National Highway Traffic Safety Administration (NHTSA) reporting shows nighttime crash rates are disproportionately higher than daytime crash rates. Despite the fact that only 25% of driving occurs at night, approximately half of fatal crashes occur during this time. Several factors contribute to the increased danger of nighttime driving, including reduced visibility under dark conditions, fatigue, and impaired drivers.
When investigating nighttime vehicle collisions, it is critical to determine the distance at which a hazard (e.g., pedestrian, animal, road debris, disabled vehicle, etc.) should have been detected by the striking driver. Establishing the detection distance is essential because a driver cannot reasonably be expected to take evasive action to avoid a collision until they can, at a minimum, detect the presence of the hazard on the roadway. Human Factors experts (often referred to as visibility experts or conspicuity experts in this context) have specialized expertise in performing these evaluations, including conducting field investigations of nighttime visibility and calculating visibility level (VL).
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Field Investigations of Nighttime Visibility
Field investigations of nighttime visibility involve a Human Factors expert conducting an inspection at the site of the collision for the purpose of observing the distance at which the hazard is detectable under the same lighting, roadway, and environmental conditions as were present at the time of the collision. To obtain equivalent ambient lighting conditions, the inspection must be conducted when the sun is in an equivalent position above or below the horizon. The position of the moon at the time of the inspection is generally of no consequence because the amount of light provided by even a full moon directly overhead is trivial in comparison to the light provided by headlights and other artificial light sources. If the collision occurred on a roadway with artificial light sources such as street lights or lights on nearby buildings, the inspection should be conducted at the collision location so that all ambient light sources that may affect detection distance are adequately accounted for. Exemplar vehicles, surrogate pedestrians, and hazards should be placed in their respective positions on or near the roadway to replicate the roadway conditions at the time of the collision. Lastly, the inspection should be conducted under similar weather conditions. Once the lighting, roadway and environmental conditions have been recreated, investigators then make observations by driving an exemplar vehicle towards the hazard and measuring the distance at which they are first able to detect the hazard in the roadway. Light measurements may be taken at the inspection to validate the investigator’s observations.
Adjusting for Expectancy
It is critical to understand, however, that the investigator’s observations during the inspection correspond to about the 95th percentile of what would be obtained in a controlled test with multiple trials and multiple participants. Meaning, approximately 95% of drivers under real-world conditions would not detect the hazard until they were closer to it. That is because the investigator knows what the hazard is and where it is located. Therefore, to calculate the distance at which a typical (average) driver would be expected to detect the hazard under real-world conditions, an investigator must approximate the median distance that would be observed under formal test conditions and then reduce that value to correct for expectancy.[1] As an example, if an investigator observed the hazard at a distance of 500 ft during the inspection, a typical driver encountering the unexpected hazard under real-world conditions would be expected to detect the hazard at approximately 165 ft.
Visibility Level
Human Factors experts can also evaluate the ability of a driver to readily detect an object at a specified distance under dark conditions by calculating the visibility level (VL)[2]. Visibility level is defined as the luminance contrast required at different background luminance levels to achieve visibility under various conditions.[3] VL is determined by the object height, viewing distance, ambient lighting conditions, headlight illumination, reflectance of the object and its background, and the age of the driver.[4]
If the collision occurred on a roadway with artificial light sources (e.g., street lights), investigators must use light measurements taken during a field investigation to calculate the luminance contrast so that all ambient light sources have been accounted for in the VL calculations. If the collision occurred on an unlit roadway under completely dark conditions, investigators can estimate luminance and reflectivity values using published data on headlamp illumination and the reflectivity of the object and its background (e.g., asphalt, clothing, etc.).
The VL is then calculated for a specified distance, object height, and the age of the driver.
A visibility level equal to 1 indicates that an object is just barely detectable to an observer. As the visibility level increases, the likelihood that the object will be detected increases. A reasonably attentive driver will likely detect an object with a visibility level of at least 6 or 7.[5] For example, a calculated VL of 12 at a specified distance of 250 ft indicates that a reasonably attentive driver would be expected to detect the hazard at a distance of at least 250 ft.
Published Detection or Recognition Distances to Hazards
Many scientific studies have evaluated detection or recognition distance to various hazards under specific nighttime visibility conditions. These published studies can be effectively used to support findings from field investigations and calculations of visibility level. However, experts should be cautious when using data from scientific studies as the sole basis for an opinion on detection distance to a hazard in a specific collision. For example, many of the studies were conducted on dark, unlit roadways and are, therefore, only applicable to a collision that occurred on a dark, unlit roadway with no other light sources (e.g., street lights, glare from oncoming headlights, etc.). In addition, many of the studies evaluated specific hazards (e.g., dark clad pedestrians, unlit vehicles, etc.) and are, therefore, only applicable to that specific hazard. Thus, while published data on nighttime detection or recognition distance to hazards can be valid, reliable sources of information, an expert should use more than one method to substantiate and support their evaluation of detection distance, if possible.
Evidence Solutions Human Factors Experts
If your case involves a nighttime collision in which a driver’s ability to detect a hazard under dark conditions is a factor, reach out to Dr. Nancy Grugle to discuss your case and whether or not it may benefit from a Human Factors investigation.
