Perception-reaction Time PAGE 49

8/11/2019 Perception-reaction Time PAGE 49

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The primary dependent variable was the time from first visibility of the obstacle until a response

was initiated by the subject releasing the accelerator. Time from accelerator release until brake

contact was also recorded. These two measures were then summed to obtain a TOTAL

RESPONSE TIME.

"SURPRISE" TRIALIn this study, subjects drove an instrumented test vehicle for several miles (purportedly for

purposes of familiarization as a prelude to a study of driving performance) until they crested a

hill and encountered an obstacle on the reverse slope. Measures were made of the time from first

sighting of the obstacle until the subject removed his/her foot from the accelerator (perception

time), and the time from accelerator release to brake contact (response time). The two valuessummed gave total time. This was referred to as the "surprise" trial.

"ALERTED" TRIALSWith the surprise phase completed, data were taken on the same subjects, under identical

conditions, except they knew the obstacle would be there. Each subject made five runs underinstructions to lift one's foot from the accelerator and tap the brake pedal as quickly as possible

when the subject saw the obstacle. These were referred to as "alerted" trials.

"BRAKE" TRIALS

At the conclusion of the alerted trials a red lamp was fastened to the front of the car hood. The

subjects were instructed that they were to release the accelerator and tap the brake as rapidly as

possible whenever the lamp came on. Five such trials were conducted as the subjects drove the

car back to the point where they had started out. These were referred to as "brake" trials.

Two groups of subjects were run in this study.

1. One group was "young," i.e., 40 years or less.

2. The second group was "older," i.e., 60 years or more.


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However, the subjects in this study were not representative of the normal driving population in

some significant respects. For example, they had been driving for only about 10 to 15 min at the


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time they encountered the obstacle, they knew they were involved in a study (although they did

not know its purpose), and they were not, as far as could be determined, under the influence of

anything (e.g., alcohol) that might alter their response times. All of these factors would favorshorter perception-response times. Therefore, a correction to the values measured in this study is

appropriate to allow for a driving population that includes persons who are relatively fatigued,

less attentive, or whose senses have been dulled by drugs of some kind.


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Other variables that were studied were obstacle height, width, and contrast, there were three

levels of each variable. The intent was to consider the obstacle in the surprise study as a mediumcondition, and bracket it with the variables in this study. Twenty-six subjects participated. All

had been involved in the surprise study. The results of this study are summarized in Figure 4.

This figure is a normal probability distribution of the total perception-response times for each ofthe seven obstacle conditions studied. The "standard" obstacle was the same as that used in the

surprise study. These data were subjected to statistical analysis. In general, it was found that the


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high and the narrow obstacles were associated with longer perception-response times than the

others, and the white and dark (high contrast) obstacles were associated with shorter perception-

response times than the others.

Perception-reaction Time PAGE 49

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