driving under adverse visibility conditions: implications for licensing of low vision drivers

Driving Under Adverse Visibility Conditions: Implications for

Licensing of Low Vision Drivers

Kent E. Higgins, Ph.D.The Arlene R. Gordon Research

Institute Lighthouse International

Conference objectives:

• To stimulate discussion regarding driving with Bioptics

• To advance and stimulate research on the utility and safety of Bioptics

A bioptic driver is one

• That cannot be corrected to 20/40 (6/12) using spectacle lenses.

• That can, however, read 20/40 (6/12) detail at the normal test distance when viewing the chart through a small telescope of sufficient magnification.

When used as a driving aid,

• the bioptic is mounted in the upper part of the spectacle (“carrier”) lens.

• the person dips their head and elevates their eye to look through the bioptic, using it as a momentary “spotting” aid.

Advantages/disadvantages of using the bioptic when driving. • Bioptic improves ability to recognize small

detail at a distance.• Bioptic severely limits the field of view to a

few degrees (e.g., 8-10o).• Bioptic can be disorienting if used for

continuous viewing.• Bioptic requires some practice.

Early debate over bioptic driving focused on problems associated with viewing through the bioptic

• The “ring scotoma”• Speed of recognition through bioptic.• Distorted motion/size-distance cues

Instead of talking about the bioptic, I am going to talk about vision through the carrier lens(es)

• the bioptic is used as a momentary spotting aid, like a rearview mirror

• most of the driving (timewise) is being done with the reduced acuity provided by the carrier lens(es).

Corrected (carrier) lens standards recommended for an unrestricted driver’s licensure (TRB, 1988)

• 20/40 (6/12) or better visual acuity

• 140 degree visual field (minimal value)

Is it reasonable to entertain the hypothesis that people may be able to drive safely when their

• Visual field is less than 140o?• Visual acuity (VA) is less than 20/40

(6/12)?

Traditional research, like that of A. Burg provides little relevant data• Method: Correlate accident records with vision

test scores in licensed drivers. r = 1.0 (perfect predictability) r = 0 (no predictability)

• Results: r’s of about 0.1.• Implications: Relationships statistically significant,

but with little practical significance• r2 values of about 0.01

Traditional research on role of vision in driving

• Typical interpretation: must have used wrong tests.

• Heretical interpretation: existing standards are so restrictive as to have eliminated accidents that could be blamed on poor vision.

Limitations of the traditional correlational approach:

• Use of already-licensed drivers restricted the range of variation in vision test scores in driving sample.

• Use of accidents restricted range of variation in driver performance.

• Difficult to infer causal relationships.

Owens et al. (1993) estimated that a U.S. driver can expect to travel

• For 102 yrs before experiencing a disabling-injury accident.

• For 3,738 yrs before becoming a traffic fatality.

Improving sensitivity of research paradigm:

• Study accident-prone drivers• Closed road driving assessments• Driving simulator training/assessment• On-road driving assessments• Research on driving at night

Study of drivers with history of accident involvement (Ball et al., IOVS, 1993)• Selected sample of elderly drivers with

known history of accident involvement• Sample also showed wide range of

variation in visual characteristics • However, no single cutoff was found that

would eliminate unsafe drivers without also eliminating many safe drivers.

Ball et al. (cont.)

• This is also the study showing that the UFOV (Useful Field of View) was better than clinical vision tests at separating the safe from the unsafe driver, where safe and unsafe were defined in terms of accident risk.

Do results of Ball et al. mean that vision (acuity, etc) is irrelevant?• No. UFOV measures speed of visual

processing, and ability to divide attention (with and without distracting stimuli), all of which are important to driving

• However, UFOV is not visually demanding in terms of acuity and visual field. Largest eccentricity tested is 30o and Ss need not be corrected for near test distance.

Closed road driving tests: Used by J. Wood & collaborators to study effect of

• Artificial visual field constriction in normal Ss (Wood & Troutbeck,1992)

• Artificial acuity degradation (Higgins, Wood, & Tait, 1998, 1999)

• Age and visual impairment (Wood, 2002)

Advantages of Closed Road Assessment:

• finer-grained analysis of variations in driver performance.

• introduction of wider range of variation in visual characteristics of drivers.

• can show relationship of vision to components of overall driving task, e.g., sign recognition, road hazard avoidance.

Wood & Troutbeck (1992) studied artificial visual field restriction• Results showed that many aspects of

closed road driving performance did not deteriorate significantly until field was restricted to less that 40o.

Higgins & Wood (1998) degraded acuity & studied road hazard avoidance

• Hazards = 1m X 2.2 m X 8 cm thick sheets of gray foam rubber “speedbumps”

• Ss told to call out when they saw them and to steer around them

• Ss tested on 5.1 km course under 5 different levels/types of acuity degradation

20/20 20/40 20/100 20/200

r = 0.78 (r2 = 0.61)

0.0 2.0 4.0 6.0 8.0 10.0MAR (arcmin)

-1.0

1.0

3.0

5.0

7.0

9.0

Roa

d H

azar

ds H

it (m

ax. =

9)

Low Contrast Road Hazard Avoidance

r = 0.51 (r2 = 0.25)

20/20 20/40 20/100 20/200

Berkeley Disability Glare Test

0.0 2.0 4.0 6.0 8.0 10.0MAR (arcmin)

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

25.0

Num

ber L

ette

rs L

ost D

ue to

Gla

re

What about drivers with real cataracts?• Owsley et al. (1999) found that they were

2.5 times more likely to be involved in an accident.

• Owsley et al. (2002) found that those who elected cataract surgery reduced risk of crash involvement by factor of 2 compared to those who did not elect surgery.

What do state vision standards mean?

• No one should drive if VA < 20/40 (6/12).

• No one should drive if visual field < 140o.

If so, states would have to prohibit driving by normally sighted people

• Under adverse weather conditions (fog, driving rain or snow, dust storms, etc.)

• At night when headlamps are the primary source of illumination.

Driving at night (a.k.a. “low vision driving”):

“We drive by night and when we do we often misjudge our

visual abilities, courting disaster” (Leibowitz & Owens,

1986)

L & O’s points re: night driving

• Accident rates go up at night.• “Focal” (hazard recognition) vision is

degraded, but we are aware of it.• Ambient” vision, which enables us to

steer, is not degraded at night.• We overdrive our headlights.

Illumination pattern of US headlights (Owens et al. 1989)

• Determined iso-illuminance contours using 0.3 ftC (3 lux) criterion

• Determined at ground level and either 27” or 43” above road surface

• 0.3 ftC = lower limit of civil twilight

55 mph

45 mph

35 mph

25 mph

U.S. LOW BEAM

0

100

200

300

400

500

-40

-20

20

40

0

55 mph

45 mph

35 mph

25 mph

-40

-20

20

40

0

0

100

200

300

400

500

U.S. HIGH BEAM

POSITION RELATIVE TO VEHICLE (FT) (L R)

TWIL

IGH

T D

ISTA

NC

E (F

T)

GROUND HEIGHT HEADLIGHT HEIGHT

(27”)

EYE HEIGHT (43”)

Ground level horizontal field limits for headlights at 185 ft (56 m)

• On low beam = about 10o

• On high beam = about 15o

VA at nighttime light levels

• Richards (1977) – 0.34 – 1.03 c/m2 is range of night driving luminance levels, i.e. well below clinical light levels.

• Sturr & Taub (1990) – many, especially older, persons would not meet the 20/40 criterion if VA tested at night driving light levels

Is it reasonable to entertain the hypothesis that people may be able to drive safely when their

• Visual field is less than 140o?• Visual acuity is less than 20/40

(6/12)?

A number of states in US consider this a reasonable hypothesis

• Keltner & Johnson, Ophthalmol, 1987• Fishbaugh, JASORN, 1995• Brilliant, et al., Essentials of Low Vision

Practice, 1999• Peli & Peli, Driving with Confidence: A

Practical Guide to Driving with Low Vision, 2002

Answer a question with a question:

• If normally sighted drivers are able to drive safely when their vision is impaired, should we not at least consider the possibility that some individuals with vision loss might also be able to drive safely and, possibly, without the bioptic?

Driving simulator/On-road studies

• I leave it to subsequent speakers to address evidence from these important types of studies.

• I will begin to close by noting that funding agencies seem to be looking more favorably at research designed to evaluate the driving abilities of people with vision loss.

One significant recent initiative:

• “Engineering Approaches to Low Vision Rehabilitation”

• NEI funded 5 year bioengineering research partnerships grant

• PI is Eli Peli of the Schepens Eye Research Institute (SERI)

Overall objectives:

• Develop devices aimed at restoring interplay of central (high-resolution) and peripheral (wide-field) vision for persons with different types and amounts of vision loss

• Test efficacy of devices through laboratory (virtual world) and field (real-world) tests of pedestrian and driving mobility.

Participating Institutions/Industry Partners

• MEEI• Lighthouse Int’l• U. of Cambridge• Boston College• U. Groningen• U. Alabama

• Boston VA• Belgian Road Safety

Institute• Ghent Univ. Hosp.• Chadwick Optical• DigiVision, Inc.• MicroOptical Group

Acknowledgement of support for KEH

• Queensland University of Technology• National Eye Institute• Department of Veterans Affairs