Is The Moth Effect Real?1
|Note. Updated 4 July 2013. This article should not be interpreted to mean that having flashing or any other lights is bad. If I'm a bicyclist, for example, I want as many lights as possible, preferably flashing. Lack of visibility and conspicuity constitutes a far greater danger than the moth-effect.|
Clark et al. studied pilot errors due to "fascination," concentration on some object or task that caused loss of voluntary control over response. They divided fascination into subcategories with the quote above exemplifying Type B-2, where pilots suffering felt drawn to a target and could not avoid the attraction. If the target is a light viewed at night, then the phenomenon is now called the "moth to the flame effect" or more often simply the "moth effect."2
The moth effect has also long been discussed by motorcycle riders, who call the phenomenon "target fixation". The NHTSA-funded "Model National Standards For Entry-Level Motorcycle Rider Training" includes target fixation as a rider hazard: "staring at the object you are trying to avoid. Associated with riders striking obstacles they were attempting to avoid." The standard warns riders to avoid such behavior. 3
Some (e. g., Younger, 1997) believe that the moth effect causes road accidents. In one common scenario, a driver inexplicably steers off the road and collides with an emergency vehicle parked on the shoulder. Presumably, the fascination/moth effect caused the driver to steer toward the brightly flashing waning lights. moth effect advocates have usually supported this belief with anecdotal accident reports supplemented by evidence (Taylor & Sucov, 1974; Murdoch & Caughey, 2004) showing that humans innately orient toward light
Despite these arguments and the reports of airplane pilots, many (e. g., Summala, Leino, & Vierimaa, 1981; Agent & Pigman, 1990), are skeptical and argue that no empirical support exists. Muttart (2004) dismissed the effect as a myth saying, "there is no credible empirical support for a theory that claims that drivers drive toward lights at night." Wells (2004) summarized the view of many by saying, "There are no known studies that have not been disproven that substantiate the actual existence of this effect in real world driving." The studies may have been unknown to people unfamiliar with scientific literature, but there is a substantial body of scientific research showing people unconsciously move in the direction of gaze.
The first attempts to validate the moth effect compared accidents rates for emergency vehicles equipped with and without warning lights. The moth effect would appear as an increase in the accident rate for equipped over unequipped vehicles. Charles, Crank, & Falcone (1990) reviewed several accident analyses and concluded that the data showed no evidence for increased accident rates in marked cars and hence no clear evidence for the moth effect.
In reality, however, these studies say very little. First, marked vehicles would be more visible. Any increase in accident rate due to the moth effect would likely be offset by a decrease in accident rate due to higher visibility. At most, the studies could reveal the cost/benefit tradeoff of the two effects. Second, marked and unmarked vehicles may have differed in other ways that confound the studies. For example, marked cars might be newer and used more often or used in different ways. Further, the respondents in the Clark et al. (1953) report believed that type B-2 fascination caused about 5% of pilot errors while Sipes, Lessard, & Heideman (1998) agreed that the moth effect causes accidents but concluded it was a relatively rare source of pilot disorientation. If these conclusions transfer to road accidents, the moth effect may simply be too small effect to rise above noise in these poorly controlled data. Lastly, the moth effect might depend on an enabling variable, such as driver fatigue or distraction that was not captured in the results. Lumping all accidents together would obscure key variables, so a simple examination of global accident rates is not likely to uncover evidence for the moth effect.
Early experimental tests of the moth effect also failed to produce support. (Helander, 1978) sought to provide direct evidence by testing the steering of drivers as they met an on coming car. He found that drivers began steering away from the oncoming vehicle but that 2 seconds before the meeting drivers deflected the steering wheel in the direction toward the approaching vehicle. The deflection maximized just as the oncoming vehicle passed, a result that was consistent with a moth effect. Helander (1978) further argued that the steering could not be a compensation for the initial steering away from the oncoming vehicle. The study has been criticized (Summala, Leino & Vierimaa,1981) because he measured only steering wheel deflection and not actual lane position. Later, the same author (Summala, 1998), showed that steering becomes generally erratic when drivers fixate eccentrically.
However, earlier research had already established a connection between steering and direction of gaze. Martin (1940) concluded that "laboratory tests demonstrated that manipulation of an airplane steering mechanism during conjugate4 gaze to one side caused involuntary pressure on the mechanism, tending to turn it toward the same side." Later, Kitamura & Matsunaga (1994) measured lane position for drivers passing stopped roadside emergency vehicles. The study tested three conditions: 1) emergency lights extinguished, 2) emergency lights operating with no specific instructions to the drivers and 3) emergency lights operating with drivers instructed to attend the lights. Drivers instructed to fixate the lights passed closer to the parked vehicle than drivers with no specific instructions and drivers who passed the vehicle when the hazard lights were extinguished. These results are the first clear, experimental demonstration of a moth effect. Moreover, they implicate driver attention as a possible enabling variable.
Recent studies provide even more compelling evidence. Readinger, Chatziastros, Cunningham, Bülthoff. H., & Cutting (2002) had drivers steer a simulator car down a virtual road while performing a Landolt C acuity task for targets fixated a various eccentricities. Drivers began steering their cars in the direction of fixation only a few seconds after the beginning of each trial. The magnitude of the lane deviation generally increased over time, although there was a tendency to make corrections giving the lane position a periodic component. The lane deviations averaged about 1.2 meters but reached distances as great as 2.5-3 meters (Chatziastros, 2006). These effects are certainly big enough to cause a driver to inadvertently leave the road. A subsequent study (Chatziastros, Readinger, & Bülthoff, 2003) replicated the original and also discovered that the effect was bigger when the scene contained few trees to provide visual information.
What Causes The moth effect?
The last two studies showed that drivers might steer off the road in the direction of their fixation. Neither study, however, employed bright lights, so it is unlikely that the "moth effect" results from an innate phototaxis. In fact, vehicles are not even necessary to produce the moth effect. One study (Cutting, Readinger, & Wang, 2002) found that pedestrians walk slightly in the direction of gaze so that their intended straight path becomes curved. Apparently, a moth effect can ocur under any form of locomotion at any time. If it isn't phototaxis, then what causes the moth effect?
The likely explanation for the moth effect is imprecision in knowing there the eyes are pointed. In order to perceive a stable world, the brain is constantly monitoring the direction of gaze. It is one of those critical mental operations necessary for survival that operates outside of awareness. If the brain did not know where the eyes were pointed, for example, the world would jump every time we made an eye movement.
We also could not know which direction were heading as we move through the world. When people look in the direction of travel, the egocentric straight-ahead direction and gaze direction are the same. When people fixate away from the direction of travel, then they must then use knowledge of eye position in order to maintain a proper sense of egocentric direction. If the calculation is correct, then the person has maintained directional constancy. Studies (e. g., Hill, 1972; Morgan, 1978) show that people are unable to maintain their sense of egocentric direction when fixating eccentrically. Instead, the sense of straight-ahead moves in the direction of fixation. In other words, the driver looking right while attempting to travel down the road straight will inadvertently steer to the right in an attempt to steer straight.
The moth effect might also be a generalization of normal curve driving. When a driver approaches a curve, he turns his eyes to look at the curve's tangent point (Land & Lee, 1994). If the road curves rightward, he then unconsciously looks rightward and uses the discrepancy between heading and gaze direction to determine the degree of steering wheel turn. In a sense, it is an intentional moth effect. The repeated practice of steering curves during normal driving creates a "sensorimotor schema", a set of automatic programmed movements that once triggered, run off with minimal supervisory control. The moth effect might occur when a driver inadvertently actives the "curve driving" schema and adjusts steering in the direction of gaze as in normal curve driving.
Conversely, people frequently look at lights by the roadside without steering off the road, so any simple appeal to phototaxis explains little. There are likely two reasons, one due to the driver and the other due to the environment. The driver factor lies in driver attentional concentration. Attention is a zero-sum game, so the more attention focused on one task, the less available to others. Concentrating attention on a target might reduce attention available to maintaining directional constancy. Even the most automatic behavior, such as maintaining lane position, still requires some degree of attention. It might also prevent the driver from noticing cues for steering correction. The perceptual narrowing might prevent the driver from monitoring road delineations in peripheral vision. The driver would not be aware they he last lost lane positioning. Similarly, the driver may fail to notice to tactile cues that occur when a driver leaves the paved roadway.
Moreover, drivers who start with less attentional resource should suffer a greater chance of suffering the moth effect. Drivers who are fatigued, bored, affected by drugs or alcohol, or older should be more prone to steer off the road. However, it is not these conditions that directly cause the result. It is the way they affect distribution of attention.
The environmenta can also promote the moth effect by reducing optic flow cues. Viewers maintain knowledge of eye position by using a combination of signals, one being the transformations of the optic flow field and other perception of visual direction (Wilkie, & Wann, 2002). At night when there are fewer objects to define the field, the knowledge of eye position suffers. The lack of optic flow likely removes the normal source of heading information, forcing the driver to rely on a sense of egocentric direction relative to a landmark - the fixated object. This explains the Chatziastros, Readinger, & Bülthoff (2003) results, which found a greater moth effect in scenes with fewer trees and other lanscape features.
Lastly, the target fixation effect of motorcycle riders may have a different cause from the moth effect in car drivers. When a motorcycle rider fixates away from his heading, he may unconciously change his body posture to lean in that direction, causing the motorcycle to turn. Motorcycle riders, for example, tend to turn their handlebars more in the direction of a head turn (Heuer & Klein, 2001). Readinger, Chatziastros, Cunningham, Bülthoff. H., & Cutting (2002) investigated whether a posture change could account for the moth effect in car drivers and concluded that it did not. However, this does not rule out the possibility that postural changes may contribute to the moth effect in some circumstances.
The "moth effect" is a myth in one sense and reality in another. The idea that drivers may steer off the road when they fixate flashing lights is likely correct, but they are not drawn to the lights like moths to a flame. Rather, they inadvertently steer rightward, which may or may not take them into collision with the roadside vehicle. Even normal, alert drivers in daylight conditions may steer in the direction of eye position during periods of intense fixation, although factors reducing attentional capacity increase the probability. The cause is likely error in judging heading under eccentric fixation when optic flow cues are minimal and when attentional focus prevents sensing of the need to correct the error. Although bright lights and fascination are not required, of course, it is impossible to rule out these factors in some accidents.
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1A version of this article appeared in Accident Reconstruction, May/June, 2006.
2The technical term for orienting toward light would be either "phototaxis" or "phototropism."
3Target fixation sometimes refers to steering toward a target straight ahead. This would not be the moth effect. It probably has a different cause, such as a response conflict between steering left vs. right.
4"Conjugate" means that the two eyes move together.
"'I had an almost overwhelming feeling that something was pulling me toward the lead plane…Sometimes I had the feeling that I could do nothing to prevent this.'" Clark, Nicholson, & Graybiel (1953), p 433.|