Perception/Response When Driving at Night

Perception-response time is the interval between when some object or condition first becomes visible to a reasonably alert driver and when the driver initiates a discernable response (e.g. foot on the brake pedal or the hands start to turn the steering wheel). This typically starts when an obstacle or event enters the driver's field of view.

The most important factor affecting perception-response time is expectation. Expectation exists in three levels: 1) Expected, 2) Unexpected and 3) Surprise. Expectancy also plays an important part in assisting or completing Identification.

Perception and Response has four stages:

  1. Detection - This starts when a hazard enters the driver's field of view and it ends when the driver has become consciously aware that something is there. Nighttime is the most common condition that can severely limit visibility, thereby making it difficult to determine when some object or condition has entered the driver's field of view. During the day, a potential hazard, such as a pedestrian standing in the road, would be visible for a considerable distance. However, at night, this pedestrian becomes gradually more visible as the vehicle approaches. The end of Detection is marked by when the driver becomes aware that the pedestrian is there.
  2. Identification - Once the driver becomes aware of something, he/she must next acquire sufficient information in order to decide what action is necessary to take, if any. The Identification stage is a frequent source of difficulty. Consequently, Identification is often responsible for lengthening the overall perception-response, particularly under adverse visibility conditions such as nighttime or adverse weather.
  3. Decision - After Identification is complete, the driver must decide what action is necessary to take, if any.
  4. Response - In this stage, the brain gives out instructions to the appropriate muscle groups to carry out the intended action (e.g. the foot lands on the brake pedal or the hands begin to turn the steering wheel).

Source: Forensic Aspects of Driver Perception and Response by Paul L. Olson and Eugene Farber.

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