Author Ellen Szubski
Susan, a 45-year-old runner, was jogging in the early morning approximately an hour before sunrise, also known as “civil twilight.” Civil twilight occurs both before sunrise and after sunset when the sun is below the horizon and the sky is partially illuminated. Susan was wearing dark athletic clothing and grey tennis shoes for her morning run. She was running on the sidewalk in a suburban area and began to cross the four-lane road at an unmarked crosswalk.
John, a 30-year-old driver, was on his way home after a night shift at work. He was driving fast with his low beam headlights on. John’s visibility of the road ahead was limited due to the partial illumination of the sky during civil twilight.
As Susan was crossing the road, she was difficult to see due to her dark clothing.
John approached the unmarked crosswalk and did not see Susan crossing until it was too late. Before impact, John attempted to brake and swerve to avoid Susan, but the collision was unavoidable. The collision resulted in Susan sustaining significant injuries.
Person running during civil twilight.
As mentioned in previous blogs, reduced visibility is one of the primary challenges of nighttime driving. Human eyes are not well-adapted to low-light conditions. The human visual system relies on light to see fine details and colors. In low luminance conditions, such as civil twilight, humans have decreased acuity (ability to see fine detail), contrast sensitivity (ability to distinguish an object from the background), accommodation (ability keep your eyes focused on an object while that object or you are in motion), reaction time, and object recognition.
Civil twilight occurs before sunrise and after sunset when the sun is still below the horizon and the sky is partially illuminated.
There are two visual pathways: one responsible for visual recognition and the other responsible for locating objects in space. As illumination levels decrease, the two pathways are affected differently. In civil twilight, performance in the visual recognition pathway is significantly degraded while performance in the visual guidance pathway remains resistant to performance decreases in illumination. The ability to locate objects in space exhibited through the ability to maintain lane position at night can cause the driver to be overconfident in the ability to recognize and see fine detail at night. This overconfidence can lead to overdriving of the headlights, meaning the driver is traveling at a speed that exceeds their ability to react within the distance illuminated by their vehicle’s headlights.
In this case study, John was overdriving his headlights. Once Susan was illuminated by his headlights and he was able to recognize that a pedestrian was in his travel path, he did not have enough time to avoid the collision. Had John been driving at a slower speed, he may have had more time to react to Susan entering the roadway. In addition, had Susan been wearing more conspicuous clothing, such as retroreflective material or active lighting, John may have been able to recognize her presence in the roadway at a greater distance, therefore allowing more time for an appropriate reaction.
Ellen Szubski, Ph.D., CXLT, CPSI, AHFP, is a human factors consultant at The Warren Group. She earned a Doctorate of Philosophy in Human Factors Psychology and a Master of Science in Applied Psychology from Clemson University. She did her dissertation on “The Influence of Pedestrian Biological Motion on Time-To-Collision Estimates at Night”. She is also a Certified XL Tribometrist, Certified Playground Safety Inspector and a Certified Associate Human Factors Professional (AHFP). Prior to entering the forensic field, Ellen planned and conducted experiments for a major bicycle manufacturer. She also conducted laser strike perception studies for the Department of Defense. Ellen applies her experience in Human Factors to the analysis of crash investigations and other personal injury matters. These matters often include collisions involving vulnerable road users and drivers, driver distraction, and slips, trips, and falls. She utilizes her knowledge of OSHA regulations, codes, and standards in her analysis of premises liability incidents and safety consulting. Ellen is a current member of the Human Factors and Ergonomics Society (HFES) and it’s Forensic Professional Technical Group.
