Reaction Time Components
When a person responds to something hears, sees or feels, the total reaction time can be decomposed into a sequence of components.
1-Mental Processing Time
This is the time it takes for the responder to perceive that a signal has occurred
and to decide upon a response.
Mental processing time is itself a composite of four sub-stages:
- Sensation: the time it takes to detect the sensory input from an object.
- Perception/recognition: the time needed to recognize the meaning of the sensation.
- Situational awareness: the time needed to recognize and interpret the scene, extract it's meaning and possibility extrapolate into the future.
-Response selection and programming: the time necessary to decide which if any response to make and to mentally program the movement.
These four stages are usually lumped together as "perception time," a misnomer since response selection and some aspects of situational awareness are decision, not perception.
2- Movement Time
Once a response is selected, the responder must perform the required muscle movement.
Several factors affect movement times. In general, more complex the movements require longer movement times while practice lowers movement times.
3- Device Response Time
Mechanical devices take time to actuate, even after the responder has acted. For example, a driver stepping on the brake pedal does not stop the car immediately. Instead, the stopping is a function of physical forces, gravity and friction.
Total stopping distance consists of three components:
1. Reaction Distance. First. Suppose the reaction time is 1.5 seconds. This means that the car will travel 1.5 x80.67 or 120.9 feet before the brakes are even applied.
2. Brake Engagement Distance. Most reaction time studies consider the response completed at the moment the foot touches the brake pedal. However, brakes do not engage instantaneously.
3. Physical Force Distance. Once the brakes engage, the stopping distance is determined by physical forces (D=S²/(30*f) where S is mph) as 134.4 feet.
Total Stopping Distance = 120.9 ft + 24.2 ft + 134.4 ft = 279.5 ft
Expectation
Reaction times are greatly affected by whether the driver is alert to the need to brake. alertness is divide into three classes:
- Expected: the driver is alert and aware of the good possibility that braking will be necessary.
- 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.
Urgency
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, weight 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 1 second or more, depending on the circumstances.
Stimulus-Response Compatibility
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.
Most people have experienced this phenomenon when going into a skid. The correct response is to turn he 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.
Age
Although most basic research finds that older people respond slower than younger ones, the data on older drivers' braking times are not entirely clear. One problem is that different studies have used different definitions of older; that is, sometime "older means 55, sometimes it could mean 70. Moreover, some studies find no slowing of reaction time with age. Instead, they conclude that the older driver's greater experience and tendency to driver slower compensate all or in part for the decline in motor skills. Never the less, I would place the slowing with age to be about 0.3 seconds for a "moderately" older driver, say 65-70. On the other hand, older drivers generally compensate for slower reaction times with reduced speeds.
Gender
Although the data are not entire clear, it seems likely that females respond slightly slower than males.
Nature of the Signal
One
Second, it is more difficult to judge motion of the object ahead if we are moving as well. The visual system must then disentangle to the retinal image motion caused by the movement of the object ahead from the retinal image motion caused by our own motion. This is far more complex a problem than judging motion of an object when we are stationary.
Third, normal expectation is that cars do not stop in the middle of the road. Reaction time, as explained above, is much slower when people encounter a low probability or unexpected event.
Visibility
On the other hand, there are some situations in which response is faster in low light. For example, light emitting sources, such as rail-highway crossing signals or brake lights, produce better reaction times at night. With no sun or skylight to reflect off of the fixture and with a darker background, the signal has higher contrast and greater visibility.
Response Complexity
More complex muscular responses take longer. For example, braking requires lifting the foot from the accelerator, moving laterally to the brake pedal and then depressing. This is far more complex than turning the steering wheel. While there have been relatively few studies of steering reaction time, they find steering to be 0.15 to 0.3 second faster. Perception times are presumably the same, but assuming the hands are on he steering wheel, the movement required to turn a wheel is performed much faster than that required to move the foot from accelerator to brake pedal.
Reaction Time At Night
The same factors affecting reaction day in daylight conditions operate at night. Light level per se, has little effect on reaction time. For example, one study found that under scotopic vision, decreasing light levels by a factor of ten only slowed reaction time by 20-25 msec (1/40 to 1/50 second.)
However, there are new variables at work. For example, a light which might have low contrast and low conspicuity during the day because background is bright could become highly conspicuous at night and produce faster reaction times. Always remember that contrast is what matters: people see contrast, not light.
Case Study
The ensuing example is taken from a real case. It demonstrates how "standard" reaction time estimates must be adjusted to situation-specific conditions.
A 73 year old male driver, Mr. Smith, broad-sided another car crossing his path from a side road in good daylight visibility. Mr. Smith stated that he had approached the intersection cautiously because he had had several close calls with vehicles cutting across that intersection.
. A hundred feet before the intersection he saw a car from the side road. What is his expected reaction time?
First, expectation has a large effect on reaction time. Since Mr. Smith was aware of previous accidents at the intersection and had experienced close calls himself. Given Mr. Smith's heightened alertness and his expectation that a dangerous situation might arise.
Complex Reaction Times
In his classic "On The Speed Of Mental Processes," Donders (1868) proposed a classification scheme that experts still use to distinguish among three different types of reaction time, simple (Type A) and more complex situations, choice (Type B) and recognition (Type C). While most of the variables affect simple and complex types in the same way, choice and recognition reaction times each add new factors that must be also be considered.
Choice reaction time (Type B) occurs when are multiple possible signals, each requiring a different response. The responder must choose which signal was present, and then make the response appropriate for that light. This requires two processes not present in simple reaction time: 1) signal discrimination - decide which signal occurred and 2) response selection - choose the response based on which signal occurred. In the classic laboratory procedure, a person sits with his/her fingers on 2 different telegraph key and waits for one of 2 different lights to flash. When a signal occurs, s/he releases the telegraph key assigned to that signal. Reaction time is again the time between light onset (signal) and release of the key (response.)
With multiple signals, the responder cannot simply detect the signal but must also recognize which signal occurred and then mentally program the correct response. These extra mental operations slow reaction. Choice reaction times slow as the number off possible signals increases according to the equation,
RT = a + b log2N
Final Comments
This article has focused on driver reaction times. While the basic principals generalize to estimating other reaction times, the exact numbers do not. Each type of reaction time has its own peculiarities that must be examined. For example, reaction time for a shooter who is tracking a target might be 0.3 second. but even this would be a function of trigger pull weight.
1This is a brief summary/elaboration of the article, "'How Long Does It Take To Stop?' Methodological Analysis of Driver Perception-Brake Times" Transportation Human Factors, 2, pp 195-216, 2000.
2See Green, M. (2009) Perception-Reaction Time: Is Olson (& Sivak) All You Need To Know?," Collision, 4, 88-95.
