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Article # 0023



Part 1 

Dr. P. F. Packman (1) and Dr. J. L. Packman (2) 

(1)     Professor Emeritus, Dept. of Mechanical Engineering Southern Methodist University,  Dallas TX 75240

(2)     Assistant Professor, Dept. of Counseling and Educational Psychology, University of Nevada, Reno, NV, 89557




People receive and absorb information in the form of stimulus.  They then process the information internally and make responses.  Response time or reaction time is a measure of the time or speed with which a person responds to an external stimulus.  This includes (1) the external initiation of the stimulus and the time for the individual to recognize or register the stimulus, (2) the time to determine the significance of the stimulus, (3) the time for the subject to identify responses, and (4) the time for the individual to carry out an adequate response, i.e. evasive movement.


Movement time deals with motor skills and motor responses as well as the accuracy and control of the motor movements. The literature in the area of motor skills is very large, and comes from a number of diverse disciplines, including engineering, biology, psychology, physical education, kinesiology and neuroscience.  In this discussion of reaction time, we concentrate primarily on the concept of a response/reaction time to external stimuli up to the point of the movement.


In the course of accident reconstruction or individual response behavior studies the nature of the response is concerned with the time taken by the individual before responding.  In many instances this is treated as a relatively trivial matter. Engineers and professional accident investigators often use a standard total reaction time of about 1.5 seconds in automobile accident reconstruction to assign driver fault and liability.  For many reasons a single number to serve all purposes can leave much to be desired.  A range of response times from 0.50 to 3.5 seconds has been references by Olson and Farber (2005).


The process by which a person responds and reacts to external stimuli is complex.  A large number of internal and external variables influence the responses of an individual.  This paper provides a brief review of some of the factors affecting response time and discusses the effects of a number of the variables known or suspected to effect the response times. It discusses the complexity of the process and provides an understanding of the basic concepts and the degree of variability.   It may be possible to evaluate many of the factors known to change response time and obtain a better estimate of the individual’s response to specific stimuli.  In addition we propose a statistical foundation for evaluating the variability of response times.


In this discussion reaction time and response time are interchangeable.


Components of Response Time


Stimulus/ Perception Time


This is the amount of time it takes for the individual to recognize that a stimulus or signal has occurred.  The stimulus can be present in a number of sensory forms: visual, auditory, smell, touch or a combination of factors.  For most people, normal situation stimuli are visual and auditory. Smell, while probably one of the most sensitive, rarely acts alone as a primary stimulus that acts as a warning.  Touch can be a strong primary stimulus acting directly on the individual, but obviously requires close contact as opposed to visual or auditory.


It is generally accepted that auditory stimuli, i.e. noise or abrupt unusual sounds are probably the most effective warning signals.  Typical auditory stimuli are the sound of a horn, a beeping backup sound, cockpit voice warnings, a bang or other percussive sounds.  The best reaction times are always faster for auditory signals rather than visual signals.  This has been confirmed by a number of laboratory experiments where the mean auditory reaction times are 140-160 ms (0.14-0.16 sec) and visual reaction times being 180-200 ms (0.18- 0.2 sec.)  (Galton 1899, Woodwoth and Schlosberg, 1954, Fieandt et al, 1956, Welford 1980,)


Visual signals can be difficult for the individual to process unless they are clear and unambiguous.  The visual signal must be perceived as a clear warning.  Laboratory or closely controlled situations are designed with minimal visual clutter.  In real-world situations the visual danger signals are not necessarily clear and sufficiently defined to produce a rapid reaction response.  Visual driving stimuli that would require a rapid reaction are often ambiguous.  The perception of a shape on the road does not immediately imply danger.  Many drivers do not “see” an object such as a motorcycle, bicycle or pothole as requiring an immediate response when compared to another vehicle, human or large animal in the immediate field of view. 


With all stimuli, it has been demonstrated that the reaction time decreases with greater signal intensity.  This decrease in reaction time with increase in signal intensity has a significant limitation. If the stimulus intensity is too intense, the reaction time increases due to a shock or startle effect.  A moderate increase in a typical sound or visual stimulus would decrease reaction time. If the shock is too loud or the flash is too bright, the individual is stunned or shocked into immobility for a brief time.  This is the concept behind of sound grenades used by SWAT teams when entering an occupied building or an airplane.


Increases in visual stimuli associated with brightness, color, visibility and clarity all decrease reaction time.  If the individual is looking directly at the visual stimuli temporary loss of sight will negate any response that can be made.


A combination of stimuli also decreases the reaction time, providing there is compatibility of stimuli and required response.  A blinking red or orange light on a shut-off button coupled with an intermittent horn or auditory signal provides a clear signal to press the button to restore the system.  Either stimulus alone would require increased visual and cognitive processing time to determine which button needed pressing.  Everyone has had the “modern” problem associated with decoupling of stimuli and response in trying to locate the blinking curser or computer arrow on their computer screen. It can often be lost in the visual clutter on the screen.  A simple change of color to contrast with all other visuals on the screen would considerably reduce the reaction time to locate the curser. 


Cognitive Time


This portion of the response time is further divided into two general components.  The first part is the time to recognize the meaning of the stimulus.  The second part is to apply the recognition/knowledge of the individual to develop an appropriate response.


The cognitive time requires some information from memory to interpret the signal as either benign or dangerous.  For some situations this stage is very rapid and the cognitive response is almost automatic.  A typical automatic response would be the response to touching a very hot surface with your finger.  The time to respond, i.e. pulling the hand off the hot surface, takes considerably less than the accepted 1.5 seconds for stimulus, cognitive and muscle movement.  Simple reaction time to touch is about 155 ms (0.155 sec.,) (Robinson, 1934) This is in the range of simple laboratory response time to auditory and visual stimuli (140-200 ms).


The reason for this short response time is the significant decrease in cognitive time required to process the stimulus of high heat. This rapid reaction would persist in non-laboratory situations. The response is simple response timing, and the real-world situation closely duplicates by the laboratory situation.   The time for muscle movement would not change from real-world situation to laboratory situations.  This simple reaction time can increase significantly if the stimulus was ambiguous, i.e. not too hot, or if there were extenuating circumstances such as other high intensity stimulus. There are often stories of individuals who discovered they had burns on their skin that occurred during rescue efforts that they did not remember receiving and did not reach to them at the time.


Muscle Movement Time


This is the time it takes to initiate muscle movement and is the final stage in the individual reaction time.  There is an additional length of time required when the muscle movement activates an electrical or mechanical device, and the device takes sometime to respond, but this is not usually considered as part of the human reaction process.   A typical mechanical reaction process outside of the human reaction time factors is the length of time (or distance) it would take for an automobile breaking system to activate the brakes and to stop a vehicle.   

Forms of Reaction Time

Simple Reaction Time


When there is only one stimulus and only one response is required, the process is called simple reaction time.  Many laboratory experiments are measurements of simple reaction times where the subject is aware that the stimulus will occur within a relatively short period of time and within a confined special region. .  Many of these studies are concerned with the organization of the brain and how different factors may influence the brain function/reaction and simple reaction times/behavior.  Professional psychological experiments typically examine 20 subjects in each sample group.  These subjects are given about 100-200 reaction tests. (Saunders 1998)  Because the test population is small, the data is usually limited and statistical analysis is limited to the mean and standard deviation to determine if there is a difference in reaction time due to the differing test factors.


There are relatively few simple reaction time conditions outside of a controlled laboratory experiment.  In most real-life conditions there are usually several different or conflicting stimuli that required significantly different responses. In most cases the occurrence of the stimulus is not anticipated.


Choice or Complex Reaction Time


This is part of a cognitive process required when a number of possible stimuli are presented to the subject.  Different responses are required depending on the type and form of the stimulus.  For example, in a laboratory environment if a letter appears on a screen, the subject is required to press a key corresponding to the letter. However if a number appears on the screen the subject may be required to speak the number.  In a real-world situation the individual may respond in a number of different ways to the same stimulus.  A choice or complex response to the single stimuli an object suddenly appearing in the driver’s vision would be (a) to step on the brakes, (b) to turn the wheel to avoid the object, (c) to do noting and hit the object and (d) to do nothing and permit the vehicle and the object to avoid each other. The response time increases with the number of possible responses. (Saunders 1998)


In a recognition reaction time experiment the applied stimuli may correspond to a pre-selected memory set or experience. If the stimuli are in the memory set of the individual, the subject is required to respond, if the stimuli does not match the memory set, then either a different response or no response is correct. Real-life recognition reaction time situations are often very complex. 


Consider a situation where a pilot is confronted with an unexpected situation.  The pilot must recognize the situation, and draw on his training and experience (memory set) to develop an adequate and correct response.  Inexperienced pilots flying general aviation aircraft under visual flight conditions can become disoriented in fog or nighttime environments.  The pilot loses visual references such as a horizon, ground lights or ground orientation references.  The aircraft can gradually enter a spin.  When (or if) the pilot breaks out into the clear while in the spin, the immediate reaction is to pull back on the yoke.  This tightens the spin and often results in structural failure. This type of loss of orientation is common and has resulted in a number of flying accidents. 


If the aircraft does not enter a spin but remains level, the loss of the horizon doe not allow the pilot to distinguish between the ground and the airspace.  The pilot can gradually fly into the terrain.  This is the presumed accident scenario that killed John Kennedy while flying over water.       


Laboratory reaction time experiments, while instructive and informative, do not necessarily correspond directly to reaction times in the non-laboratory environment.   Real-world stimuli/reactions are not simple and the choices available to the individual can be complex, misleading and counter-intuitive.  Training or lack of training can play a major role in the variability of reaction times among individuals.  In laboratory experiments when the same subject is used for 100-200 reaction time measurements their later response would be quicker than the first time experimental response.  The initial subject response would logically be closer to the real-world unlearned response time. 



Variables Affecting Reaction Times

in Simple Reaction Time Experiments


While simple reaction time laboratory experiments rarely duplicate real-world situations, these experiments are used to provide significant insight into brain function and disfunction.  Simple reaction time tested in a laboratory environment typically ranges from 150 to 250 ms (0.15 to 0.25 sec.).  A value of 200 ms is a fair average for a mixed population.  A number of variables are known to modify simple reaction times, but for the most part these changes are less than 100 ms.


The literature notes that for college-age individuals the simple reaction times as tested in the laboratory have been about 0.19 seconds (190 ms) for light stimuli and 0.16 sec (160 ms) for sound stimuli (Galton, 1899, Welford 1980, Brebner and Wellford, 1980, Fieandt et al, 1956).  Simple auditory or simple tactile reaction time is about 40 ms faster than visual reaction time. 


When the visual stimulus is above the threshold for vision the simple reaction time increases to about 200 ms even for very small and brief light flashes.  It was reported that the difference between simple reaction times to light and sound stimuli could be minimized if the stimulus intensity was increased. (Kohfeld, 1971)


Reaction times decrease if the visual and auditory stimuli are increased in duration and in intensity. (Froeberg, 1907, Wells, 1913)


Visual stimuli presented in the central field of view produce responses about 15-30 ms faster than those presented in the peripheral field of view. (Keele, 1986)


Age alone appears to have little effect on simple reaction time.  There is a modest slowing after age 60.  Much of the increase in reaction time associated with aging concerns decreases in visual or auditory acuity and perception of the stimulus. 


A study reported on the University of Idaho website (2006) indicates that the range of reaction time for drivers to recognize a road hazard and move their foot to apply the vehicle brake system varies from less than 1 second to up to 3.5 seconds.  In the design of roadways, the civil engineer must provide a continuous safe sight distance equal to or greater than the stopping sight distance.  The designers assume that 90% of the driving population can react to visual stimuli within 2.5 seconds.  The distance traveled during the driver reaction time is calculated by multiplying the vehicle speed ft/second) by the brake reaction time (2.5 seconds).  The distance required to brake the vehicle from the posted speed is then added to the reaction time distance to obtain the total stopping sight distance.  It is important to note that the brake reaction time of 2.5 seconds is a design reaction time and not the 1.5 seconds used by the accident reconstructionists.

Simple vs. Recognition vs. Choice Reaction Time


Simple reaction time is shorter than recognition reaction time. Choice reaction time is the longest of the three (Donders, 1868)  Simple reaction time averaged about 220 ms, recognition reaction times averaged about 384 ms.  The more complex the stimuli the longer the reaction time. (Brebner and Welford, 1980) For example if there are two auditory signals, but only one signal requires a response, the reaction time would be longer than a subject confronted with single tone.  The more responses possible, the longer the reaction time.  If the initial reaction time for a single response is 0.20 seconds, three possible responses increase the reaction time to 0.40 seconds, while 5 possible choices increase the time to 0.50 seconds. The reaction time continues to increase to 0.65 seconds for as much as 10 possible responses. 


Article # 0023         TEST QUESTIONS:

1.   The time or speed with which a person responds to an external stimulus is called the ____.

  1. response time 

  2. reaction time

  3. reflex time

  4. a. and b.

2.   Engineers and professional accident investigators often use a standard total reaction time of about _____ in automobile accident reconstruction to assign driver fault and liability.

  1. 1 second

  2. 1.5 seconds

  3. 2 seconds

  4. 2.5 seconds

3.   Reaction times for auditory signals are _____  than for visual signals.

  1. faster

  2. slower

  3. more easily measured

  4. more consistent

4.   Which of the following are components of response time?

  1. Cognitive Time

  2. Muscle Movement Time

  3. Stimulus/ Perception Time

  4. All of the above

5.   When there is only one stimulus and only one response is required, the process is called ____.

  1. recognition reaction time

  2. cognitive reaction time

  3. simple reaction time

  4. none of the above

6.    Reaction times decrease if the visual and auditory stimuli are 

  1. increased in duration

  2. increased in intensity

  3. combined, provided the stimuli are compatible with the required response

  4. All of the above

7.   If the stimulus intensity is too intense, the reaction time increases due to a _____.

  1. immobility effect

  2. startle effect.

  3. SWAT effect.

  4. none of the above

8.   The increase in reaction time associated with aging is mostly due to ....

  1. perception of the stimulus

  2. decreases in auditory acuity

  3. decreases in visual acuity

  4. All of the above

9.   What is the effect on response time to visual stimuli presented in the central field of view verses those presented in the peripheral field of view

  1. 15-30 ms slower

  2. 15-30 ms faster

  3. 200 ms faster

  4. 500 ms faster

10.   Simple auditory or simple tactile reaction time is about ____ms faster than visual reaction time. 

  1. 200

  2. 120

  3. 40

  4. 5

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