Effects of Different Levels of Refractive Blur on Nighttime Pedestrian Visibility

Abstract

Purpose: The aim of this study was to systematically investigate the effect of different levels of refractive blur and driver age on nighttime pedestrian recognition and determine whether clothing that has been shown to improve pedestrian conspicuity is robust to the effects of blur.

Methods: Nighttime pedestrian recognition was measured for 24 visually normal participants (12 younger mean = 24.9 ± 4.5 years and 12 older adults mean = 77.6 ± 5.7 years) for three levels of binocular blur (+0.50 diopter [D], +1.00 D, +2.00 D) compared with baseline (optimal refractive correction). Pedestrians walked in place on a closed road circuit and wore one of three clothing conditions: everyday clothing, a retro-reflective vest, and retro-reflective tape positioned on the extremities in a configuration that conveyed biological motion (known as “biomotion”); the order of conditions was randomized among participants. Pedestrian recognition distances were recorded for each blur and pedestrian clothing combination while participants drove an instrumented vehicle around a closed road course.

Results: The recognition distances for pedestrians were significantly reduced (P < 0.05) by all levels of blur compared with baseline. Pedestrians wearing biomotion clothing were recognized at significantly longer distances than for the other clothing configurations in all blur conditions. However, these effects were smaller for the older adults, who had much shorter recognition distances for all conditions tested.

Conclusions: In summary, even small amounts of blur had a significant detrimental effect on nighttime pedestrian recognition. Biomotion retro-reflective clothing was effective, even under moderately degraded visibility conditions, for both young and older drivers.

Uncorrected refractive error is the leading cause of reversible visual impairment in adults, with the prevalence of refractive visual impairment increasing significantly with increasing age.¹ Large numbers of individuals drive with uncorrected refractive errors, which account for 80% of drivers whose vision fails to meet the legal limit for driving.² There are also potentially large numbers of drivers who drive with refractive errors that are not optimally corrected, but whose visual acuity (VA) still meets the commonly adopted licensing requirement for driving of 20/40. 

Although the functional impact of uncorrected refractive errors on reading,³ falls risk,⁴ and quality of life⁵ has been investigated, the effects of uncorrected refractive errors on real-world driving performance and safety, particularly in terms of how these refractive errors interact with aging, are poorly understood. Nighttime driving, in particular, presents a significant safety risk, where fatality rates at night are three times higher than in the daytime,⁶ and seven times higher for pedestrian fatalities.⁷ There is strong evidence that visibility is a key causative factor in these high nighttime fatality rates,⁸ and drivers often fail to recognize and respond to pedestrians from a safe distance at night.⁹ The impact of refractive blur on pedestrian recognition has not been explored, with the exception of a recent study by our laboratory,¹⁰ where we demonstrated for a group of young drivers that one level of blur that reduced VA to approximately 20/40 had a significant detrimental impact on nighttime pedestrian recognition distances. Importantly, the effects of different levels of refractive blur on driving performance and safety are largely unknown. 

This study aimed to determine how refractive blur and aging affect pedestrian recognition under nighttime conditions, so as to provide practical evidence-based guidelines for practitioners who prescribe optical corrections for driving and to inform the community and policy makers about the implications of these findings for road safety. We also wished to determine whether pedestrian clothing configurations that take advantage of the biological motion phenomenon are robust to the effects of blur, given that our previous studies have suggested that they confer a conspicuity advantage in the presence of modest levels of visual impairment that reduce VA to the threshold required for driver licensing (20/40).¹⁰ 

Methods

Participants

Participants included 12 younger (mean age, 24.9 ± 4.5 years; range, 18–30 years; 7 males and 5 females) and 12 older drivers (mean age, 77.6 ± 5.7 years; range, 69–86 years; 10 males and 2 females). All participants were licensed drivers and reported that they drove regularly. All participants met the minimum Australian drivers' licensing criteria for binocular VA of 20/40⁻² or better in both the younger (mean VA = −0.10 ± 0.07 logMAR) and older groups (mean VA = −0.03 ± 0.08 logMAR). The study followed the tenets of the Declaration of Helsinki and was approved by the Queensland University of Technology Human Research Ethics Committee. Participants were given a full explanation of the nature of the study, experimental protocols, and possible consequences of the study, and written informed consent was obtained, with the option to withdraw from the study at any time. 

Visual Conditions

Pedestrian recognition at night was measured under four visual conditions, with the participants wearing their optimal distance visual correction (referred to as the baseline condition), and under three different levels of blur, consisting of the baseline refractive correction plus either +0.50 diopter sphere (DS), +1.00 DS, or +2.00 DS binocular spherical blur. For all conditions, participants drove while wearing goggles that did not restrict the binocular field of view below that of driver licensing standards in Australia of a horizontal extent of 110 degrees.¹¹ The goggles incorporated the participant's optimal distance spectacle correction using standard wide aperture 38-mm diameter trial lenses as well as the additional spherical lenses for the three blur conditions. 

Visual acuity and letter contrast sensitivity were also measured in a randomized order for the four visual conditions in a laboratory-based session at the vision and driving laboratories at Queensland University of Technology. High-contrast distance VA was assessed at 6 m both binocularly and monocularly using a Bailey-Lovie logMAR chart, which was scored on a letter-by-letter basis (−0.02 log units per letter correct) until a line of letters was reported incorrectly. Letter contrast sensitivity was determined binocularly using the Pelli-Robson chart, where participants were asked to guess letters until a line of errors was made, and scored on a letter-by-letter basis (0.05 log units per each letter correct). Room lighting was provided by a combination of fluorescent and incandescent lights; the luminance of both charts was 125 cd/m², as measured with a Topcon BM7 Colorimeter (Itabashi-ku, Tokyo, Japan). 

Pedestrian Recognition

Driver testing was conducted at night on a closed road circuit that has been used in previous studies.^10,12–15 The circuit is representative of a rural road, and includes hills, bends, curves, intersections, lengthy straight sections, and standard road signs and lane markings but does not include any artificial lighting; a 1.8-km loop of the circuit was used for this study. Experimental sessions were conducted only on those nights when the road surface was dry and there was no rain. 

The experimental vehicle was an instrumented right-hand drive 1997 Nissan Maxima with automatic transmission and halogen headlights set on low beam, given that drivers are generally reliant on low-beam headlights. A dual-camera, parallax-based video measurement system was used to determine the distance at which the participant (as a driver) first recognized the presence of the pedestrian.¹² 

An experimenter acted as the pedestrian and walked in place at the end of a 400-m straight section of roadway that started and finished at approximately the same elevation but featured a dip halfway along its length (Fig. 1). The pedestrian was not surrounded by any visual clutter or lighting. A series of four flashing light-emitting diodes (LEDs) and four retro-reflective bollards were positioned around the circuit to increase the instances of flashing lights and retro-reflective material being presented to the driver at heights similar to that of the pedestrian; this ensured that drivers could not rely on the presence of lights or retro-reflective material to signify the presence of a pedestrian, but had to be sure that a real person (pedestrian) was present. To reduce drivers' expectation that a person would always be in a single location, a second experimenter cycled in place at a corner at the opposite side of the circuit on a bicycle that had flashing white-front and red-rear LEDs; however, data for this cyclist are not reported due to the limited sighting distance available at this location. 

Figure 1

 

Schematic diagram of the layout of the driving circuit.

Clothing Conditions

For each driving lap, the experimenter acting as a pedestrian wore one of three clothing conditions. To represent a range of pedestrians differing in conspicuity, clothing configurations that have been shown to provide enhanced pedestrian recognition (incorporating retro-reflective materials) were included, as well as everyday street clothing.¹⁶ The street condition consisted of a light gray long-sleeve T-shirt along with a pair of light gray track pants. The vest condition consisted of a black track top and pants along with a lightweight fluorescent yellow cloth vest with 50-mm-wide silver retro-reflective (Scotchlite 8910 Silver Fabric; 3M, St. Paul, MN, USA) material on the shoulders, front and back. The area of retro-reflective material facing the oncoming driver in this condition was approximately 925 cm². The third condition, biological motion (biomotion), included the vest outlined above plus 50-mm-wide silver retro-reflective strips (Scotchlite 8910 Silver Fabric; 3M) around the elbows, wrists, ankles, and knees. The area of retro-reflective material facing the driver in this condition was approximately 1300 cm². 

The pedestrian wore each of the three clothing types four times, once for each of the blur conditions, resulting in a total of 12 data collection laps and one lap in which the pedestrian was absent to reduce expectancy effects. In addition, an initial (practice) lap familiarized the driver with both the vehicle and the circuit. The order of the clothing and blur conditions was randomized. On each lap, the experimenters walked or pedaled in place as the test vehicle approached; this allowed for the inclusion of naturalistic motion and ensured the safety of the experimenters (as they were at a known location). Each of the experimenters was equipped with a two-way radio, as was the experimenter in the test vehicle. All communications regarding clothing and whether the pedestrian/cyclist was present or absent for a given lap was conducted between laps and outside of the vehicle, so that the participant could not hear the conversations. The main dependent variable was the driver's response distance to the primary pedestrian, which was defined as the distance from the test vehicle to the pedestrian at that moment when the response pad was pressed to indicate recognition of the presence of the pedestrian at the side of the road. 

The pupil diameter of each participant was measured at the beginning of testing using a NeurOptics VIP-200 Pupillometer (Irvine, CA, USA). This was conducted on the closed road circuit within the research vehicle with the headlamps on low beam. Three measurements were taken and the average was recorded. 

Participants were instructed to follow the specified route, which was the same for each trial, to drive at a comfortable speed, and to press a large circular (12-cm diameter) luminous dash-mounted response pad (and announce “person!”) as soon as they recognized that a pedestrian was present ahead; the distance measurement was taken from when they first pressed the response pad. Trials in which the pedestrian was not recognized were scored as 0 m. To ensure a more accurate level of response, participants were instructed not to press the response pad or call out “person!” unless they were confident that what they had seen was an actual person. In an effort to increase driver workload, we also instructed participants to read out aloud all road signs that were encountered around the circuit; importantly, there was only one sign on the long stretch of the circuit before the pedestrian; these road signs are unlikely to have significantly affected the pedestrian-recognition distances. Performance on this sign-recognition task was not recorded or analyzed. 

Results

The baseline high-contrast VA was significantly better in the younger cohort (−0.10 ± 0.07 logMAR) compared with the older cohort (−0.03 ± 0.08 logMAR) of participants (t₂₂ = −2.18, P < 0.05), and this significant difference was also evident for letter-contrast sensitivity, with the younger cohort having contrast sensitivity of 1.89 ± 0.08 log units, compared with that of the older cohort of 1.78 ± 0.01 log units (t₂₂ = 2.941, P < 0.01). There was a significant main effect of blur on VA (F_3,20 = 60.06, P < 0.001), but not age (F_1,22 = 1.019, P = 0.324), with a significant interaction between blur and age (F_3,20 = 3.61, P = 0.031) (Fig. 2), such that higher levels of blur had a greater effect on the VA of the younger compared with the older participants. Blur also significantly reduced letter-contrast sensitivity (F_3,20 = 8.25, P = 0.001), as did age (F_1,22 = 18.15, P < 0.001), but there was no significant interaction between blur and age (F_3,20 = 1.027, P = 0.402). 

Figure 2

 

Group means and SEs for VA as a function of refractive blur for the younger and older age groups.

The group mean pupil diameter measured in the vehicle under the nighttime conditions of the driving circuit of the older participants was significantly smaller than that of the younger participants (5.08 ± 0.94 vs. 6.83 ± 0.67 mm; t₂₂ = 5.257; P = 0.001). 

A three-way multivariate mixed ANOVA with factors of refractive blur (four levels: baseline, +0.50, +1.00, and +2.00), clothing (three levels: street, vest, and biomotion), and age (young and older) revealed that there was a significant main effect of blur (F_3,20 = 22.26, P < 0.001). Higher blur levels were associated with significantly reduced visibility distances, with all pairwise differences being significant: 89.15 m for baseline, 70.28 m for +0.50 DS, 51.62 m for +1.00 DS, and 27.72 m for +2.00 DS. There was also a significant main effect of age (F_1,22 = 13.82, P = 0.001), such that the younger participants were able to recognize the pedestrian at longer distances than the older participants; pedestrian-recognition distances were halved when considered across all blur and clothing conditions for the older participants relative to the younger participants (39.8 vs. 79.6 m). In addition, there was a significant two-way interaction between age and blur (F_3,20 = 6.94, P = 0.002). Figure 3 shows that the ability of the older participants to recognize pedestrians was significantly worse than the younger participants for all visual conditions, but that older participants were less affected by blur than were the younger participants. For the younger participants, there was a significant decline in recognition distance with increasing blur for all pairwise comparisons, with the exception of the baseline versus the +0.50 DS condition. For the older participants, only the baseline and the +2.00 DS conditions were significantly different from one another. 

Figure 3

 

Group means and SEs for nighttime pedestrian recognition distances as a function of refractive blur collapsed across clothing conditions for the younger and older age groups.

There was also a main effect of clothing type (F_2,21 = 57.35, P = 0.001), showing that overall, the pedestrians wearing the biomotion clothing were recognized at longer distances than when wearing the vest, and were in turn recognized at a longer distance when wearing the vest than when wearing street clothing (all pairwise differences significant at P < 0.05); overall recognition distances were approximately nine times greater for pedestrians wearing biomotion clothing compared with street clothing (111.2 vs. 12.6 m). There was also a significant interaction between pedestrian clothing and age (F_2,21 = 4.96, P = 0.017) (Fig. 4), with the biomotion clothing resulting in longer recognition distances for the younger cohort. Last, there was a significant interaction between blur and clothing (F_6,17 = 7.48, P < 0.001) (Fig. 5). Under all blur conditions, the biomotion clothing was seen at the longest distance, followed by the vest and finally the street clothing (all pairwise differences significant at P < 0.05); however, the magnitude of the difference was reduced at higher levels of blur (Fig. 5). 

Figure 4

 

Group means and SEs for nighttime pedestrian recognition distances as a function of clothing collapsed across blur conditions for the younger and older age groups.

Figure 5

 

Group means and SEs for nighttime pedestrian recognition distances as a function of clothing and blur for the younger and older drivers combined.

Discussion

The ability to recognize pedestrians at nighttime was impaired by increasing levels of refractive blur for both older and younger participants. However, the younger participants were significantly better at recognizing pedestrians at nighttime while driving, although at higher levels of blur, the difference between the younger and older participants was diminished. We also found that pedestrians can make themselves significantly more recognizable to oncoming drivers, even in the presence of blur, by wearing retro-reflective strips attached to their moveable joints, to create the perception of biological motion. 

The surprising finding that even small amounts of refractive blur (+0.50 D) can have a significant detrimental effect on nighttime pedestrian recognition is important given that many drivers fail to wear their optimum refractive correction when they drive² and that uncorrected refractive error is the most common cause of visual impairment in adults older than 40 years.^1,17 The findings add to our previous data that showed that refractive blur (mean blur of +1.30 ± 0.06 D), which reduced VA to the equivalent of 20/40, also significantly reduced nighttime pedestrian recognition.¹⁰ Here we show that even smaller amounts of blur make a difference in the ability of drivers to recognize pedestrians at night. 

Our finding that older drivers' recognition ability was reduced compared with younger drivers supports that of previous studies of both pedestrians^12,18 and cyclists,¹⁴ demonstrating that older drivers would recognize pedestrians at approximately half the distance of that of a younger driver. Interestingly, Luoma et al.¹⁸ found smaller age-related decreases in nighttime pedestrian-recognition distances, which may relate to the fact that their participants acted as passengers (seated in either the passenger seat or the back of the vehicle), and were not drivers. The participants in our study drove under realistic conditions with various visual distracters, while also performing a sign-detection task. Importantly, the poorer ability of the older drivers to recognize pedestrians is likely to be related to changes in visual function, especially age-related changes in VA and contrast sensitivity, which are exacerbated under low luminance conditions,¹⁹ together with the increased complexity of the driving task. 

Although the introduction of blur significantly diminished the recognition distances of younger participants, the decline in performance of older participants was much less pronounced and with +2.00 D blur, the performance of the younger and older cohorts was similar. The difference in response to blur may have at least partly resulted from the smaller pupil diameter of the older participants (5.1 vs. 6.8 mm), as measured under the driving conditions in the field. Pupillary miosis has the effect of reducing retinal illumination, but also enhances image contrast²⁰ and increases the depth of field.²¹ Indeed, older persons are reported to show better visual performance than younger persons when exposed to defocus blur,^22,23 which has been suggested to result from a combination of pupillary miosis and neural compensation,²³ and there is evidence that the neural processes that mediate adaptation to blur remain intact with increased age.²⁴ However, the difference in response to blur by older individuals is an area worthy of further investigation. 

Although blur predictably affected both VA and recognition distances in a more or less monotonic fashion, it is notable that the primary interaction reported here (the greater effect of blur for younger participants) was evident to a much greater extent for pedestrian-recognition distances than for VA, and that the effect of blur on recognition distance was highly dependent on pedestrian clothing. Thus, there is not a simple correspondence between the effects of blur on VA and recognition distance, implying that other higher-order functions must also be involved. 

Pedestrian clothing also was found to have a highly significant impact on the ability of the oncoming drivers to recognize pedestrians. Overall, the biomotion clothing had a substantive visibility advantage over the other clothing configurations, where pedestrians wearing biomotion clothing were visible at approximately twice the distance of those wearing a standard vest and approximately nine times those wearing street clothing. The benefits of the biomotion clothing were reduced for the older drivers and under blurred conditions, but were still significant. Importantly, although the vest and biomotion clothing in this study did not have equal amounts of retro-reflective material, it is more likely that the advantage of biomotion clothing over that of the vest is the result of the perceptual salience of biological motion, rather than the amount of retro-reflective material. Studies have repeatedly revealed a strong biomotion advantage even when retro-reflective surface area is held constant across conditions.^12,13,25 Our finding of the important conspicuity benefits of biomotion clothing supports and adds to that of previous research,^10,12,26 and importantly highlights its superiority to other clothing configurations in the presence of a range of levels of blurred vision, even when VA is significantly degraded. 

An advantage of the approach taken in this study is that the only factor that varied between tests was the visual status of the participants that was manipulated through the use of blurring lenses. It also was possible to minimize the effects of practice on the tests by randomizing the order in which the blurring lenses and pedestrian clothing conditions were worn. There are, however, inherent limitations in simulating the effects of blur, in that while the use of simulated visual impairments allowed us to isolate the effects of vision, it is recognized that the effects observed may not exactly reflect those of people who have longer-term experience of living with refractive blur. There is evidence that individuals can adapt to some extent to the presence of blur and that the time course of short-term adaptation is approximately 6 minutes, with any improvement leveling off after this period.²⁷ Because the participants in our study were exposed to each of the blur conditions for at least 6 minutes before testing, their responses are likely to represent those of a person who is largely adapted to their refractive blur; however, we cannot rule out the possibility that adaptation over much longer periods of time may further reduce the impact of blur on performance. 

Another factor that should be noted when considering the generalizability of our findings to that of the wider driving population, is the bias toward males in our sample, particularly in the older group. However, this higher number of male volunteers is consistent with previous studies that report that older male drivers are more confident about driving at night and are much less likely to avoid night driving than female drivers in the same age group,²⁸ suggesting that our sample does provide a reasonable representation of the sex balance of older drivers at night. 

The study outcomes provide strong evidence that pedestrian conspicuity at night is decreased by refractive blur, even for amounts as small as +0.50 D blur. The biomotion configuration of reflective clothing represents a highly beneficial increase in visibility that is effective even under degraded viewing conditions resulting from blur and improves the conspicuity of pedestrians for both older and younger drivers. These results suggest that optimum refractive correction should be prescribed for driving, especially for those who drive at night, so as to maximize the ability to recognize hazards and other road users. They also indicate the need for greater emphasis by licensing and health care authorities of the importance of wearing appropriate optical corrections when driving, particularly at night. In addition, the findings have implications for the design of road systems to accommodate the potential for blurred vision from uncorrected refractive errors. 

Acknowledgments

The authors express appreciation to Queensland Transport and Main Roads for allowing the use of the facilities at the Mt. Cotton Driver Training Centre and to the staff of the Mt. Cotton Centre for their generous cooperation and support. 

Supported by an Australian Research Council Linkage Grant. 

Disclosure: J.M. Wood, None; R. Marszalek, None; T. Carberry, None; P. Lacherez, None; M.J. Collins, None