurations. This sequence was as follows:
The procedures for this study, including training, experimentation, and debriefing, were conducted in two shifts every night (first shift, 7:45 to 11 p.m.; second shift: 11:30 p.m. to 2:30 a.m.). Participants who usually worked and drove late at night ran in the second shift to minimize the possibility of fatigue. Other participants ran during the first shift. Participants received payment for the total number of their participation hours at the end of the experimental session.
Data for this research were contained in one data file per VES configuration per participant. All the data collected for the 20 participants were merged into a single database that included objective and subjective data. An analysis of variance (ANOVA) was performed to evaluate drivers' detection and recognition performances with each of the different configurations. PROC ANOVA was used in SAS® statistical software to compute the ANOVA. Table 5 shows the full experimental design model used in the data analysis.
The main effects that characterized this study were VES configuration (VES), driver's age (Age), and type of object (Object). A Bonferroni post hoc analysis was performed for the significant main effects (p < 0.05). Post hoc analyses assisted in the identification of experimental levels that were responsible for the statistical significance of the main effects. Note that the significance of a main effect or interaction does not make all levels significantly different. A detailed discussion of post hoc tests is referred to Winer, Brown, and Michels.(18)