the visual and driving performance of monocular and ...

Accid. Anal. & Prev. Vol. 23, No. 4. pp. 225-237. Printed in Great Britain.

1991

cool-4575191 $3.00 + .Ml 0 1991 Pergamon Press plc

‘THE VISUAL AND DRIVING PERFORMANCE OF MONOCULAR AND BINOCULAR HEAVY-DUTY TRUCK DRIVERS* A. J. MCKNIGHT, D. SHINAR,? B. HILBURN National Public Services Research Institute, 8201 Corporate Drive #220, Landover, MD 20785, USA (Received 1 March 1990) Abstract-This study compared the performance of 40 monocular and 40 binocular tractor-trailer drivers on measures of both visual and driving performance. On the visual measures, the mononuclear drivers were significantly deficient in contrast sensitivity, visual acuity under low illumination and glare, and binocular depth perception. They were not significantly deficient in static or dynamic visual acuity, visual field of individual eyes, or glare recovery. Driving measures of visual search, lane keeping, clearance judgment, gap judgment, hazard detection, and information recognition showed no differences between monocular and binocular drivers. Monocular drives were poorer than binocular drivers only in sign reading distance in both daytime and nighttime driving. This decrement correlated significantly with the binocular depth perception measure. There were large individual differences within each group for most of the visual and driving performance measures. It was concluded that monocular drivers have some significant reductions in selected visual capabilities and in certain driving functions dependent on these abilities, compared with binocular drivers. However, monocular drivers are not significantly worse than binocular drivers in the safety of most day-to-day driving functions. Implications of these findings and the large individual differences within each group are discussed.

INTRODUCTION Visual defects,

like many other physical handicaps, pose a dilemma for agencies that regulate access to the public highways. Safety to the public demands that the opportunity to operate motor vehicles on the public highways be granted only to those who are able to do so safely. This is particularly true of those who compile high annual mileage in commercial trucks and buses, which, because of their weight and limited maneuverability, present a particularly great hazard to the public. On the other hand, there is the freedom of citizens to drive vehicles in the pursuit of their livelihoods, so long as it does not intrude upon the rights of others. This right extends to those with physical handicaps, so long as those defects do not endanger others. Problem

In the case of the monocular driver, the operational problem that confronts regulatory agencies is the need to resolve conflicting conclusions drawn from two different sources of evidence. The conclusion that monocular drivers present a safety hazard (Bleakley 1974) is based primarily on analytical analyses of the visual demands placed upon drivers of heavy vehicles (Henderson and Burg 1973; Shinar 1978a). Empirical research in support of these analyses shows that monocular drivers are deficient in many of the visual functions believed to be critical to safe operation of heavy vehicles. The weakness in the research and in the conclusions derived from it is the lack of any direct connection between the deficiencies resulting from monocular vision and accidents involving heavy vehicles. Causal inferences are drawn from the conditions surrounding the accident rather than any established relationship between monocular vision and frequency of accidents. Thus, in an analysis of the causes of large truck accidents, Shinar (1978b) concluded that in 20% of the accidents one of the probable accident causes was improper lookout (mostly during lane changes), delayed recognition (mostly of car ahead *This work was performed under contract to the Federal Highway Administration, Transportation. ton ieave from Ben Gurion University of the Negev, Beer Sheva, Israel. 225

U.S. Department

of

226

A. J. MCKN~GHTet al.

slowing down), or misjudgment (mostly the location of the end of the trailer relative to the roadway or other vehicles). The opposing view is that summarized by Bartow (1982), who, based on a comprehensive literature review, noted that while many of the functions analyzed were found to be related to accidents, those in which monocular drivers are most clearly deficientlimited visual field and lack of binocular depth perception-were not among them. The weakness of this conclusion stems from the tenuous relationship that exists between any single driver function and numbers of accidents. Because of the multiplicity of factors that influence accidents, it is very difficult to discern the effect of any particular driver characteristic.

The role of various visual functions in safe driving has been studied for over 50 years. It has been only in the last decade that any attempt has been made to measure the full array of visual functions and to relate them to the definitional criterion of safetyaccidents. Among the more comprehensive studies are those of Henderson and Burg (1974); Shinar, Mayer, and Treat (1975); and Shinar (1977) in which batteries of tests were used to assess a wide array of specific visual functions and the results correlated with accident experience (cf. Booher 1978; Bartow 1982). ~o~oc~~~~iry. The data on the relation between monocularity and accident involvement is quite sparse and far from definitive. The studies demonstrating poorer performance of monocular drivers have all suffered from methodological flaws acknowledged by the authors themselves. Keeney (1981) noted that monocular drivers in his study were drawn from a “Driver Limitation Program” and not necessarily a representative sample of monocular drivers. Rogers, Ratz, and Janke (1987) found that monocular drivers (defined as having less than 6160 acuity in the worse eye) in California had significantly higher accident and violation rates than visually unimpaired heavy vehicle drivers, but the latter were more likely to underreport accidents. This is because out-of-state accidents and convictions do not appear on the California records, and monocular drivers are prohibited from interstate driving; so their exposure to hazards within the state lines is greater and their opportunity for underreporting is less than that of binocular drivers. Finally, a study of the relationship between vision and general aviation accidents revealed that monocular pilots appear to be overinvolved in accidents in some years but not in others (Dille and Booze 1982). Visual field. The importance of visual field derives from the fact that much of the visual information needed by drivers to operate safely first appears in the periphery. Many binocular drivers may also suffer restrictions in visual field and they have been shown in some studies to correlate slightly-but significantly-with accidents (Lauer et al. 1939; Brody 1941; and Burg 1967, 1968,1974). Other studies by Shinar (1975,1977), Hills and Burg (1977), and Council and Allen (1974) did not find that field-deficient drivers had more accidents, though Council and Allen found that more of the accidents sustained by the field-deficient drivers came from their visually deficient side. All of these studies measured only the horizontal extent of the visual field. The most compelling evidence for the criticality of visual field for safe driving was provided by Johnson and Keltner (1983), in a large study of 10,~ license renewal applicants in California. They found that while drivers with visual field loss in one eye evidenced no more accidents and convictions that normal drivers, those with field loss in both eyes had twice the number of accidents and three times the number of convictions as normal drivers matched for age and sex. Such a finding is of particular relevance here, since a monocular driver with a visual field restriction in the remaining eye would, by Johnson and Keltner’s definition, have visual field restriction in both eyes. Their study is noteworthy both because of their large sample and because it relied on a diagnostically valid perimeter for the evaluation of visual field in all axes rather than just the horizontal plane.

Driving performance

of monocular and binocular truck drivers

227

De~~~~~~ce~f~o~. Binocular depth perception (stereopsis) has been largely dismissed as a factor in driving because of the many monocular cues to distance and the fact that stereopsis provides little advantage in distance perception for objects beyond six meters when the viewer’s head is allowed to move. Nonetheless, there are at least three reasons for considering it in the study of monocular drivers: 1. lack of stereopsis is one of the major deficiencies distinguishing monocular from binocular drivers; 2. stereopsis may be required in some of the tasks required of heavy vehicle operators, e.g. having to judge clearance from parked vehicles; 3. while stereopsis may not be necessary to perceiving depths beyond a distance of 6 m, it can conrri~ufe to depth perception for distances up to 185 meters (Schmidt 1966). Acuity. In the general population binocular drivers may be expected to have greater acuity for two reasons: (i) people with binocular vision have slightly better acuity when tested with both eyes than with the better eye alone (e.g. Horowitz 1949); (ii) binocular vision has shown to be superior to monocular vision in several acuity-related tasks, including letter identification and detecting camouflaged targets, and more so under reduced levels of illumination than with optimal illumination (Jones and Lee 1981). Static acuity-low but significant correlations between accidents and degraded visual acuity have been found wherever sample sizes have been sufficiently large to provide reasonable assurance that true relationships will manifest themselves. It is the small number of drivers with poor acuity (after correction) who are really responsible for what relationship there is. This is probably one reason why the relation between acuity and accidents is strongest among older drivers, where the proportion of drivers with poor acuity is the greatest. Dynamic a&&---of the two forms of acuity, static and dynamic, the latter has evidenced the stronger relationship to accidents (Henderson and Burg 1974; Shinar 1977). Because dynamic visual acuity has not been used in licensing, and because it deteriorates more rapidly with age, there are greater individual differences among drivers in their dynamic visual acuity than in their static acuity (Shinar 1978~). Acuity in reduced ~i~u~i~ario~-acuity under optimal and low illumination do not correlate highly with each other (Shinar 1977). Consequently, it is not surprising that driving-related studies of acuity have included acuity under reduced levels of illumination (Henderson and Burg 1974; Shinar, 1977) and acuity for low contrast targets (Allen and Lyle 1963; Keeney 1967). Jones and Lee (1981) found that binocular people perform worse on several acuity-related tasks when performing them under monocular viewing conditions, particularly under conditions of reduced illumination. Acuity under glare- the detection distance for an unlit car or a pedestrian can be reduced by over 50% in the presence of glare (Shinar 1984). However, no statistically significant overall relationship has been found between glare sensitivity and accidents by Burg (1967), Henderson and Burg (1973, 1974), or Shinar (1975, 1977). There is no reason to expect monocular drivers to have greater glare sensitivity than their binocular counterparts. However, Wolbarsht (1977) found that most one-eyed drivers tend to have elevated glare thresholds in the remaining eye. Confrusf ~e~siti~ify. Contrast sensitivity is particularly relevant to nighttime driving because one of the most important effects of reduced nighttime illumination and glare is a reduction in contrast. Recent studies by Ginsburg et al. (1982, 1983) found contrast sensitivity a better predictor of skill in target detection and simulator landings than both photopic and scotopic acuity, which did not correlate at all with performance on a flight simulator. Shinar and Gilead (1987) also found a strong relationship between contrast sensitivity and complex target detection. Although there is no reason to a priori associate monocularity with reduced contrast sensitivity, the absence of the benefits of binocularity noted for acuity, may impair contrast sensitivity too.

228

A. J. MCKNIGHTet

al.

Objectives The objectives

of this study were to (i) analyze and identify those aspects of visual performance that might be affected by monocularity and the particular driving functions of heavy vehicle operators that are likely to be significantly affected by the reduced visual performance; (ii) based on the above analysis, identify and formulate specific measures of visual and driving performance, and (iii) conduct an empirical study to compare the performance of monocuiar and binocular heavy vehicle drivers on the above measures. METHOD

This study assessed the effect of monocularity upon driving empirically by comparing the performance of 40 monocular and 40 binocular drivers in carrying out a variety of tractor-trailer driver tasks. Subjects The subjects in the study were 40 monocular

and 40 binocular heavy truck drivers. The initial search for monocular drivers was based on the Maryland Motor Vehicle Administration driver records. Since monocularity is not listed in the driver records, all Class A drivers restricted to driving with outside mirrors were contacted. This approach provided us with 33 of the monocular drivers. The rest were recruited through notices posted in truck stops, letters sent to 300 ophthalmologists practicing in the Maryland, Virginia, and Washington D.C., and a newsletter mailed to independent truckers in the Northeast area. Once the monocular driver sample was obtained, a sample of 40 binocular drivers, matched for age (which correlates with many visual functions) and truck driving experience (which could affect the performance measures) was obtained from the Maryland driver license files after inte~iewing a total of 1200 binocular drivers without any known visual limitations. All participants were paid for their participation. Average age for the monocular drivers was 46.5 and for the binocular drivers it was 44.3 (the difference was not statistically significant). In terms of exposure, the monocular drivers averaged 58,259 km in the previous year, and binocular drivers averaged 61,633 km in the previous year (the difference was not statistically significant). Within age and total driving exposure matches it was impossible to obtain a very close match of the total truck driving experience. The monocular drivers averaged 21.5 years of truck driving experience whereas the binocular drivers averaged 16.8 years of truck driving experience. However the difference was not statistically significant and it is unlikely that a five-year difference would have a great effect for drivers averaging over 16 years of experience. Whatever effect exists would benefit the monocular drivers. Visual measures

Eight measures of visual performance that were considered relevant to driving were included in the battery: static visual acuity, dynamic visual acuity, acuity under low levels of illumination, glare resistance, glare recovery, visualfield, depth perception, and contrast sensitivity. The operational definition of each of the visual performance measures and the measuring device used to measure it are summarized in Table 1. The procedures used in the test administration conformed to the manufacturers’ instructions (e.g. the AA tests, the Lafayette tests, and the Arden Plates), or were based on standard clinical practices for threshold measurements (e.g. Snellen Chart). For the Dynamic Visual Acuity test, a Landolt ring moved across the screen at 15 degreesfsec, its size decreasing every 5 trials. Threshold was determined at the highest level in which the subject correctly identified the direction of the opening on four of the five trials. Driving measures

The driving performances selected for this study were identified by comparing the visual deficiencies of monocular drivers with the requirements of heavy vehicle operation

Driving performance Table 1. Visual performance

229

of monocular and binocular truck drivers measures and their associated measuring devices

Visual performance

Measuring device

Stuficvismzlacuity-The ability to see stationary objects clearly.

Snellen Chart-The standard eye chart was selected because of its widespread use.

Dynamic visual acuity-The

Projected Moving Images-Slides

Low i~l~i~tion

AAA Night Vision Tester-This

Glare resistance--The

AAA Night Vision Tester-The

Glare recovery-The speed with which the eye recovers from the presence of glare.

AAA Night V&ion Tester-After

V&al field-The

Lafayette Perimeter-The

Depth perception-The ability to perceive the relative distance of two objects from the eye, using both binocutar and monocular cues of depth perception.

Lafayette Depth Perception Tester-Using

Confrasf sensitivity-The

Arden Plates-This

ability to see objects clearly when they are moving relative to the viewer.

acuity-The ability to see clearly at low levels of iIlum~nation (e.g., darkness).

ability to see clearly under low illumination in the presence of glare.

size of the visual field around the point of fixation as measured separately for each eye. Measurement was confined to the horizontal plane - 15, + 15 degrees since it is only stimuli in this plane that are important to driving.

ability to perceive contrast between a figure and background.

containing Landolt rings were projected into a mirror mounted on a turntable. The revolving mirror caused the rings to sweep across a screen at the rate of 1.5degrees per second.

device consisted of a box in which subjects view a series of 6112 Landoh rings through a peephole. Illumination is slowly increased until subjects can correctly identify the position of the gap in the ring.

procedure is the same as that used to test acuity under low illumination, except that the Landolt rings must be viewed in the presence of glare from a small bulb.

the glare resistance trials, the illumination is set at the subject’s low illumination threshold and the interval between the time the glare source is turned off and time the ring can be correctly read is recorded as the glare recovery time.

subject’s chin is placed in a chin rest and the eye is fixated on an object straight ahead. A stylus is used to bring a stimuhts into the periphery from the rear. Subjects report the moment at which they detect the stimulus. The perimeter around the subject’s head allows the visual angle to be read. Recordings were taken on a horizontal plane as well as 15 degrees above and below this plane.

long cords, subjects adjust the fore-aft position of two sticks until they are adjacent to one another. The task was performed at a distance of 6m, a distance at which binocular cues have been found to become ineffective.

is a series of 7 plates containing patterns of differing frequency (distance between the lines making up the patterns). The level of contrast between the pattern and background is progressively increased until subjects report being able to perceive the pattern.

as identified in Truck and bus driver task analysis (Moe et al. 1973) and Curriculum for training tractor-trailer drivers (McKnight 1981). The driving behaviors that appeared most likely to lead to improper heavy vehicles by monocular drivers were: Lane keeping is dependent

operation

of

to some extent on peripheral vision since the cues are often (Denton, 1980; Shinar, McDowell, and Rockwell 1974). Spatial judgment involves the ability to judge the relative distance between the truck and other objects on and off the road and among those objects themselves. Reduced acuity and lack of stereopsis can impair this ability. Behaviors measured here were gap judgment-distance judgments from, or gaps between, oncoming vehicles when peripheral

A. J. MCKNIGHTet al.

230

preparing to make left turns across traffic; judging the distance from, or gap between, vehicles on an intersecting path when approaching an intersection or when preparing to cross or enter the flow of intersecting traffic; judging the distance of an overtaken vehicle when preparing to return to a lane, and clearance judgment-judging the lateral distance of a structure or another vehicle when maneuvering in tight quarters; and judging the distance between the rear of the vehicle and a fixed structure while backing. Mirror checks involve seeing if monocular drivers compensate for limitations in visual field by more frequent mirror checks to detect overtaking vehicles. Information recognition is the process of detecting and extracting relevant stimuli from the environment. Stimuli for which recognition distances can be measured include warning signs, traffic control signs, and route guidance signs. Driving performances that were measured in the street test included lane keeping, gap judgment, clearance judgment, information interpretation, and mirror checks. For those aspects of driving performance that could be measured more accurately and efficiently in an off street environment, without losing their validity, a separate off street test was conducted. This test measured clearance judgments to the side and behind and information recognition. The driving performance measures and the visual performance measures that they may be related to are listed in Table 2. Performance was measured both during the daytime and at night in order to permit any deficiencies in acuity under low levels of illumination, glare resistance, and glare recovery to manifest themselves in the measured driving performance. Equipment

and facilities

The heavy truck used in the study was a standard tractor-trailer combination, a GMC Astro Cab-over-Engine tractor and a 13.72m enclosed cargo trailer. The tractor had a sleeper berth in which the data recording equipment was installed. To record both the street-test driving situations as they occurred and the drivers’ responses to them, three cameras were used. A forward-facing video camera was mounted on a shock absorber pad in the sleeper berth behind the driver’s seat and zoomed to its maximum extent in order to encompass a go-degree field of view. It recorded mirror checks, rejected gaps, tractor lane keeping, and responses to information signs. Time was measured by counting video frames. The second camera was a rearwardfacing video camera mounted 1.83m above the floor of the trailer. It measured the distance of vehicles following behind, length of accepted gaps, clearance behind after Table 2. Driving nerformance Driving performance

measures

Driving measure

Related visual task

Lanekeeping-The ability to maintain the position of the trailer within lane boundaries

Trailer lane excursions

Static visual acuity

Gap Judgment-The

Acceptance/rejection of gaps when crossing, entering, or making a left turn across traffic

Visual acuity, depth perception

Mirror Checks-Use

Duration of mirror fixations during lane changes and merges

Visual search

Clearance Judgment-The

Performing an alley dock maneuver

Visual acuity, depth perception

Information Recognition-The

Responding to lane markings and to signs created to call for an immediate response

Visual acuity

ability to judge distance from other vehicles of head and eye movement to compensate for limitations in visual field ability to judge distance beween the trailer and structures behind ability to correctly read and interpret signs at a distance

Driving performance

of monocular and binocular truck drivers

231

lane changes and merges. The third camera was a super-8 motion picture camera used for nighttime measurements only. It replaced the rearward-facing video camera, because the latter was too sensitive to the glare from the following vehicles. To maximize ecological validity of the performance measures, a test route over public streets was selected-one that provided a full array of driving performances to be assessed and sufficient replication of each situation to provide a reliable sample of the performances measured. The entire 56km route selected contained a mixture of sections of freeway, urban, suburban, and rural streets where drivers were required to make turns. Since it was desirable that the behaviors measured would reflect driving habit as well as driving skill, a sample of behavior was observed under conditions in which the drivers did not know they were being tested. This was done by informing the drivers at a certain point that the test was completed and asking them to return to the test site. In fact, the return route constituted a portion of the test. To be convincing, the subjects were given no guidance as to test route. However, there was only one possible route to the termination point. The subjects had no way of knowing that the video recording equipment was still operating. Later analysis disclosed no practical or significant differences in responses under the two conditions and the data were merged in the final analysis. For the offstreet test, the University of Maryland driving range was used. It consisted of 0.97km of road including a straightway, intersections, merges, and parking stalls. Two exercises occurred in the off-street area: clearance judgment and sign recognition. The clearance judgment exercise consisted of an “alley dock” maneuver in which subjects backed the trailer into a stall approximately 4m wide created by cement curbs. A barricade rising 1Sm was placed at the end of the alley and subjects were instructed to back as close to it as possible without striking it. Distance from the barricade at the end of the maneuver was measured along with time to complete the maneuver and whether or not the barricade was struck. The information recognition test took place on a circular track quadrisected by two roads at ninety degrees to one another (north-south, east-west). Subjects drove around the circle turning into the crossroads as directed by the test administrator. Each of the two crossroads was traveled in each of the two directions. At the end of each crossroad segment was a sign directing them to perform an observable act (swerve, hit brake, etc.). Applicants were instructed to respond to each sign as soon as they were able to read it. An assistant test administrator outside the vehicle observed the position of the tractor cab at the moment the subject responded and measured the distance from that point to the sign. This became the offstreet measure of recognition distance. RESULTS

Visual performance

of monocular

and binocular drivers

The results of the mean performance levels of the monocular and binocular drivers on all the vision tests are summarized in Table 3. The two groups did not differ significantly from each other in their static visual acuity, dynamic visual acuity, visual field (when measured for each eye individually), and the glare recovery time. The added advantage of the binocular drivers’ two eyes did not manifest itself in a significant difference in binocular acuity. When the visual acuity in the seeing eye of the monocular drivers was compared to the binocular acuity of the binocular drivers (which was consistently better than their acuity in each of the eyes alone) the difference between the two groups was still not statistically significant. The total visual field was obviously much smaller for the monocular drivers (extending on the average only 145 degrees) than for the binocular (extending 173 degrees). This difference is obviously highly significant both statistically and practically. The groups did differ from each other in their contrast sensitivity, depth perception, minimal illumination for night vision, and glare resistance. In all of these tests, the performance of the binocular drivers was better than that of the monocular drivers. The

A. J. MCKNIGHT et al.

232 Table 3. Summary of performance

on the driving tasks for the monocular and binocular drivers Monocular

Test

Binocular

Mean

SD

Mean

SD

Significance

614.2 615.0 614.6

611.2 611.5 611.4

615.1 615.5 614.2

611.5 611.9 60.0

n.s. n.s. n.s.

617.5

611.1

617.5

6.08

n.s.

85.3 59.9 145.3 11.3

4.42

85.0 59.0 172.6 10.1

5.7 6.6 6.2 1.73

ns. n.s. p < .01** t = 2.92 p < .Ol

1. Static Acuity

Right eye Left eye Both* 2. Dynamic visual acuity (target size) 3. Visual field (0 plane f. 150”) Temporal Nasal Totai 4. Contrast sensitivity*** 5. Depth perception (error in cm.)

3.46

::; 2.09 1.52

1.65

1.26

t = 9.72

p < .Ol 6. Night vision***

34.2

13.3

28.7

11.5

t = 1.89

p < .05 7. Glare resistance**

83.8

18.5

70.3

22.5

a. Glare recovery time (set)

22.7

23

26.9

27

t = 2.80 p < .Ol ns.

*For monocular driver this entry is the same as the single-eye acuity. **This difference is highly significant since there was no overlap at aif between the two groups. **‘In arbitrary units as specified on the testing device.

largest decrement in performance of the monocular drivers was obtained for the depth perception, where the monocular drivers erred by more than twice as much as binocular drivers. Note that this test is so designed as to eliminate as many binocular cues as possible and it was positioned at a distance at which binocular cues would become relatively ineffective. Still, even at that distance, binocular drivers benefitted from their two eyes significantly. Less dramatic but still possible practical significance was the poorer performance of the monocular drivers in their night vision and glare resistance. In both cases, performance was measured in terms of the minimal levels of illumination needed to resolve targets and, for both measures, the monocular drivers required approximately 20% more light than the binocular drivers. The poor performance of the monocular drivers in their contrast sensitivity, although statistically significant, was not so large as to suggest an impairment that would manifest itself in driving measures. For all three measures, the differences between the means of the two groups were less than one standard deviation within each group.

The mean performance levels and standard deviations on each of the driving performance measures are summarized for the monocular and binocular drivers in Table 4. They are discussed below in order of the presumed sensitivity of the performance measure relative to the expected effect of monocularity: information recognition, visual search, lane keeping, clearance judgment, and gap judgment. Information recognition was measured in the offstreet test by noting the reading distance to signs requiring particular behaviors (such as “swerve,” “sound horn”) and in the orisfreet test by noting the latency of response to a lane marking (“right turn only”) that required an immediate response. The o@treet test showed significant differences between the two groups both during the day and during the night, with the binocular drivers having a reading distance that was on the average 13% (5.6m) farther than the monocular drivers during the day and 12% (3.0m) farther at night. Although

Driving performance Table 4. Summary of performance

of monocular and binocular truck drivers

233

on the visual tasks for the monocular and binocular drivers Night

Day Driving Task

Monoc

Binoc

Signif

Monoc

Binoc

Signif

Recognition distance (m) Signs Markings:

41.8 15.8

47.4 15.2

p < .05 n.s.

25.5 *

28.5 *

p < .05

Mirror checks (per km) Single lane Multi lane

18.1 11.1

13.5 14.8

n.s. n.s.

* *

i *

Lane keeping (% success)

77

78

n.s.

Clearance Judgment Time (min) stops (II) Contacts (n) Distance (m) Struck dock (%)

2.14 2.05 53 11.9 14

2.40 1.55 SO 13.7 6

n.s. n.s. Il..%

1.85 1.57 .78

ns. n.s.

I.5

2.4

ns.

Gap Errors (%) Rejected safe Accepted unsafe Crossing/center Lane change

26 32

n.s, n.s,

79

84

* *

1

ns.

:

2.03 1.34 .90 5 5

n.s. n.s. n.s. n.s.

3.8

1.6

n.5.

24 31

22 43

ll.S.

n.s. n.s.

*Driver response data could not be collected at night.

these differences were significant, note that they are relatively small compared with the standard deviation within each group. In the ansrreer test, there was no difference between the two groups in terms of the time at which they initiated a lane change and the point at which the lane marking required a lane change. On mirror checks, there was no significant difference between the two groups in the number of mirror checks per mile. The idea that the monocular drivers would compensate for the reduced visual field by increasing their frequency of mirror checks was not borne out in the statistical analysis. In fact, binocular drivers seemed to have a slightly higher rate of mirror checks though this was not statistically significant. Lane keeping performance was measured by noting the percentage of turns (right turns and curved road segments) in which both the tractor and the trailer did not cross the lane markings. It can be seen from Table 4 that, in most cases, both groups maintained their vehicles within the lane and the differences between the groups, both during the day and during the night, was statistically nonsignificant and numerically very small. Clearance judgement was measured by performance on an alley dock maneuver. Performance measures for the alley dock maneuver included the number of stops made prior to final docking, the number of contacts made with street and alley barriers and delineators, the final distance from the dock, the number of drivers who struck the dock, and the total time taken. There were no significant differences between the two groups on any of these five measures either during the daytime maneuvers or during the nighttime maneuvers. Gap judgment was measured by noting the number of times that drivers rejected gaps in cross traffic that were in excess of eight seconds. A greater number of such gaps for the monocular drivers would indicate that they either cannot judge distances properly or are more conservative. The results indicated no significant difference in the total number of gaps rejected that were safe (i.e. in excess of eight seconds) either during the daytime or during the nighttime. More critical are the situations where drivers accept gaps that are unsafe (i.e. less than two seconds). Again there were no significant differences between the monocular and binocular drivers in the acceptance of such short gaps either when crossing or entering traffic or when changing lanes either in the daytime or in the nighttime.

A. J. MCKNIGHTet al.

234

The relationship between performance driving measures

on the visual measures and performance

on the

Having found significant differences between the monocular and binocular drivers on selected visual measures and selected driving measures, we examined the correlations between the two sets of variables. The purpose of this analysis was to see if there exists a correspondence between performance on particular visual measures and performance on particular driving measures. The correlations between each of the visual performance measures and the two driving performance measures were calculated separately for the monocular and binocular drivers. For the monocular drivers, the only significant correlations were between depth perception and daytime sign reading distance (r = 0.39, p -C .05) and between depth perception and nighttime sign reading distance (r = 0.57, p < .05) (both measured in an offstreet driving situation). For the binocular drivers, none of the visual measures correlated significantly with either one of the driving measures .

The presence of a significant correlation between sign recognition and depth perception for the monocular drivers and its absence for the binocular drivers is puzzling. The lack of correlation among the binocular drivers is not due to a restriction in variance that often leads to reduced covariance and consequently to a lower correlation. As can be seen in Tables 3 and 4, the variance for the binocular drivers in depth perception and sign recognition is not much smaller than it is for the monocular drivers. It is most likely that sign recognition in the offstreet environment involved particular aspects of visual performance that are more critical for monocular drivers than they are for the driving performance of binocular drivers. What these aspects are is impossible to discern from the data. Visual screening

measures

One objective of the study described was to ascertain whether it might be possible to develop a screening process that would identify those monocular drivers whose visual deficiencies rendered them unsafe, allowing those who could operate heavy vehicles safely to be licensed for interstate operation. Such a screening process would resolve the dilemma described at the beginning of this paper. The most direct screening measure would be the one driving measure that significantly differentiated between monocular and binocular drivers-the sign recognition measure. Such a measure would have the inherent validity of (i) measuring a driving function that is critical to safety and (ii) distinguishing reliably between monocular and binocular drivers. Screening out those monocular drivers who had to get within two and a half meters of a sign before they could recognize what it said would produce a distribution of response distances among the monocular drivers that roughly paralleled that of the binocular drivers. The number of drivers who would have been screened out is nine, a little less than a quarter of the 40 monocular drivers. Of course, the system used to measure sign recognition would not make a very practical license screening test. A better alternative would be some visual measure that correlated highly with sign recognition. Among those visual measures studied, the only one showing a significant correlation with sign recognition was depth perception. The correlation ranged from .4 in the daytime to .6 at night. While these correlations are moderately high, they are far from establishing the depth perception measure as a substitute for a measure of sign recognition. If one used the depth perception measure as a screening device, it would be necessary to screen out some 65% of the monocular drivers (26 out of 40) to produce distributions of day and night sign recognition distances approximately that of binocular drivers. This amounts to almost three times the number of drivers as would be screened out using a sign recognition test itself.

DISCUSSION

It is evident that the monocular and binocular tractor-trailer drivers differ from one another in some measures of both visual and driving performance. With respect to visual

Driving performance

of monocular and binocular truck drivers

235

measures, the monocular drivers involved in this study evidenced reduced visual performance in contrast sensitivity, depth perception, night vision, and glare resistance. While lack of vision in one eye obviously reduced the total visual field of monocular drivers, the visual field of the remaining eye was as great as that found in the corresponding eye of binocular drivers. No significant differences were found in static acuity, dynamic acuity, or glare recovery. deficiencies

of monocular

drivers

None of the tasks performed in the highway traffic environment showed significant differences between monocular and binocular drivers. Differences among individual drivers were very large. However, the lack of correlations between most visual measures and driving measures shows that the latter were far more influenced by differences in the subject’s style of driving than they were by his visual performance. It is possible that the visual deficiencies of monocular drivers might manifest themselves in rare safetyrelated events and that significant differences between monocular and binocular drivers might have been observed over many thousands of hours of driving. However, the failure to find such differences in a cumulative total (across all subjects) of 120 hours of street driving suggests that, with respect to safety of operation, the differences among drivers within each category of drivers far exceed those between the two categories. In the offstreet environment, variability in performance associated with traffic conditions or individual styles of driving was minimized, allowing the influence of visionrelated factors to be more clearly observed. Specifically, monocular drivers performed significantly worse that binocular drivers in their ability to correctly read and interpret roadway signs designed to be legible at a distance of 45m during daylight. The decrement was approximately 12% during the day and 13% under night conditions. No consistent difference in perfo~ance was observed in the exercises designed to assess possible decrements in clearance judgment. On the surface, in light of the significant correlation between depth perception and sign recognition distance, it would appear that unsafe monocular drivers can be identified on the basis of their ability to judge depth alone. However, there are reasons to be skeptical about such a conclusion. First, binocular cues of depth are minimal at 6m and beyond. It is difficult to see how such cues could have aided binocular drivers in outperforming their monocular counterparts. Secondly, it is hard to see how binocular cues of depth could be responsible for the ability to read and interpret signs at distances up to 46m and not be associated with any of the measures of clearance judgments. A more likely explanation is that the depth perception measure simply provides a better means of assessing how well drivers can recognize information in a real-life environment than some of the other measures used in this study. This is supported by our and others’ (Jones and Lee 1981) findings that monocular people, as a group, perform worse than binocular people on various measures of visual performance. In the present study, the superior ability of binocular drivers to equalize the distance of the two rods in the depth perception test may be simply a result of the visual summation that is afforded by the two eyes; i.e. the ability to see each of the rods somewhat better than the monocular drivers. In short, the test of depth perception may provide a more sensitive test of the ability to “see clearly” than Snellen acuity, which does not provide a sensitive continuous measure of performance. Further research utilizing other measures of driving performance and possibly new measures of visual performance may identify relations that were not observed in the present study. In addition to sign reading, other candidates for information interpretation measures might be the detection of distant vehicles, pedestrians, and other vehicles when changing lanes; and the speed with which “targets” in mirrors can be acquired and perceived. The need for a screening process The lack of any demonstrated relation between monocular vision and accidents, along with the failure to find any significant difference between monocular and binocular AAP23:4-c

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drivers on a road test, undermines empirical support for the current practice of disqualifying all monocular drivers from operating in interstate commerce. It is true that monocular drivers as a group did significantly worse than binocular drivers on a task designed to reflect vision deficiencies. However, such a finding forms a questionable basis for disqualifying monocular drivers as a group. Physically handicapped drivers are not excluded as a group even though it would be possible to devise a test in which their disabilities would place them, as a group, at a disadvantage-e.g. an emergency swerving test. Those drivers who can demonstrate their ability to handle a tractor-trailer in normal operation are granted waviers from physical restrictions. On that basis, monocular drivers deserve a screening system that will permit a portion of the monocular driver population who can operate tractor-trailers safely, to do so on interstate roads. If it is the ability to read signs at great distances that really counts, why not assess it through measures of driving performance rather than measures of visual performance? Any predictor (e.g. depth perception) that has less than a perfect correlation with the criterion (e.g. sign recognition) must screen out a larger number of drivers to achieve the same result as the use of the criterion measure itself. The rationale behind the use of any screening test is that it assesses some human characteristic that is essential to a range of performance variables. However, in the present study only one vision test was significantly related to only one driving performance variable. If the present sample of monocular drivers were tested directly on the signs test, simply screening out drivers who had to get closer than 30m in day time to respond to the signs would have raised the mean performance level of the remaining monocular drivers to that of the binocular drivers. And, it would have done so by screening out only 9 monocular drivers rather than the 23 that would have had to be screened out by the depth perception test in order to achieve the same performance level on the sign recognition test. The obvious shortcoming of the above solution is that the proposed road test would be both expensive and not necessarily sufficiently inclusive of all driving tasks that may be affected by lack of binocular vision. If the finding of Rogers et al. (1987) that monocular heavy-vehicle drivers have higher accident and conviction rates than binocular drivers is valid, then the present findings suggest that there is a need to assess the monocular drivers’ visual functioning capabilities more closely. We found large variations among monocular drivers and several significant differences between them and binocular drivers. The challenge now is to identify those visual performance measures that significantly correlate with measures of safe driving skills. The present results provide a demonstration of the validity of this approach and the first step in that direction. Acknowledgement-This study was carried out under contract to the Federal Highway Administration. The authors wish to thank Dennis McEachen, who managed the contract, for his counsel and assistance. The authors also wish to thank Arthur H. Keeney and James E. Baily for their assistance in the development of visual measures. REFERENCES Allen, M. J.; Lyle, M. Relationship between night driving ability and the amount of light needed for a specific performance needed on a low contrast target. J. Am. Optom. Assoc 34: 1301-1303; 1963. Bartow. P. The monocular driver: A review of distant visual acuity risk analysis data. Bartow Associates, Inc. Washington, DC: Federal Highway Administration; September 1982. Bleakley, R. L. The monocular driver. Washington, DC: Federal Highway Administration. Office of Contracts and Procurement (HCP-32); 1974. Booher, H. R. Effects of visual and auditory impairment in driving performance. Hum Factors, 20: 307-320; 1978. Brody. L. Personal factors in safe operation of motor vehicles. New York: New York University, Center of Safety Education; 1941. Cited in Burg (1964). Burg, A. An investigation of some relationships between dynamic visual acuity, static visual acuity, and driving record. Report No. 64-18. Los Angeles: University of California, Department of Engineering; April 1964. Burg, A. The relationship between vision test scores and driving record: General findings. Report No. 67-24. Los Angeles, CA: University of California, Department of Engineering; June 1967. Burg, A. Vision test scores and driving record: Additional findings. Report No. 68-27. Los Angeles: University of California, Department of Engineering; December 1968. Burg, A. Visual degradation in relation to specific accident types. Report No. UCLA-ENG-7419. Los Angeles: University of California, Institute of Transportation and Traffic Engineering; March 1974.

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Council, F. M.; Allen, J. A., Jr. A study of the visual field of North Carolina drivers and their relation to accidents. Chapel Hill, NC: North Carolina University, Highway Safety Research Center; 1974. Denton, G. G. The influence of visual pattern on perceived speed. Perception 9: 393-402; 1980. Dille, J. R.; Booze, C. F., Jr. The prevalence of visual deficiencies among 1979 general aviation accident airmen. Aviat Environ. Med. 53:179-182; 1982. Ginsburg, A. P.; Evans, D. W.; Sekuler, R.; Harp, S. A. Contrast sensitivity predicts pilots’ performance in aircraft simulators. Am. J. Optom. Physiof. Opt. 59: 105-109; 1982. Ginsburg, A. P.; Easterly, J.; Evans, D. W. Contrast sensitivity predicts target detection field performance of pilots. Proceedings of the Human Factors Society, 27th Annual Meeting. Santa Monica, CA: Human Factors Society; 1983:269-273. Henderson, R. L.; Burg, A. The role of vision and audition in truck and bus driving. (TML)-5260/000/000. Santa Monica: System Development Corporation; December 1973. Henderson, R. L.; Burg, A. Vision and audition in driving. Final Report No. DOT-HS-801-265. Santa Monica, CA: System Development Corporation; 1974. Hills, B. L.; Burg, A. A reanalysis of California driver vision data: General findings. Report no. TRRL-LR768. Crowthorne, England: Transport and Road Research Laboratory; 1977. Horowitz, M. W. An analysis of the superiority of binocular over monocular visual acuity. J. Exp. Psychol. 39: 581-596; 1949. Johnson, C. A.; Keltner, J. L. Incidence of visual field loss in 20,008 eyes and its relationship to driving performance. Arch. Ophthalmol. 101: 371-375; 1983. Jones, R. K.; Lee, D. N. Why two eyes are better than one: The two views of binocular vision. J. Exp. Psycho1 [Hum. Percept.] 7(l): 30-40; 1981. Keeney, A. H. Ophthalmology in driving. Transctions of the Pacific Coast Auto-Ophthalmology Society Annual Meeting 48: 167-178; 1967. Keeney, A. H. The dilemma of the monocular driver. Am J Ophthalmol91: 801-803; 1981. Lauer, A. R.; DeSilva, H. R.; and Forbes, T. W. Report to the Highway Research Board. Unpublished mimeo report, 1939. (Cited in Burg 1964.) McKnight, A. J. Curriculum for training tractor-trailer drivers, Contract DTFH61-80-C-00037. National Public Services Research Institute, prepared for the Bureau of Motor Carrier Safety; December 1981. Moe, G. L.; Kelley, G. R.; Farlow, D. E. Truck and bus task analysis. FH-11-7616. Washington, DC: National Highway Traffic Safety Administration; May 1973. Rogers, P. N.; Ratz, M.; Janke, M. K. Accident and conviction rates of visually impaired heavy-vehicle operators. Report No. CAL-DMV-RSS-87-11. Sacramento. CA: California Denartment of Motor Vehicles: January 1987.’ Schmidt, I. Visual considerations of man, vehicle, and the highway: Part I. SP-279. Wa~endale, PA: Society of Automotive Engineers; March 1966. Shinar, D. Practice effects on simple and compiex visual skills. Paper presented at the Annual Meeting of the American Academy of Optometry, Columbus, OH, 1975. Shinar, D. Driver visual limitations diagnosis and treatment. Final report. B278-884. Washington, DC: National Highway Traffic Safety Administration; 1977. Shinar, D. An evaluation of heavy vehicle driver visual requirements. Chapel Hill, NC: University of North Carolina, Highway Traffic Research Center; 1978a. Shinar, D. An analysis of the causes of large-truck accidents. Chapel Hill, NC: University of North Carolina, Highway Safety Research Center; 1978b. Shinar, D. Psychology on the road: The human factor in traffic safety. New York: John Wiley & Sons, Inc.; 1978~. Shinar, 13. Actual versus estimated nighttime pedestrian visibility. Ergonomics 27: 863-871; 1984. Shinar, D.; Gilead, E. Contrast sensitivity as a predictor of complex target detection. Proceedings of the 31st Meeting of the Human Factors Society. Santa Monica, CA: Human Factors Society; 1987:1194-1197. Shinar, D.; Mayer, R. E.; Treat, J. R. Reliability and validity assessments of a newly-developed battery of driving-related vision tests. Proceedings of the 19th Annual Meeting of the American Association for Automotive Medicine, San Diego, CA, November 1975. Shinar, D.; McDowell, E. D.; Rockwell, T. H. Improving driver performance on curves on rural highways through perceptual changes. Columbus, OH: Systems Research Group, The Ohio State University; 1974. Wolbarsht, M. L. Tests for glare sensitivity and peripheral vision in driver applicants. J. Saf. Res. 9:128-139; 1977.