Dr. Marc Green, PhD
This article should not be interpreted to mean that human perception-reaction time is 1.5 seconds. There is no such thing as the human perception-reaction time. Time to respond varies greatly across different tasks and even within the same task under different conditions. It can range from .15 second to many seconds. It is also highly variable. In many cases, the very concept of perception-reaction time simply doesn’t apply2.
1. A “standard” or “generally accepted” PRT cannot and does not exist;
2. Exact PRT values are almost always impossible to determine due to lack of data, to the impossibility of knowing when to start timing and to the general difficulty of going from the simplified research world to the real-world;
3. A PRT cannot be determined by cookbook methods such as “Olson”, AASHTO or a computer program;
4. Specifying PRT without specifying deceleration holds little value, since stopping depends on both. Drivers often trade them off. Braking at maximum possible deceleration cannot be assumed; and
5. PRT generally does not explain why a collision occurred. It is not a cause, but rather a symptom to be explained. The real cause lies in the answer to the question, “Why was the PRT insufficient?” By example, imagine that your car stops. Why? The gas gauge points to empty. Is that why the car stopped? No. Your car does not stop because the gas gauge needle points to empty. The guage is only an overt symptom and indicator, of being out of gas. The car stopped because it was out of gas, not because the gas gauge’s needle position. PRT is like the gas gauge. The empty tank is like low visibility, misplaced action boundary, response conflict, violated expectation, driver impairment, etc.
In many cases, the speed with which a person can respond, “reaction time,” is the key to assigning liability. It is common practice for accident reconstructionists simply to use a standard reaction time number, such as 1.5 seconds, when analyzing a case. In fact, reaction time is a complicated behavior and is affected by a large number of variables. There can be no single number that applies universally.
Reaction time is a surprisingly complex topic. Unfortunately, most “experts” used canned numbers without a good appreciation for where the numbers originate, how they were obtained or the variables that affect them. Moreover, there are several distinct classes of reaction time, each with somewhat different properties. In this article, I briefly describe some keys issues. The discussion focuses primarily on driver reaction time.
Reaction Time Components
When a person responds to something s/he 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. For example, it is the time required for a driver to detect that a pedestrian is walking across the roadway directly ahead and to decide that the brakes should be applied. Mental processing time is itself a composite of four substages:
- Sensation: the time it takes to detect the sensory input from an object. (“There is a shape in the road.”) All things being equal, reaction time decreases with greater signal intensity (brightness, contrast, size, loudness, etc.), foveal viewing, and better visibility conditions. Best reaction times are also faster for auditory signals than for visual ones. This stage likely does not result in conscious awareness.
- Perception/recognition: the time needed to recognize the meaning of the sensation. (“The shape is a person.”) This requires the application of information from memory to interpret the sensory input. In some cases, “automatic response,” this stage is very fast. In others, “controlled response,” it may take considerable time. In general, novel input slows response, as does low signal probability, uncertainty (signal location, time or form), and surprise.
- Situational awareness: the time needed to recognize and interpret the scene, extract its meaning and possibly extrapolate into the future. For example, once a driver recognizes a pedestrian in the road, and combines that percept with knowledge of his own speed and distance, then he realizes what is happening and what will happen next – the car is heading toward the pedestrian and will possibly result in a collision unless action is taken. As with perception/recognition, novelty slows this mental processing stage. Selection of the wrong memory schema may result in misinterpretation.
- Response selection and programming: the time necessary to decide which if any response to make and to mentally program the movement. (“I should steer left instead of braking.”) Response selection slows under choice reaction time when there are multiple possible signals. Conversely, practice decreases the required time. Lastly, electrophysiological studies show that most people exhibit preparatory muscle potentials prior to the actual movement. In other words, the decision to respond occurs appreciably faster than any recordable response can be observed or measured.
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. For example, it takes time to lift the foot off the accelerator pedal, move it laterally to the brake and then to depress the pedal.
Several factors affect movement times. In general, more complex movements require longer movement times while practice lowers movement times. Finally the Yerkes-Dodson Law says that high emotional arousal, which may be created by an emergency, speeds gross motor movements but impairs fine detailed movements.
3 Device Response Time
Mechanical devices take time to engage, 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.
Here’s a simple example. Suppose a person is driving a car at 55 mph (80.67 feet/sec) during the day on a dry, level road. He sees a pedestrian and applies the brakes. What is the shortest stopping distance that can reasonably be expected? Total stopping distance consists of three components:
- 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.
- 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. There is an additional time required for the pedal to depress and for the brakes to engage. This is variable and difficult to summarize in a single number because it depends on urgency and braking style. In an emergency, a reasonable estimate is .3 second, adding another 24.2 feet3.
- 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
Almost half the distance is created by driver reaction time. This is one reason that it is vital to have a good estimate of speed of human response. Below, I give some values which I have derived from my own experience and from an extensive review of research results.
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.
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.
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 itsimple, reflexive uture 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.
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, sometimes “older means 50, 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 drive slower compensate all or in part for the decline in motor skills. [Note Added. Aging effects in PRT depend heavily on the task. For simple,reflexive responses, healthy older people show little slowing. For complex and/or low visibility tasks, however, they can be much slower.]
Although the data are not clear, it seems likely that females respond slightly slower than males.
Nature of the Signal
In the examples cited above, the driver detected a distinct signal such as a brake light, the appearance of a clear obstacle in the path, etc. Some braking cues are subtler and more difficult to detect, causing slower braking times.
One of the most difficult situations occurs when a driver must detect motion of the car immediately ahead, its acceleration or deceleration. Accidents frequently occur because the driver fails to notice that the car ahead has stopped and does not apply brakes until it is too late.
The general problem involves estimating time-to-collision (TTC. It is a tough problem for several reasons. One is that it is much more difficult to judge motion toward or away from you than it is to judge motion of something which cuts across your path. It’s simply a matter of optics. Humans, in part, sense motion by registering the movement of an object image projected on the retina, the light-sensing portion of the eye. The movement of the object’s image is much smaller with motion toward/away than with motion cutting across the frontal plane.
Second, it is more difficult to judge motion of the object ahead if we are moving as well. The visual system must then disentangle the retinal image motion caused by the movement of the object ahead from the retinal image motion caused by our own “egomotion.” This is far more complex a problem than judging motion of an object when we are stationary.
Third, the 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.
Reaction time increases in poor visibility. Low contrast, peripheral viewing, bad weather, etc. slow response. Moreover, virtually all reaction time studies have been performed in high light, photopic visibility conditions. At night in urban areas, vision operates in the mesopic range, so there is mixed rod-cone activation. The few existing data suggest that reaction time sharply increases as the rods become the primary photoreceptor.
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 the fixture and with a darker background, the signal has higher contrast and greater visibility.
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 the 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 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 the 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.
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 also be considered.
Choice reaction time (Type B) occurs when there 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 keys 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 of possible signals increases according to the equation,
RT = a + b log2N
where a and b are constants and N is the number of alternatives. The equation has two terms. The “a” constant is simply the “irreducible minimum” reaction time in the situation. (The variable part is called “the reducible margin.”) The relationship between RT and the number of alternatives is nonlinear – doubling the number of alternatives does not increase RT by a factor of 2 but rather by the log of the number of possible signals.
In Type C, or “recognition,” reaction time, there are multiple possible signals but only one response. In this case, the responder makes the response when one stimulus occurs but withholds response when the other(s) appears. The standard lab version of this paradigm has a subject with his/her fingers on 1 telegraph key and waits for one of x different lights to flash. When the signal light occurs, s/he releases the telegraph. If one of the nonsignal lights occurs, then the subject should make no response. This is sometimes called the “go, no-go” paradigm. Reaction times are invariably longer than for simple reaction time. A good example would occur when a police officer confronts a “suspect.” The officer sees something in the suspect’s hand and must make a go (shoot) or no-go (don’t shoot) decision.
This article has focused on driver reaction times. While the basic principles 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. (2017). Roadway Human Factors: From Science To Application. Tucson: Lawyers & Judges Publishig.
3I have made some simplifications here. First, some braking occurs during during the brake engagement period. This is best calculated by assuming that braking is half the maximum during the period. Recent data, however, suggests that the period is longer than than the 0.3 second described. Second, drivers do not always depress the brake pedal to maximum or brake in a single continuous movement, so full brake engagement may never occur. Third, vehicles with air brakes require an additional component, “brake lag”. Depending on the setting, air brakes have a .03 to .08 second lag before they engage. Most calculations use a nominal lag value of 0.5 seconds, adding another 40 feet to stopping distance.