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Now You See Me, Now You Don’t

April 26, 2017


Have you ever wondered why a driver couldn’t avoid hitting a pedestrian crossing the road at a marked crossing? What about the driver who was going full speed and rear-ended thestopped truck ahead without slowing down? These are examples of drivers not seeing hazards on the roadway in time, possibly due to visual issues. These visibility issues create situations in which drivers may not detect what may appear to be very noticeable hazards.

In the context of motor vehicle collisions, it is common to talk about a driver’s perception-response time. This is the time that elapses while a driver perceives the nature of a hazard, decides what to do and then implements a response to that hazard (e.g. braking or steering the car). However, there is another equally important time interval that is rarely mentioned in the aftermath analysis of motor vehicle collisions and that is detection interval. The detection interval is the time it takes for a driver to identify a hazard after it has come into the driver’s field of view.

The detection interval is important because if a driver does not detect the presence of a hazard, then perception-response cannot occur. Once a hazard is identifiable as an immediate problem, the driver’s perception-reaction can begin. The length of the detection interval can vary and is completely dependent upon the incident circumstances. For example, a large obstacle on the roadway that comes into view over the crest of a hill during daylight hours will result in a relatively short detection interval. However, a darkly dressed pedestrian stepping into a busy roadway under nighttime conditions may result in a longer detection period.

There are several factors that can affect a drivers’ ability to detect hazards. An object’s contrast, or conspicuity, with its background provides the greatest discernment for a driver to discriminate a hazard. In daytime, differences in colour, pattern and shading are often better for a driver to visually identify and categorize object, including those that present themselves as hazards. However, at nighttime, only brightness contrast is available for a driver to detect the presence and nature of objects in their field of view. As such, the function of vehicle headlights is to provide sufficient luminance contrast so that objects can be detected. Environmental conditions such as rain, fog, snow and glare can obscure vision that decreases the relative contrast of an object with its background. Also, lots of visual clutter can also make it difficult for a hazard to stand out from its background.

The location within a driver’s field of view where a hazard appears also affects a driver’s ability to detect it as a hazard. There is a notable difference on whether a hazard is directly in front of the driver’s focused view or offset to the side in the driver’s peripheral vision. The larger the distance from the center of the observer’s field of view, the longer the detection interval and longer overall response time.

Drivers expect certain things to happen, or things to be configured in certain ways on the roadway. For example, flashing lights at a crosswalk alert a driver to a potential pedestrian crossing their path. However, when something unexpected happens, it is more difficult for the driver to perceive and then react. In most accident situations, drivers are not expecting the hazard.

Reconstruction of a motor vehicle collision including analysis of the lighting and environmental conditions, an assessment of driver visibility and the ability of the driver to detect a particular hazard can help provides answers to questions of liability, clarify reasons for driver behaviour, identify contributing factors and identify contributory negligence.


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