PERCEPTION/REACTION DEFINITIONS OF INTEREST
Perception: (1) The act or process of perceiving. (2) The result or effect of perceiving. In traffic accident investigation issues, it can be considered to be the general process of detecting some object or situation, such as a hazard, and comprehending its significance.
Point of Possible Perception: The place and time at which the hazard could have been perceived by a normal person.
Point of (Actual) Perception: The point where a situation is comprehended or perceived as a hazard.
Perception delay: The time involved from the point of possible perception to the point of actual perception. Inattention or distractions may cause or contribute to perception delay.
Perception Distance: The distance traveled during perception delay.
Reaction: The voluntary or involuntary response to a hazard or other situation that has been perceived.
Reaction Time: The length of time from when a person perceives a given situation as being a hazard to when he reacts to his perception. The reaction time of a driver may properly be referred to as a perception-reaction time. If a person's reaction time is unknown, 1.5 seconds may be used for daytime investigation purposes and 2.50 seconds for nighttime.
Reaction Distance: The distance traveled during reaction time.
Simple Reaction: Simple reaction is a response to an expected situation, such as responding to a traffic light. Approximately 0.75 second is required for this response.
Complex Reaction: Complex reaction is a reaction involving a decision, such as when the driver has to decide quickly whether to step on the accelerator or the brake pedal. Reaction time in these situations may be as high as 3 seconds or more.
Simple Reaction Time. Simple reaction time involving a non-complex response, such as touching the horn, may be as low as 0.75 second. The reaction time required to apply the brake pedal after a situation is perceived is from 1 - 3 seconds for most people.
Older drivers have longer reaction times than do young drivers. It is generally considered that at about forty years of age, simple reaction times begin to increase to the extent that at about seventy years of age, a driver's reaction time may increase by as much as 50 percent.
Recent series of studies in Texas measured reaction time required to brake for expected and unexpected events while driving. The authors concluded that all drivers are capable of responding by braking to an unexpected object on the roadway within 2.0 seconds. In addition, Olson, in his book Forensic Aspects of Driver Perception and Response, indicates that PRT of 1.5 seconds is sufficient for most drivers to respond, "given a clear stimulus and a fairly straightforward situation." [Source: Traffic Engineering Handbook, ITE, (5th ed.) 1999.]
The AASHTO Policy Manual on Geometric Design of Highways and Streets, indicates that, based on the relevant literature, the minimum brake reaction time should be at least 1.64 seconds but that, since much of the research was done under conditions that were better than many driving situations, the figure should be 2.5 seconds to take into account the complexity of driving and account for the majority of drivers. This may still be inadequate for the most complex situations. [Source: Traffic Engineering Handbook, ITE, (5th ed.) 1999.]
The above definitions are further explained, as part of the "Series of Events" of a traffic accident, in Rivers, R.W.: Technical Traffic Accident Investigators' Handbook - A Level 3 Reference, Training and Investigation Manual (2nd ed.), and Rivers new EVIDENCE MANUAL (2006), as well as in additional information, references and sources, regarding this subject matter. See:
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