Profile photo of 74

Not trying to dispute what happens in your class because there are many variables that affect the process. I’ve copied a section of an article that provides insight into the matter for everyone not familiar with the dynamics involved. This article is based on driving and accidents which most of the studies are derived.

You’ll notice that “expected” reaction times are around .5 seconds. Reaction to “unexpected” events goes up to 1.25 seconds and a “surprise” reaction time is 1.5 seconds. There are other components to the process listed below that affect the outcome and I suggest everyone read the complete article and digest the information. His last comment is that it may take .3 seconds recation time for a shooter tracking a target.


“Response speed depends on several factors so there can be no single, universal reaction time value. Here is a list of factors which affect reaction time. In all cases, the times assume daylight and good visibility conditions.


Reaction times are greatly affected by whether the driver is alert to the need to brake. I’ve found it useful to divide alertness into three classes:

Expected: the driver is alert and aware of the good possibility that braking will be necessary. This is the absolute best reaction time possible. The best estimate is 0.7 second. Of this, 0.5 is perception and 0.2 is movement, the time required to release the accelerator and to depress the brake pedal.

Unexpected: the driver detects a common road signal such as a brake from the car ahead or from a traffic signal. Reaction time is somewhat slower, about 1.25 seconds. This is due to the increase in perception time to over a second with movement time still about 0.2 second.

Surprise: the drive encounters a very unusual circumstance, such as a pedestrian or another car crossing the road in the near distance. There is extra time needed to interpret the event and to decide upon response. Reaction time depends to some extent on the distance to the obstacle and whether it is approaching from the side and is first seen in peripheral vision. The best estimate is 1.5 seconds for side incursions and perhaps a few tenths of a second faster for straight-ahead obstacles. Perception time is 1.2 seconds while movement time lengthens to 0.3 second.

The increased reaction time is due to several factors, including the need to interpret the novel situation and possibly to decide whether there is time to brake or whether steering is a better response. Moreover, drivers encountering another vehicle or pedestrian that violates traffic regulations tend to hesitate, expecting the vehicle/pedestrian to eventually halt. Lastly, there can be response conflict that lengthens reaction time. For example, if a driver’s only possible response requires steering into an oncoming traffic lane (to the left) there may be a hesitation.


People brake faster when there is great urgency, when the time to collision is briefer. The driver is travelling faster and/or the obstacle is near when first seen. While brake times generally fall with greater urgency, there are circumstances where reaction time becomes very long when time-to-collision is very short. The most common situation is that the driver has the option of steering into the oncoming lane into order to avoid the obstacle. The driver then must consider alternative responses, braking vs. steering, weigh the dangers of each response, check the left lane for traffic, etc.

Cognitive Load

When other driving or nondriving matters consume the driver’s attention, then brake time becomes longer. For example, on a winding road, the driver must attend more to steering the car through the turns. Another major load on attention is the use of in-car displays and cell phones. There is no doubt that both cause delays in reaction times, with estimates ranging from 0.3 to as high a second or more, depending on the circumstances.

Stimulus-Response Compatibility

Humans have some highly built-in connections between percepts and responses. Pairings with high “stimulus-response compatibility” tend to be made very fast, with little need for thinking and with low error. Low stimulus-response incompatibility usually means slow response and high likelihood of error.

One source of many accidents is the human tendency to respond in the direction away from a negative stimulus, such as an obstacle on a collision course. If a driver sees a car approach from the right, for example, the overwhelming tendency will be to steer left, often resulting in the driver steering right into the path of the oncoming vehicle. The stimulus-response capability overrides and the driver simply cannot take the time to observe the oncoming car’s trajectory and to mentally calculate it’s future position. In short, the driver must respond to where the car is now, not where it will be at some point in the future.

Most people have experienced this phenomenon when going into a skid. The correct response is to turn the wheel in the direction of the skid, but it takes practice and mental concentration to avoid turning the wheel away from the skid, which is the high compatibility response.

Psychological Refractory Period

Following a response, people exhibit a “psychological refractory period.” During this period, new responses are made more slowly than if there had been no previous behavior. For example, suppose a driver suddenly steers left and then right. The steer-right response will occur more slowly because it immediately followed the steer-left.