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How Perceptual and Cognitive Factors
are Involved in a Car Accident:
A Case Study

By: Vanessa Dominguez & Marc Gentzler | Mentor: Dr. Andrew P. Daire


A second factor that may have contributed to this accident is glare. Glare is created when a light source or surface is much brighter than the rest of the visual field. Disability glare results when a small angle exists between the glare source (i.e. sunlight or artificial light) and a target of concern resulting in reduced capability to see the target. Psychological or discomfort glare is produced by the sensation of discomfort (Olson & Farber, 2003).

The effects of glare will increase when the light source increases, the background light decreases, and the angle between the line of sight and the direction of the light source decreases (Alferdinck, 1996; Alferdinck & Varkevisser, 1991). A direct relationship exists between the amount of glare and contrast detection performance. With increasing glare, the driver has reduced ability to perceive small contrasts (Theeuwes, J., Alferdinck, J. W. A. M., & Perel, M., 2002). Drivers experience disability glare from oncoming driver's headlights.

A small glare angle from an oncoming headlight indicates a large distance between the vehicles. As the distance closes, the angle between the glare source and the target (i.e., roadway) increases causing the eyes to move into zones where lower levels of light are used (Olson & Farber, 2003). According to Olson and Farber, when vehicles are separated by 100 to 200 feet on a two-lane road, "the maximum loss of forward visibility for low-contrast targets to the right of the vehicle is about 20%" (p. 57). Recovery from the glare can start once the vehicles are less than 100 to 200 feet apart. However, as Olson and Farber (2003) point out, the glare effect is amplified when there are multiple oncoming vehicles, as was the case in this particular accident.

As for discomfort glare, the relationship proves less direct. Discomfort glare is due to the adaptive function of the eye, the luminance at the eye from the glare sources, and the angular relationship between the glare sources and the target. This creates more subjectivity in the measurement of discomfort glare. Theeuwes, Alferdinck, and Perel (2002) researched the effects of discomfort glare on driving behavior. Their results indicated that even relatively low glare sources caused a significant drop in detecting simulated pedestrians along the roadside, causing participants to drive significantly slower on dark and winding roads.

Discomfort glare depends on task difficulty. In other words, discomfort glare is judged more uncomfortable on a road with a more difficult task than on one with an easier task. This relationship suggests that discomfort glare affects driving behavior.

Glare likely contributed to the described car accident by causing the driver of Vehicle A to experience both disability and discomfort glare. There were three main sources of glare during the incident: from the headlights of Vehicle B that were on the side of the road (right side of Vehicle A), from Vehicle C's headlights in the south lane (left side of the driver of Vehicle A), and that of Vehicle A's own headlights. The combined sources of glare likely had significant damaging effects on driver performance. The driver of Vehicle A had greater difficulty distinguishing the oncoming vehicles especially given there were more than one source of glare.

In most standard cars, low-beam headlights point slightly down and to the right (Olson & Farber, 2003). This is to reduce glare for oncoming cars, pointing the light beam slightly away from the driver. With Vehicle B placed on the side of the road facing toward the driver of Vehicle A on his right side, the lights were pointing directly toward him. The human factors development of headlights pointing to the right actually caused more glare in this situation. Further, the weight of the newspapers found in the back of Vehicle B likely tilted the car slightly upward, pointing the light more directly toward his eyes. As the driver of Vehicle A approached Vehicle C in the correct lane going in the opposite direction, the angle between the glare of Vehicle C's headlights and of Vehicle A began to increase, possibly decreasing the effects of glare. However, it is unknown as to when Vehicle C passed Vehicle A, or when the headlights of Vehicle B were turned off. The headlights of Vehicle A shining on the road and on other objects possibly caused some glare as well. The glare may have been amplified by the wet roads, turning the road into a more reflective surface. This could have also made the glare worse originating from Vehicle B and C. When the driver of Vehicle A was approximately 100 to 200 feet away from Vehicle B, even if he had recovered from the glare, the adjustment would have occurred too late to react successfully.

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