The conspicuity of pedestrians at night: a review

Transcription

1 REVIEW The conspicuity of pedestrians at night: a review Clin Exp Optom 2016; 99: Richard A Tyrrell* PhD Joanne M Wood PhD DAlfredOwens PhD Stephanie Whetsel Borzendowski PhD Ashley Stafford Sewall PhD *Department of Psychology, Clemson University, Clemson, South Carolina, USA School of Optometry and Vision Science, Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Queensland, Australia Whitely Psychology Laboratories, Franklin & Marshall College, Lancaster, Pennsylvania, USA Applied Building Sciences, Inc, Charleston, South Carolina, USA Turner-Fairbank Highway Research Center, McLean, Virginia, USA tyrrell@clemson.edu Submitted: 8 January 2016 Revised: 29 May 2016 Accepted for publication: 24 June 2016 DOI: /cxo Drivers visual limitations are a leading contributor to night-time traffic crashes involving pedestrians. This paper reviews the basic changes in vision that occur at night for young and old visually healthy drivers, as well as those with common ocular pathology. To maximise their safety at night, pedestrians should be conspicuous. That is, beyond being simply visible (detectable as an ambiguous object), they should attract the attention of drivers and be readily perceivable as pedestrians. Research has established that the conspicuity of pedestrians can be optimised by attaching retroreflective markings to the pedestrian s extremities. Doing so highlights the pedestrian s biological motion, which facilitates the accurate perception of a person; however, retroreflective markings on the torso (for example, vests) are less effective. Importantly, behavioural evidence indicates that most road users drivers and pedestrians alike are not aware of the limitations of night vision. For example, drivers typically overdrive the useful range of their headlight beams and under-use their high beam headlight setting. Further, pedestrians overestimate their own conspicuity at night and fail to appreciate the extent to which their own conspicuity depends on their clothing. The widespread misunderstanding of the challenges associated with night driving reflects a lack of awareness of the fundamental limitations of night vision. Educational interventions are needed to ameliorate these dangerous misunderstandings and to improve the safety of all road users at night. Key words: driver s vision, night driving, pedestrians Pedestrians are highly vulnerable road users at any time of day but when illumination is low their vulnerability increases even higher due to the normal limitations of drivers vision at night. This review provides an overview of the peer-reviewed scientific literature that addresses the visibility and conspicuity of pedestrians at night. After describing the frequency of collisions with pedestrians, we examine the critical role of visual factors in these collisions. We summarise the visual changes that occur at night and briefly discuss vehicle headlighting. We review the research methods that have been used to quantify drivers abilities to recognise and respond to the presence of pedestrians at night. We also emphasise the importance of clothing reflectance and discuss the evidence that positioning retroreflective markings on pedestrians extremities is particularly valuable because such markings create highly salient information for perception of biological motion. Finally, we examine the misunderstandings that typical road users have concerning fundamental limitations of night vision and the ways in which such misunderstandings increase the dangers to pedestrians at night. THE MAGNITUDE OF THE PROBLEM The World Health Organization (WHO) estimated that pedestrians account for 22 per cent of all road deaths globally, with more than 270,000 pedestrian fatalities per year. 1 In 2013, there were 4,735 pedestrians killed and 66,000 injured in traffic crashes in the USA with pedestrians representing 14 per cent of all traffic fatalities. 2 In Australia, pedestrian fatalities represented 13.7 per cent of all road fatalities in More pedestrian fatalities in the USA occur in urban areas (73 per cent in 2013) than in rural areas (27 per cent) and most (69 per cent in 2013) occur away from intersections. 2 The pedestrian safety problem is increasing, particularly in developing nations. 4 Of particular interest in the present context is the fact that pedestrian fatalities are over-represented in low illumination conditions. The WHO reports that worldwide, a high percentage of pedestrian deaths and injuries occur at night. 1 In 2001 it was estimated that 200,000 pedestrians were killed in night-time traffic collisions globally. 4 More recently, the National Highway Traffic Safety Administration reported that 72 per cent of all pedestrian fatalities occurred in the dark in the USA in RELEVANCE OF VISUAL FACTORS Although this review focuses on the relevance of visual factors in collisions between vehicles and pedestrians at night, it is acknowledged that most crashes result from a complex blend of factors. For example, fatigue and alcohol consumption are also presumed to vary between daytime and night-time conditions; however, the critical relevance of visibility to crashes involving pedestrians has been confirmed by careful analyses of crash databases. 5 7 These analyses have successfully isolated conditions in which visibility varies systematically, while other factors remain consistent. For example, Owens and Sivak Optometry Australia Clinical and Experimental Optometry 99.5 September

2 studied the crash patterns in the Fatality Analysis Reporting System, a database that contains census information from all fatal traffic crashes in the USA. The authors focused on 104,235 crashes that occurred during what they termed the twilight zones daily periods that, when examined over the course of a year, included periods of high and low illumination in similar proportions. As a result of fluctuations in the times of sunrise and sunset, the twilight zones comprise two three-hour time periods (one in the morning and one in the evening) that are light during the summer and dark in the winter. By examining the frequencies of fatal crashes during these twilight zones throughout the year, Owens and Sivak 5 found that crashes involving pedestrians or bicyclists (but not other types of crashes) were over-represented in darkness despite the fact that time of day remained constant. Also, because the darker seasons are associated with colder weather, it is likely that fewer pedestrians and cyclists are active and exposed to traffic. Moreover, unlike other types of fatal crashes, drivers alcohol consumption was a minor contributor to fatal collisions with pedestrians and cyclists. Owens and Sivak 5 concluded that poor visibility at night is a major contributor to fatal crashes with low-contrast hazards, like pedestrians and bicyclists. A follow-up study by Sullivan and Flannagan 6 found a similar pattern when they analysed how fatality rates were affected by daylight savings transitions, when the relationship between time of day and illumination is suddenly and artificially altered. They found a strong impact of illumination on pedestrian fatalities; they estimated that pedestrians are three to seven times more vulnerable in the dark than in daylight, whereas single-vehicle run-off-the-road crashes were unrelated to light conditions. Further, Owens and Brooks 7 found that older drivers show an even stronger association between illumination and involvement in fatal crashes with pedestrians and bicyclists. This suggests that age-related changes in a driver s visual function at night result in increased risk of being involved in a fatal crash with a pedestrian or bicyclist. Taken together, these analyses established that the normal visual limitations experienced by drivers at night are a key contributing factor to fatal pedestrian crashes. 8,9 VISION IN TWILIGHT To describe the diurnal transitions between daylight and darkness, early astronomers defined three stages of twilight. 10 These stages are, from brightest to darkest, civil twilight (from when the upper edge of the sun is tangent with the horizon until when the centre of the sun is six degrees below the horizon), nautical twilight (the sun is six to 12 degrees below the horizon) and astronomical twilight (the sun is 12 to 18 degrees below the horizon). The changes in visual performance are most dramatic during civil twilight. In clear atmospheric conditions, ambient illumination during civil twilight ranges from approximately 330 lux down to about 3.3 lux. 11 This range of illumination corresponds to the transition between photopic (cone) and scotopic (rod) vision. Civil twilight lasts approximately 30 minutes at mid-latitudes, although the exact duration varies with latitude and time of year. 12 Although less research has focused on vision during civil twilight relative to nighttime conditions, a meta-analysis of psychophysical studies revealed that during civil twilight relatively small changes in luminance can result in large changes in visual performance. 11 At the bright end of civil twilight, photopic luminances prevail and both foveal acuity and contrast sensitivity approach optimal daytime levels; however, at the dark end of civil twilight, scotopic luminances predominate and visual acuity and contrast sensitivity are degraded to less than 20 per cent of their daytime levels. VISION AT NIGHT Changes in visual abilities at night are well understood. For example, under night driving conditions, drivers adaptation levels are typically at the lower end of the mesopic luminance range, 13 their pupils are relatively dilated and their vision is mediated by both rod and cone mechanisms. 14 These changes affect important aspects of visual performance while driving at night, especially the ability to recognise low-contrast objects. Under low luminances, visual acuity is reduced both centrally and peripherally. 20 Contrast sensitivity also diminishes, with both peak sensitivity and the high spatial frequency cut-off moving to lower spatial frequencies. 21,22 Letter contrast sensitivity also decreases at low luminance levels and this is exacerbated by increased age. 23 Temporal processing and motion perception are also impaired at night because of the poor temporal processing characteristics of the rods, with velocity perception being degraded under scotopic conditions. 24,25 Colour vision and hue perception are also reduced under low luminance levels, 26,27 given that colour is mediated by the cones, which function at mesopic and photopic light levels, with the rods also influencing colour perception through rod-cone interactions. 28 Importantly, the changes in visual function at night are not uniform. Indeed, some basic functions of peripheral vision, which depend on wide-field stimulation, remain efficient in scotopic conditions. Visually induced feelings of self-motion (vection), for example, are as powerful in scotopic as in photopic conditions. 29 Similarly, vehicle guidance, measured by steering ability in wide-field driving simulators, is nearly as accurate in scotopic as in photopic conditions. 30,31 As will be discussed, these functions are important for driving and their continued efficiency in night conditions may contribute to drivers misperceptions of their visual abilities at night. Effects of age and ocular disease The reduction in visual function at low luminances is exacerbated in older adults, where the amount of light reaching the retina is reduced (due to senile miosis and media opacification), along with age-related changes in retinal and retino-cortical neural projections. 32 The age-related declines in visual acuity 17,33 and contrast sensitivity are exaggerated under low luminance conditions, 34 with reductions in mesopic contrast sensitivity being evident 10 years prior to decreases in photopic contrast sensitivity. 23 Vehicle guidance (steering) is also diminished in older drivers at night. 31 These age-related changes are likely to underlie the problems that older adults report under low light levels 35,36 and they may contribute to older drivers reluctance to drive at night. 37,38 Eye diseases are also more common in older populations, 39,40 including glaucoma, cataracts and age-related macular degeneration which have been reported to exacerbate the visual problems reported under low luminance levels. 36,41 45 Relevant visual functions Traditional measures of high-contrast photopic visual acuity are poor predictors of night-time visual function and driving ability. 46 Alternative tests have been advocated that involve reductions in target contrast and luminance levels to provide information regarding low luminance functional ability, 43,47 as well as insight into visual loss in ocular disease. 44,48 Individual Clinical and Experimental Optometry 99.5 September Optometry Australia

3 differences in photopic visual acuity do not predict variations in night driving recognition ability, with mesopic measures being better predictors; 18 however, measurement of mesopic visual acuity may be variable due to variations in chart luminance at these low levels, which can significantly affect visual acuity. 16,49 A recent study reported that testing visual acuity at 0.75 cd/m 2 gave higher repeatability; 19 whether this level has advantages in terms of predicting functional capacity of night drivers has not yet been tested. Contrast sensitivity measured under photopic or mesopic conditions appears to be more relevant than acuity to overall functional capacity, particularly at night, given that the visual environment consists of objects of different sizes and contrasts. Indeed, photopic contrast sensitivity better predicted the ability to recognise night-time road workers than either visual acuity or glare sensitivity 50 and was more highly associated with driving performance in a study that simulated visual impairment 51 and again in a separate study that varied headlamp beam illumination. 18 Interestingly, mesopic contrast sensitivity exhibited better predictive power than photopic measures for night recognition; 18 this is relevant given that photopic and mesopic contrast sensitivity are not necessarily related, even in visually healthy adults. 52 Thus, mesopic contrast sensitivity provides useful information regarding low luminance visual functioning that is not captured by standard photopic measures. Other measures of visual performance, including visual attention, 53 detection of motion, 24,54 peripheral vision 55,56 and patterns of optical flow 57 may also provide valuable insights into the visual skills critical for night driving. Motion sensitivity was a significant predictor of pedestrian recognition at night even after controlling for age. 58 In the same study, selective visual attention was also significantly correlated with night-time recognition distances, although this relationship was partially explained by age. 58 While the link between these alternative measures of visual function and performance during night driving is promising, further exploration of this area involving larger samples and other aspects of night-time driving performance is warranted. HEADLIGHTING Motor vehicles include carefully designed headlights to compensate for the normal limitations of night vision. Design standards limit the total amount (luminous intensity) and the spatial distribution of light. 59,60 While detailed photometric descriptions of headlight characteristics are available, 61 a spatial analogue of civil twilight provides a holistic description of the useful range of headlight illumination. 11,62 This twilight envelope method involves measuring the spatial extent over which a pair of headlamps provide illuminance of 3.3 lux or more (that is, bounded at the dark limit of civil twilight). This approach was developed to describe the useful range of headlight beams on a scale that is related to the shift from photopic vision near the headlights to scotopic vision farther down the road at the edge of the twilight envelope. Standards for headlights were developed to balance two conflicting goals maximising a driver s ability to see ahead and minimising the problem of glare experienced by other road users. Having both low and high beam settings allows drivers to choose when to emphasise one goal over the other. Although headlamp designs vary from vehicle to vehicle, 62 a universal feature of low beams is that the hot spot is aimed relatively low and is laterally biased toward the near (passenger-side) shoulder of the road rather than straight ahead. While this reduces glare problems for oncoming motorists, this advantage comes at the expense of reduced visibility for objects or persons in the lane of travel or near the centre of the roadway. 63 Indeed, the asymmetrical aiming of low-beam headlights (which distribute more light to the passenger side than to the driver side) has been shown to have safety consequences, where night pedestrians who are illuminated by the driver s side of the low beam pattern are at elevated risk (for example, during left turns in the USA). 64 Although high beam use greatly increases the distance at which drivers respond to pedestrians, 65,66 drivers typically rely on their low beam headlamps even when traffic conditions are optimal for using high beams Drivers reluctance to use high beams is problematic and suggests that they fail to appreciate the visibility problems that accompany reliance on low beams. 70 This issue will be addressed later in this paper. Table 1 presents a quantitative comparison of visibility distances with low and high beam headlights using the dark limit of civil twilight (three lux) as the cut-off criterion. The values, which are derived from the median values of market-weighted data, show that, compared to high beams, the visibility distance with low beams is much shorter and strongly biased toward the passenger-side shoulder which limits glare for oncoming drivers. 61 Low beams project 30 per cent farther at the near shoulder (89 metres) than straight ahead and more than 200 per cent farther than the illumination (41 metres) toward the oncoming lane. In comparison, the visibility distances with high beams are substantially longer in all directions, that is, 1.6 times longer than low beams toward the near shoulder, 2.1 times longer straight ahead and 2.7 times longer toward the oncoming lane. Emerging developments in headlight technology may provide new and important benefits to drivers. Adaptive headlamps, for example, can change their aim and/or modify their output in response to current traffic and roadway conditions. 71 Jermakian 72 estimated that adaptive headlamps have the potential to prevent 140,000 crashes in the USA annually. Whether these benefits are realised remains to be seen. A recent study found that drivers who approached low reflectance objects that were positioned on the interior corners of a curvy road responded to the objects from a farther distance when their vehicle featured adaptive low beam headlamps that pivoted during cornering compared to when the same headlamps did not pivot; 73 however, the extent to which they would contribute to enhanced safety for pedestrians is not yet clear. As headlight technology continues to develop, it will be important to assess empirically the extent to which it enhances visibility of low-contrast road hazards. Studies of the effects of new headlight technology on drivers abilities to perceive pedestrians at night will be particularly valuable. EXPERIMENTAL APPROACHES It is useful to consider the relative merits of the various methods that have been used to quantify drivers abilities to recognise and respond to pedestrians at night. Methods range from tightly controlled laboratorybased experiments to those using low- or high-fidelity driving simulators to studies on actual roadways that are either closed or open to the public. All approaches have important advantages and disadvantages and none is ideal. When considering the risk of positioning pedestrians on roadways, one might conclude that driving simulators are an ideal research tool. Simulators have the advantage of allowing tight experimental control while eliminating physical risk and allowing virtual 2016 Optometry Australia Clinical and Experimental Optometry 99.5 September

4 Low beam High beam Increase relative to low beam Five metres toward oncoming lane 41 metres 110 metres 2.7 Centre of lane ahead 68 metres 145 metres 2.1 Five metres toward near shoulder 89 metres 144 metres 1.6 Table 1. Visibility distances with low- and high-beam headlights in three directions relative to the centre of the vehicle. Values are based on the dark limit of civil twilight (three lux) for median luminous intensity for passenger vehicles sold in the USA in the year that a pedestrian will be present at a particular location, to optimise safety, pedestrians need to be maximally conspicuous. Researchers have used complex driving routes, catch trials (during which no pedestrians are present) and between-subjects designs in an effort to prevent participants from knowing when and where a pedestrian will be encountered and to ensure that conspicuity, rather than visibility, is assessed. 66,79,81 pedestrians to be positioned anywhere, even directly in front of the driver. An alternative laboratory-based approach involves recording video clips from inside a real vehicle, as it is driven past pedestrians. These video clips can then be replayed to participants in a laboratory setting to measure how and when they respond. 74,75 With both simulator and video-based studies, important technical limitations are inherent in all video display systems. These limitations prevent accurate rendering of the luminance and the spatial and temporal resolution of night-time driving environments. 76 For example, even modern high-definition displays cannot faithfully reproduce the extremely wide range of luminance levels encountered by drivers at night, from actual darkness to the high intensities of opposing headlamps. 76 Thus one cannot assume that simulators provide realistic dark adaptation levels or that the luminance and contrast of pedestrians replicate real-world conditions. Such limitations are likely to distort participants abilities to recognise pedestrians. Most research on the visibility of pedestrians at night has been conducted outdoors. Researchers have used open and closed roadways to address issues of pedestrian visibility since the 1930s. 77,78 Using roads that are open to the flow of traffic has the advantage of high validity associated with realistic traffic conditions, but this comes at the cost of a reduction in experimental control. Using a between-subjects design and positioning a test pedestrian at the side of an open road provides additional validity and reduces problems associated with participants knowing in advance where to look to see the test pedestrian. Yet problems associated with variations in traffic flow can be problematic. An alternate approach, adopted to overcome this issue, uses roads with low traffic density and discards trials in which extraneous vehicles happen to be present From the perspective of a participant, the test situation is no different from a typical drive, yet the procedure ensures that data are not confounded by the presence of other vehicles which could add varying degrees of glare and distraction. Using closed roads provides the key advantage that researchers can control conditions in ways that are not possible on public roads. For example, Wood and colleagues have had notable success using a closed road (a circuit that includes a realistic network of rural roads) to study drivers abilities to recognise pedestrians, 66 road workers 50 and bicyclists. 82 They have implemented simulated construction sites, 83 low- and high-clutter environments 84 and situations in which dark foam obstacles are positioned on the road surface in the test vehicle s laneoftravel. 51 They have also manipulated headlamp intensity 85 and have simulated several forms and severities of visual impairment, including optical blur and cataracts. 86,87 The results of these studies will be summarised later in this review. Regardless of the research setting, the issue of expectancy can be problematic for any study of drivers abilities to recognise the presence of pedestrians. Drivers who expect to encounter a pedestrian in a given location are likely to respond from a longer distance than drivers who are surprised by the pedestrian s presence. 77,88,89 Drivers who expect to encounter a pedestrian may have different visual scanning patterns prior to encountering a pedestrian and are likely to respond more quickly once a pedestrian is perceived. Indeed, most authors differentiate between conspicuity and visibility, in that a pedestrian is said to be conspicuous when he or she is recognised as a pedestrian, even by a driver who had no advanced warning or special expectation of encountering a pedestrian in that particular location. 90 Conspicuous pedestrians grab a driver s attention. A pedestrian is said to be visible when he or she is detected by a driver who has reason to expect a pedestrian to be present. Because drivers are not typically informed in advance IMPORTANCE OF CLOTHING CHARACTERISTICS Low reflectance clothing Brightness contrast, the difference in luminance of a pedestrian relative to the background, is the most critical variable affecting pedestrian conspicuity at night. 66 While contrast is affected by illumination (ambient lighting, street lighting and headlighting), the reflectance of the pedestrian s clothing is the factor that most influences contrast. Unfortunately, the worst-case scenario of a pedestrian wearing dark clothing is all too common. In 1977, Bhise and colleagues 91 measured the clothing reflectance of participants across the USA at different times of year and found that most wore clothing that had a reflectance of 10 per cent or less (comparable to dark grey). More recently, Hagle and colleagues 92 found that pedestrians underuse contrast-enhancing visibility aids. Numerous studies 66,75,79,84 have documented that drivers typically respond to dark-clad pedestrians at distances that are dangerously short. In one closed-road study, 79 per cent of drivers failed to perceive a dark-clad pedestrian who was walking in place (that is, making walking-like movements without changing position) on the opposite shoulder of the road despite the fact that the drivers had normal vision and knew that their ability to see pedestrians was being tested. 84 The mean distance at which the drivers responded to the black-clad pedestrian was only 2.6 metres. Another study that used a similar method 66 found similar results (60 per cent failed to respond; a mean response distance of 5.6 metres). Moreover, when the pedestrian wore all white clothing 25 per cent of drivers failed to respond and the mean response distance was 40.3 metres. Other researchers have also found highly reflective clothing to be helpful; 88,93 however, most clothing fabrics diffusely reflect only a small portion of the headlight illumination Clinical and Experimental Optometry 99.5 September Optometry Australia

5 toward the driver s eyes and therefore, even white clothing provides limited contrast against a dark background. Retroreflective materials Retroreflective materials, unlike diffuse reflectors, are engineered to reflect incident light directly back toward the driver. These materials are commonly used in highway environments for signage, lane delineators and warning devices, and they provide an elegant way of enhancing contrast because they capitalise on the illumination from approaching headlights. Early studies of the benefits of retroreflective materials for pedestrians used small reflective disks called dangle tags, 88,94,95 which yielded mixed results. Shinar 95 found that drivers who had been informed in advance that the small reflective tags represented pedestrians, responded from distances that were roughly twice as long as responses from drivers who had not been informed in advance about the dangle tags. Similarly Blomberg, Hale and Preusser 94 reported a mean pedestrian detection distance of 162 metres for the tags but mean recognition of the pedestrian at a distance of only 44 metres. Retroreflective materials have also been incorporated into safety vests and standards specifying their use have been developed. 96 Several studies 66,74,75,79,82 84,94,97 have evaluated the potential benefits of retroreflective vests for pedestrian conspicuity at night. Most of these studies have found that, similar to dangle tags, vests provide only moderate improvements in pedestrian conspicuity. Despite the fact that the vests can be detected at long distances, the bright reflections are often not perceived as a pedestrian. 66,74,75,79,82 84 In one open-road study, for example, mean response distances were less than 20 metres, when the test pedestrian wore a retroreflective vest; 79 however, two closed-road studies, found larger advantages of retroreflective vests. 94,97 It is likely that methodological differences played a role, notably the use of within-subjects design, which provided advance knowledge that a pedestrian would be present in one of two locations on the test route. Biological motion In the 1970s, Johansson 98 discovered that observers readily perceive humans and their actions, when only the actor s major joints are visible ( biological motion or biomotion ). Johansson 98 attached reflective markings to the primary joints of actors and then recorded brief movies. After editing the films to show only the points of light, Johansson 98 demonstrated that the movement of only the reflective joints was sufficient for viewers to recognise the actors as humans engaged in specific actions, like walking, cycling, dancing, throwing, et cetera. Johansson s discovery of our perceptual sensitivity to biomotion stimuli led to a flood of basic research 99 into this special case of perception of complex structure-from-motion. Researchers have discovered that observers of point-light displays can recognise actions with presentation times of only 100 to 200 milliseconds, 100 even when the dots are presented within clouds of randomly moving distractor dots. 101 Awide range of complex personal characteristics are easily perceived from point-light displays, including the gender of actors, 102 an actor s personal identity, if s/he is familiar to the observer 103 and even the emotional state of the point-light actor. 104 Biomotion actors are perceived by newborn human infants, 105,106 as well as by other species The powerful perceptual characteristics of biomotion suggested a new approach to enhancing pedestrian conspicuity at night. Blomberg, Hale and Preusser 94 were the first to include biomotion markings in a study of pedestrian conspicuity, finding that retroreflective marking of a pedestrian s ankles, waist, wrists and head resulted in recognition distances that were 26, 28 and 67 per cent longer than retroreflective vest, flashlight and dangle tag conditions, respectively. A subsequent study, inspired by Johansson s research, used video recordings to investigate the benefits of biomotion markings for pedestrian recognition. 75 Participants in that study viewed videos that had been recorded when driving through several public road environments at night. Their task was to depress a brake pedal when they observed a pedestrian near the road. Participants responded significantly earlier to pedestrians wearing biomotion markings on the major joints than for pedestrians wearing a retroreflective vest. Later studies on closed and open roads 79,80,83 also found sizable conspicuity advantages when retroreflective elements were positioned on pedestrians extremities. In one closed road study, pedestrians walked in place on the side of the road while wearing black clothing, white clothing, black clothing with a retroreflective vest or black clothing with retroreflective straps on the ankles,knees,waist,shoulders,elbowsand wrists. 66 The total surface area of the retroreflective markings was equal in the vest and biomotion conditions but their conspicuity was strikingly different: drivers responded to the biomotion configuration from a distance that was 3.4 times greater (when using low beams) or 2.7 times greater (when using high beams) than the distance at which they responded to the vest configuration. In an open road study, Balk and colleagues 79 compared the conspicuity of a pedestrian who was either standing still or walking in place on a footpath, while wearing either all black clothing or black clothing with one of four different configurations of retroreflective markings (Figure 1). Regardless of whether the pedestrian stood still or walked in place, the vest (which included a rectangular patch of retroreflective material) provided no conspicuity benefit relative to the black clothing condition (Figure 2). Yet when the pedestrian walked in place, his conspicuity was significantly and substantially increased in every condition that included retroreflective markings on the extremities (Figures 1C E and Figure 2). A recent study 81 confirmed and extended the research on biomotion markings to electroluminescent panels. These panels can provide a useful supplement to retroreflective markings since electroluminescent panels are visible even when a pedestrian is not illuminated by approaching headlamps (for example, when roadway curvature results in headlamps being aimed away from a pedestrian). To our knowledge, only one study has failed to find an advantage of biomotion markings for pedestrian conspicuity at night. 74 This laboratory-based study required participants to respond to video clips that had been recorded in a visually cluttered traffic setting at night. Finding no biomotion advantage over a vest, Moberly and Langham 74 concluded that the perceptual advantage of biomotion markings may be diminished in visually cluttered settings. Motivated by this negative finding, Tyrrell and colleagues 84 investigated the effects of clutter under more realistic conditions using a closed-road circuit, in which participants drove past pedestrians who were either surrounded by extraneous retroreflective stimuli or not. That study confirmed the strong advantage of retroreflective markings on the pedestrian s extremities regardless of whether visual clutter was present. Consistent with that finding, other studies found that workers in cluttered road construction sites are more conspicuous when the workers wore 2016 Optometry Australia Clinical and Experimental Optometry 99.5 September

6 Figure 1. The five clothing configurations used by Balk and colleagues. 79 (A) Black, (B) vest, (C) ankles, (D) ankles and wrists, (E) full biomotion. Configurations B E each included 302 cm 2 of retroreflective material that faced the approaching vehicle. The test pedestrian either stood still or walked in place. biomotion markings in addition to a retroreflective vest. 50,83 Collectively, the evidence indicates that while contrast is important for conspicuity, the distribution of contrasting elements on the pedestrian is critical. As a result of drivers perceptual sensitivities to biomotion, retroreflective markings on a pedestrian s extremities are far more effective than markings that are confined to the pedestrian s torso. In this context, the application of biomotion to enhance the conspicuity of pedestrians at night provides a clear example of the benefits of applied science being informed by basic visual science. 110 OTHER VISUAL FACTORS A variety of additional environmental, situational and visual factors influence drivers abilities to see and respond to pedestrians at night. While there is empirical evidence for the importance of some of these factors, others await systematic investigation. The visual challenges faced by older drivers have been summarised earlier. At night, Figure 2. Mean distance (and one standard deviation) at which participants in the Balk and colleagues study 79 responded to the presence of a pedestrian on an adjacent footpath. The pedestrian wearing one of the five clothing configurations in Figure 1 either stood still or walked in place. The test vehicle always used low beams. older drivers respond to the presence of pedestrians from shorter distances and are more likely to fail to respond. 50,58,66,85,111 In one study, the mean distance at which older drivers responded to a pedestrian was only 58 per cent of the mean distance at which younger drivers responded. 66 In another study, older drivers responded to the presence of a road worker at a mean distance that was less than half the distance of younger drivers. 50 Oncoming headlamp glare can also impair drivers abilities to respond to pedestrians at night. Closed and open road studies demonstrate that glare reduces the likelihood that drivers can recognise real 66,80,86 as well as simulated pedestrians at night 111 and these effects are exacerbated by visual impairment. 86 Also, drivers underestimate the impact of glare on visual function and on their abilities to recognise pedestrians at night. 80,112 Uncorrected refractive error is one of the leading causes of reversible visual impairment. 113 Simulator and closed-road driving studies indicate that while steering accuracy and lane-keeping are relatively robust to even high levels of blur, 30,31 night-time road sign and pedestrian recognition are negatively affected by blur and following refractive surgery. 86,87,114,115 Even low levels of refractive blur reduce drivers recognition of pedestrians at night, 87 reinforcing the importance of refractive error correction for night-time driving. Cataracts are a leading cause of reversible visual impairment in older adults. 116 While the effects of true cataracts on night-time driving have not been explored, studies using Clinical and Experimental Optometry 99.5 September Optometry Australia

7 simulated cataracts have found reductions in the ability to respond to pedestrians at night, despite slower driving speeds. 51,86 Importantly, the detrimental impact of cataracts is greater than for optical blur, even when visual acuity is reduced by the same amount; this likely results from the additional reductions in contrast sensitivity and increased light scatter that are associated with cataracts. 86 The impact of other visual impairments (for example, visual field defects, glaucoma) on drivers abilities to respond to pedestrians at night has not been studied. SELECTIVE VISUAL IMPAIRMENT AND THE NEED FOR EDUCATION The research summarised thus far provides a basis for understanding why the risks to pedestrians are elevated at night. Too often motorists simply cannot see the vulnerable road user in time to avoid a collision. This explanation raises another puzzling question: why do most road users drivers, pedestrians and cyclists alike fail to take appropriate precautions to ensure their own safety at night? From a scientific standpoint, the fundamentals of night vision have been widely explored. Extensive research has shown that visual recognition functions (for example, acuity, contrast sensitivity and object recognition), referred to as focal vision, deteriorate progressively as illumination decreases to the dark limit of civil twilight; 10 however, visual changes at night are not uniform. For example, visual guidance of actions is evident in the behaviour of most nocturnal animals as well as in humans whose eyes have fully adapted to darkness. 117,118 This ambient visual capacity is served by the rod receptor system and magnocellular pathways to the midbrain (superior colliculi), which project to the parietal cortex ( dorsal stream ). Leibowitz and Owens 119 proposed that drivers risky behaviours at night may be due to selective degradation of focal vision, while ambient vision maintains high levels of function. 119 According to their hypothesis, drivers do not appreciate their visual limitations for two reasons: 1. most visual stimuli that drivers see routinely (road signs, lane delineation, lighting on other vehicles, instrumentation) have been engineered to be bright enough for cone-based focal vision at night, and 2. steering the vehicle remains easy and accurate because ambient vision continues to function well at night. From this perspective, drivers are unaware that they are selectively blind for any lowcontrast object that might enter their path. Simulator studies have confirmed that drivers can steer their vehicles efficiently even when luminances are reduced to low scotopic levels, 30,31 thus reinforcing their confidence at night. Meanwhile, pedestrians also fail to appreciate the extent to which drivers vision is degraded at night. In this case, a key factor is differential dark adaptation. Drivers vision never fully adapts to darkness because they are viewing the bright but limited portion of the roadway illuminated by their headlights. Unlike drivers, pedestrians adapt to low light (scotopic) conditions and therefore, can see much of the nocturnal environment, including their own body. From the pedestrians perspectives, the headlights of an oncoming vehicle appear to be extremely bright, leading them to assume that they are visible when in fact they are not. Researchers have repeatedly demonstrated that pedestrians overestimate their own visibility to approaching drivers at night. 78,80,93 95,120,121 Pedestrians also fail to appreciate the extent to which their conspicuity depends on their clothing 121 and their judgments of their own conspicuity are not significantly reduced even when headlamp intensity is reduced by 97 per cent. 122 Together, the selective degradation of drivers vision and the superior dark adaptation of pedestrians provide a heuristic explanation of why road users fail to behave with greater caution at night. This explanation is grounded on the notion that drivers, pedestrians and cyclists do not understand the limitations of night vision. These misunderstandings increase the danger to all road users at night. The need to educate road users about basic aspects of night vision seems clear. Drivers, pedestrians and bicyclists who better understand the visual limitations that exist at night may alter their behaviour in ways that mitigate the risks. Drivers with a better appreciation of their own visual limitations at night (that is, a more accurate meta-perception ) may be more likely to use their high beams appropriately and to drive more slowly at night. Pedestrians who better understand drivers visual challenges may be more likely to appreciate that they are not conspicuous to approaching drivers and therefore, take responsibility for their own safety by avoiding walking or jogging along the shoulder of a road and by wearing biomotion markings. Evidence indicates that educating pedestrians about fundamental aspects of night vision can be beneficial. In one experiment, Tyrrell, Patton and Brooks 123 measured college students estimates of their own visibility at night, while they wore different clothing configurations. Those who had heard a single lecture about night vision, which had been given to a large audience roughly seven weeks earlier, estimated their visibility to be significantly less than students who had not heard the lecture, despite the fact that there was no apparent connection between the lecture and the data collection. Tyrrell, Patton and Brooks 123 confirmed this for a group of high school driver education students. In another study, 124 a carefully designed lecture given to an audience of frequent outdoor exercisers resulted in the participants having a significantly improved understanding of conspicuity issues and a greater interest in (and willingness to purchase) conspicuityenhancing athletic garments. Together, these findings provide encouraging preliminary evidence that learning about fundamental aspects of night vision can convince pedestrians that they are not as visible to oncoming traffic at night as they had thought. Although not yet tested, it seems possible that knowledge of the actual limitations of night vision may also induce drivers to behave more safely at night. Because educational interventions in this domain have the potential to provide substantial safety benefits at relatively low cost, it seems clear that additional research is needed to guide their development and to quantify their potential benefits. In particular, studies are needed that measure the extent to which specific behavioural changes of drivers (for example, high beam usage) and pedestrians (for example, acquiring and wearing biomotion markings, actively avoiding exposure to traffic at night) can be triggered by educational interventions. SUMMARY Strong evidence reveals that a key cause of many night-time crashes with pedestrians are the visual limitations that even visually healthy drivers experience at night. Pedestrians often appear as low contrast, inconspicuous and unexpected hazards that can be challenging to detect and difficult to recognise from safe distances. The visual challenges faced by drivers at night are exacerbated when pedestrians wear low-reflectance clothing and when drivers experience glare Optometry Australia Clinical and Experimental Optometry 99.5 September

8 The visual challenges are even greater for older drivers and for those with visual impairment. Although pedestrians can be made more conspicuous to drivers at night when drivers use high-beam headlamps, typical drivers over-rely on low beams. Unfortunately, road users typically fail to appreciate the normal limits of vision at night. Both pedestrians and drivers overestimate the distance at which drivers can recognise the presence of pedestrians and underestimate the conspicuity value of reflective clothing. These flawed metaperceptions increase the risk to all road users. Efforts to educate road users about fundamental aspects of night vision are needed. Driver education and public service announcements have the potential to be both beneficial and cost-effective. A key finding from basic perceptual science that human observers possess a strong perceptual sensitivity to biological motion provides a valuable means of optimising pedestrian conspicuity at night: pedestrians can dramatically enhance their own conspicuity at night by wearing retroreflective markings on their extremities. This strategy is effective and inexpensive. Unfortunately, retroreflective markings are too often positioned only on vests, providing only modest gains in conspicuity while making pedestrians believe that they are conspicuous. The fact that retroreflectors positioned on a pedestrian s major joints (ankles, knees, elbows and wrists) are far more valuable than when the same retroreflectors are positioned on the torso, highlights the important distinction between visibility and conspicuity. Making a pedestrian visible (that is, detectable but perceptually ambiguous) is not sufficient to ensure safety. Using biological motion to make a pedestrian conspicuous easily recognisable by a driver who was not expecting to encounter a pedestrian is far more useful. Clearly there is much more to learn about the issues surrounding the conspicuity of pedestrians at night and more research is needed. Although most pedestrian fatalities occur at night, research into the visual aspects of driving has concentrated on daylight conditions. Among the issues that merit future research are continued efforts to explore how individual differences in the visual capabilities of drivers affect the ability to recognise the presence of vulnerable road users at night. Of particular interest is the extent to which uncorrected refractive errors or visual pathology (especially those that are often experienced by licensed drivers including macular degeneration and cataracts) and driver age affect night-time pedestrian recognition. Further, the ongoing development of headlight technology should be validated by quantifying the ability to assist drivers to recognise the presence of pedestrians at night from a safe distance. Indeed, quantifying drivers abilities to recognise pedestrians at night may be the most valuable index for use when searching for an optimal balance between maximising forward visibility and minimising disability glare experienced by other road users. In addition, new ways to assist pedestrians to maximise their own night-time conspicuity are needed. Because retroreflective markings can only be valuable when they are purchased and worn, research is needed into effective ways to convince pedestrians to use these approaches. As mentioned previously, educational interventions are needed. To maximise their efficacy, the development of these interventions should be guided by empirical evidence and must be validated experimentally. Such efforts could stimulate both consumer demand for conspicuity-enhancing garments and the production of such garments by clothing manufacturers. Further, developing (and not-yet-developed) wearable technology could prove useful in making pedestrians conspicuous in ways that are practical to manufacture and convenient to wear. 81 Finally, while it is possible that the introduction of autonomous vehicles could affect pedestrian safety at night in ways that cannot be anticipated, it is important to note that as long as some vehicles are controlled by human drivers, it will be important to maximise the conspicuity of pedestrians at night and to encourage pedestrians to minimise the risks associated with interacting with traffic at night. REFERENCES 1. World Health Organization. Make walking safe: A brief overview of pedestrian safety around the world. Geneva: World Health Organization, National Highway Traffic Safety Administration. Traffic Safety Facts, 2013: Pedestrians. Washington, DC: Report No: DOT HS , Bureau of Infrastructure Transport and Regional Economics. Pedestrians and road safety. Report No: 70. Canberra, Rumar K. A Worldwide Perspective on Future Automobile Lighting. University of Michigan Transportation Research Institute, Owens DA, Sivak M. Differentiation of visibility and alcohol as contributors to twilight road fatalities. Hum Factors 1996; 38: Sullivan JM, Flannagan MJ. The role of ambient light level in fatal crashes: inferences from daylight saving time transitions. Accid Anal Prev 2002; 34: Owens DA, Brooks JC. Drivers Vision, Age and Gender as Factors in Twilight Road Fatalities. Ann Arbor, Michigan: The University of Michigan, Transport Research Institute, Rumar K. The basic driver error: Late detection. Ergonomics 1990; 33: Sivak M, Luoma J, Flannagan MJ et al. Traffic safety in the U.S.: Re-examining major opportunities. J Safety Res 2007; 38: Leibowitz HW, Owens DA. Can normal outdoor activities be carried out during civil twilight? Appl Opt 1991; 30: Owens DA, Francis EL, Leibowitz HW. Visibility distance with headlight: a functional approach. Society of Automotive Engineers; SAE Technical Paper Series No Warrendale, Pennsylvania, p Anon. Washington, DC: The United States Naval Observatory. Available at: mil/faq/docs/rst_defs.php Af ed. 13. Plainis S, Chauhan K, Murray IJ et al. Retinal adaptation under night-time driving conditions. In: Vision in Vehicles VII, Plainis S, Murray IJ, Charman WN. The role of retinal adaptation in night driving. Optom Vis Sci 2005; 82: Arumi P, Chauhan K, Charman WN. Accommodation and acuity under night-driving illumination levels. Ophthalmic Physiol Opt 1997; 17: Johnson CA, Casson EJ. Effects of luminance, contrast and blur on visual acuity. Optom Vis Sci 1995; 72: Sturr JF, Kline GE, Taub HA. Performance of young and older drivers on a static acuity test under photopic and mesopic luminance conditions. Hum Factors 1990; 32: Wood JM, Owens DA. Standard measures of visual acuity do not predict drivers recognition performance under day or night conditions. Optom Vis Sci 2005; 82: Lin RJ, Ng JS, Nguyen AL. Determinants and standardization of mesopic visual acuity. Optom Vis Sci 2015; 92: Bedell HE. Eccentric regard, task and optical blur as factors influencing visual acuity at low luminances. In: Night vision, current research and future directions. National Research Council Symposium proceedings, Woodhouse JM, Barlow HB. Spatial and temporal resolution and analysis. In: Barlow HB ed. The Senses. Cambridge: Cambridge University Press, p Peli E, Arend L, Labianca AT. Contrast perception across changes in luminance and spatial frequency. J Opt Soc Am A Opt Image Sci Vis 1996; 13: Puell MC, Palomo C, Sanchez-Ramos C et al. Normal values for photopic and mesopic letter contrast sensitivity. JRefractSurg2004; 20: Gegenfurtner KR, Mayser H, Sharpe LT. Seeing movement in the dark. Nature 1999; 398: Gegenfurtner KR, Mayser HM, Sharpe LT. Motion perception at scotopic light levels. J Opt Soc Am A 2000; 17: Pokorny J, Lutze M, Cao D et al. The color of night: Surface color perception under dim illuminations. Vis Neurosci 2006; 23: Zele AJ, Cao D. Vision under mesopic and scotopic illumination. Front Psychol 2014; 5: Clinical and Experimental Optometry 99.5 September Optometry Australia