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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-04-149
Date: December 2005

Enhanced Night Visibility Volume XVIII Overview of III

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Recall that Phase II of the ENV project included a discomfort glare evaluation of 11 different headlamp configurations (ENV Volume VII). The primary focus was on rating the discomfort glare of UV–A as compared to other VESs; however, it is difficult to fully understand the effects of oncoming vehicle safety without a direct disability glare evaluation. The two types of glare have different physiological origins, and factors that affect one type often do not affect the other.(1) Therefore, a disability glare evaluation in combination with a discomfort glare evaluation was needed to determine what effect the newer headlight technologies have on oncoming drivers. The study was performed as a 5 (VES) by 2 (Driver's Light Adaptation Level) by 2 (Pedestrian Location) by 3 (Age) mixed-factor design.

This chapter discusses these independent variables, the dependent variables, and the key findings.


Vision Enhancement Systems

For this study, VESs were always two headlamps without supplemental systems. VES was a within-subjects variable. Both baseline headlamps (an HLB and an HID) from the Phase II discomfort glare study were included as VESs. The disability glare study used three additional HID headlamps so that differing intensities and beam patterns could be compared:

  • High/narrow: higher intensity with narrow beam pattern (HID).
  • High/wide: higher intensity with wide beam pattern (HID).
  • Low/wide: lower intensity with wide beam pattern (baseline HID).
  • Medium/medium: mid-level intensity with medium beam pattern (HID).
  • Low/narrow: low intensity with narrow beam pattern (baseline HLB).

These glare sources were positioned on a static frame in the opposite lane from the participant, simulating an oncoming vehicle.


Driver age was the only between-subjects variable. It included the same three gender-balanced age ranges used in most of the ENV studies: a younger group (18 to 25 years), a middle-aged group (40 to 50 years), and an older group (65 years and older). Ten participants from each age group were involved in the study.

Driver Light Adaptation Level

At night, a driver's eye will adapt to the ambient lighting condition. This adaptation level will change the ability of the driver to perceive objects as well as affect glare sensitivity. For this study, driver light adaptation level was a within-subjects variable that included a low (0.15 lux (lx)) and high level (0.45 lx). The light adaptation level of the driver was varied by using a dimmable light source inside the vehicle, placed across the top of the instrumentation panel.

Pedestrian Location

Pedestrian location was also a within-subjects variable. The locations at which pedestrians walk in the roadway significantly affect their visibility in the presence of glare relative to the driver. Two pedestrian locations were used in this study—one near the centerline and the other near the right edgeline. Both locations were 15.2 m (50 ft) behind the oncoming glare headlamps, and the pedestrians wore white clothing.


Three dependent variables were collected during this study: pedestrian detection distance, discomfort glare rating, and driver illumination level in lux.

During the study, the participants drove toward the glare sources. For the disability glare portion, participants indicated when they could see the pedestrian as they approached. For the discomfort glare portion, participants were asked to rate the discomfort of the glare.

During the disability portion of the study, the in-vehicle experimenter flagged the data when the participants verbally reported detecting the pedestrian and flagged the data again when the vehicle passed the pedestrian's location on the road. Calculations of pedestrian detection distances used these data flags. When necessary, videotape from the in-vehicle data collection system was used to verify detection distances using a post hoc video analysis.

During the discomfort glare portion of the study, participants rated the discomfort they experienced from oncoming headlamps. The participant's rating was an overall rating of the approximately 305 m (1,000 ft) approach toward the VES. After stopping past the VES, the participants rated the overall discomfort experienced from the glare by using the deBoer scale, a nine-point scale with verbal anchors for each of the odd numbers as follows: (1) Unbearable, (3) Disturbing, (5) Just Acceptable, (7) Satisfactory, and (9) Just Noticeable.(2)

Illuminance readings were collected at approximately the driver's eye level every tenth of a second throughout both the disability and the discomfort portions of this study, allowing illuminance data to be collected at the point of detection as well as during the discomfort ratings.


The primary focus of this research effort was to evaluate the discomfort and disability glare associated with headlamps of different intensities and beam patterns. The results indicated that beam intensity (i.e., maximum light output) affected disability and discomfort glare more than beam pattern. Specifically, VESs with higher maximum output had shorter pedestrian detection distances and were rated as more discomforting. In general, the results showed that discomfort glare corresponded to disability glare; oncoming VESs that were rated as more discomforting were the same VESs that restricted detection distances.

The participant's age did not indicate a significant difference in discomfort glare ratings, but the detection distance for pedestrians did decrease significantly as participant age increased. The driver light adaptation level appeared to have no effect on the glare rating and the detection distance for pedestrians. Finally, the pedestrian location near the right edgeline was detected from almost twice as far away as the pedestrian location near the centerline.


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