Driving and Low Vision - What Optometrists Must Know

An eyecare provider must have multiple considerations when deciding whether or not a patient is fit to drive. Each state has a distinct set of parameters for which a person can obtain a driver’s license, including visual acuity (VA) and visual field (VF).

While safety is at the forefront of every person’s mind in the context of driving with low vision, scientific data does not support the belief that low vision driving will lead to an increased accident rate.

Another important consideration is quality of life. Not only is a loss of driving privileges a loss of autonomy, it can limit healthcare access and lead to social isolation and economic hardships.

There is no general consensus on what vision is needed for a person to be a safe driver. Vision requirements are not federally regulated and there is a significant lack of uniformity across states. In all states, there is a set of requirements―VA and for most, also visual fields VF―for driving with an unrestricted license.

In most states, there is a secondary set of requirements that govern granting a restricted license to those who fail the unrestricted license standards. However, these secondary regulations vary widely in regards to VA, VF, location and time of day restrictions, and allowances for bioptic telescopes.

In addition to VA and VF, other visual components may have an important, if not more important, effect on driving ability. Contrast sensitivity and visual attention are more consequential than visual acuity, yet not a required measurement. Cognitive ability and mental state, not to mention hearing and motor ability, are also crucial for the safe handling of a motor vehicle.

In this article, we will discuss the range of vision requirements for restricted licenses in the United States in regards to VA and VF. We will consider the other visual components (and other criteria) that may play a role in driving safety. We will explore the science behind the belief that low vision drivers may be at an increased rate for accidents. Lastly, we will elaborate on how a loss of driving privileges can affect the quality of life of a patient and how eyecare practitioners can alleviate that strain.

How visual acuity affects driving

In 40 of the 51 jurisdictions (including District of Columbia), VA is required to be 20/40 or better for unrestricted licenses.

According to the Federal Highway Administration, US road signs are designed based on sight-distances for drivers with 20/30 vision.

The premise is that drivers need to be able to see highway signage (such as speed limits, stop signs, exit signs on interstate) within a certain time frame to make safe vehicle control decisions. Thus, requiring licensed drivers to have a certain minimal visual acuity that increases the likelihood the driver will be able to read highway signs in the time frame they need to make decisions and execute motor responses.

However, there are several states where 20/40 is not the minimum, including Florida where 20/70 vision permits unrestricted driving. Fifty jurisdictions specify a minimum VA below which driving is not permitted under any circumstance; however, the VA levels vary widely (four states have a minimum of 20/40 while three states specify a minimum of 20/200).¹

According to Peli, there is no known justification for any of these VA levels and numerous studies have found no association (or only a weak association) between VA and crash rate.¹ Owsley posits that VA-related driving skills (ie. sign recognition) does not play a large role in the safe operation of a vehicle. VA may be important for route planning, but not critical for collision avoidance.

Visual field requirements for driving

Only 36 states specify a VF requirement, ranging from 20° (Wisconsin) to 150° (Maine) on the horizontal meridian. Only two states specify a vertical minimum, Kentucky (25°) and Utah (20°). According to Peli, the field of view through a common car windshield only allows 15° above and below the center of the windshield. Only 12 states specify a minimum VF requirement for a restricted license. Many of these states require the use of outside mirrors; however, these rear view outside mirrors only allow viewing the area behind the driver and does not provide frontal field expansion.¹

While most recent studies have found an impact of visual field on driving performance, we do not know the level of loss that is consistent with dangerous driving. In driving assessments, only patients with moderate to severe peripheral VF loss are found unsafe to drive.¹ Owsley states that large individual differences exist and individual assessments of driving skill, rather than comprehensive prohibitions, should be recommended. Drivers with VF defects that are deemed to be safe drivers tend to engage in more scanning behavior compared to unsafe drivers with VF defects.³

How contrast sensitivity impacts driving

Contrast sensitivity (CS) is not currently a licensing requirement in any state. However, multiple studies have shown a correlation between impaired CS and a recent history of crash involvement. In a study of patients with cataracts, CS impairment was strongly associated with recent crash history. For those who elected cataract surgery, their risk of future crash involvement was reduced by 50%. CS measured under photopic conditions was also a better predictor of recognition of road signs, obstacles and pedestrians while driving at night than photopic VA.

Visual attention and other considerations

The task of driving involves visual sensory abilities such as VA and CS, and the simultaneous use of central and peripheral vision, in a visually cluttered environment. Controlling a vehicle involves the execution of primary and secondary tasks (both visual and non-visual) through a rapidly changing environment. The driver is often uncertain when and where the next critical visual event may occur and must react in a timely manner to it.

Visual attention became recognized in the role of driver safety in the 1990’s when there was an increased interest in the elevated crash rates of older drivers as compared to middle-aged drivers; it was about double. Using the useful field of view (UFOV), Ball et al found that older adults with impaired divided attention abilities were more likely to report driving difficulties. In addition, poor performance in the UFOV by older drivers was associated with a history of increased number of motor vehicle collisions in recent years.

The implication of these studies is that visual attention and visual processing speed are critical to safe driving skills and may be better screening tests than such visual sensory values as VA for identifying crash-prone older drivers.

More than five million Americans age 65 or older have some form of dementia such as Alzheimer’s disease. This number is expected to grow to 13.8 million by 2050. 50% of patients with Alzheimer’s continue to drive up to three years after diagnosis. Multiple studies have shown that even mild cognitive impairment can significantly affect driving performance and increase involvement in traffic accidents.

Given that driving is a multi-sensory, reaction-time dependent task, other senses also play an important role in the ability to drive, including hearing and motor skills.

While an eyecare provider is dedicated to the task of evaluating vision, he or she must also consider hearing loss and mobility restrictions and decide if these areas may limit the patient’s safe driving abilities.

Low vision drivers and safety

Statistics show that in the US, crash rates for drivers under the age of 18 are four times higher than drivers between 30 and 79. Moharrer et al found that older drivers with mild cognitive impairment and normal vision had a hazard ratio of six times compared with age-matched normal controls.¹¹

Several studies by Burg and Hills in the 1960’s and 1970’s revealed that for young and middle-aged California drivers, there was no relationship between poor visual acuity and motor vehicle collision involvement. More recent studies by Rubin and Cross in 2007 and 2009, did not find a significant relationship between visual acuity and motor vehicle collision involvement rates either.

Research supports the theory that visually impaired drivers tend to drive less and in more familiar surroundings.¹¹ Therefore, the driving patterns of the visually impaired diminishes any excess risk posed on a per capita basis.¹

Bioptic telescopes

Bioptic telescopes are miniaturized telescopes mounted on spectacles that allow drivers with reduced acuity to discern higher detail at further distances. Thirty-six states allow driving with bioptic telescopes and two prohibit it; eighteen states require a road test with the telescope, while twelve states require special driver training for bioptic drivers.¹

One of the concerns with bioptics is the limited field of view due to the ring scotoma caused by the enlarged image of the telescope. Another concern is the inattention blindness that comes from performing two tasks at once (similar to using a cell phone, changing the radio station).

The accident rate of bioptic drivers in some states have been studied and reveal a slightly higher accident rate: 2.2x in California, 1.34x in Texas, 1.2x in Illinois. To put this into perspective, the accident rate for 16 year old drivers is 18x.¹

Audible GPS devices are a good alternative, due to their ready availability and affordability, compared to a bioptic telescope. A GPS device will afford the driver more attention on the road as well as relay information regarding traffic around them, while reducing the need to read street signs.¹⁷

Quality of life

Because driving is the primary and preferred mode of travel for most Americans, being a driver has a large impact on health and well-being. Cessation of driving can lead to a number of adverse outcomes including decreased health-related quality of life, increased likelihood of depression and social isolation, reduced access to healthcare services, and increased likelihood of placement in long-term care.¹¹

Driving alternatives

Suggesting alternative transportation should always be a part of the conversation if discussion driving cessation with a patient. Some current options are using family and friends, taxi services, ride-share applications such as Uber or Lyft, shuttle buses, and public transportation. Local agencies for aging and rehabilitation are great resources for alternative transportation options, so consider tapping into these resources.

Driving legalities

As doctors, we have a duty to warn, which states that “failure to warn a patient of conditions that create a risk of injury will be upheld as a cause of action against eyecare providers when it can be shown that the failure to warn is the proximate cause of an injury.” This basically states that if a patient with a visual impairment has a motor vehicle accident related to their vision, the eyecare provider can be held responsible for the accident if the patient was not adequately educated regarding their risks.

We have a responsibility to warn patients if their vision is no longer legal or safe to operate a motor vehicle, recommend they abstain from driving, and record such in the patient records.

Clinical responsibilities

Regardless of the patient’s level of vision, it would be prudent to ask if they are still driving. The answer to this question may be surprising. Ask the conditions that they may be driving: daytime, nighttime, near home, highways, etc. Watch the patient as they navigate in the room; if they are struggling to find the exam chair or go through the doorway, this may also cause a limitation in operating a motor vehicle. Consider their cognition and mental capacity as case history is done and the exam is performed.

If the patient has visual or cognitive limitations that may affect their ability to safely operate a motor vehicle, it is our professional responsibility, and legal duty, to share that information with the patient. The American Medical Association (AMA) Physician’s Guide to Assessing and Counseling of Older Driver’s recommends evaluation of risk factors for drivers over 65 including vision, cognition and motor skills. If there are concerns, a referral to the state DMV/DOT for a formal driving assessment is recommended.

Conclusion

A person’s ability to drive is not dependent on age, VA or VF alone. We, as eyecare providers, are at the forefront of determining who is fit and not fit to drive. We are responsible to inform our patients if their vision is below the state’s legal standards, but also understand the repercussions of driving cessation. We should advocate for those who have reduced VA or VF but have the compensatory skills to drive safely. Finally, we as a profession should strive to advocate for a national standard for vision requirements for safe operation of a motor vehicle.

References

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Federal Highway Administration. Manual on Uniform Traffic Control Devices. 2003 Edition, Revision 1. Washington DC: US Department of Transportation; 2003.
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Owsley C, Stalvey BT, Wells J, Sloane ME, McGwin G Jr. Visual risk factors for crash involvement in older drivers with cataract. Arch Ophthalmol. 2001 Jun;119(6):881-7. doi: 10.1001/archopht.119.6.881. PMID: 11405840.
Owsley C, McGwin G Jr, Sloane M, Wells J, Stalvey BT, Gauthreaux S. Impact of cataract surgery on motor vehicle crash involvement by older adults. JAMA. 2002 Aug 21;288(7):841-9. doi: 10.1001/jama.288.7.841. PMID: 12186601.
Wood JM, Owens DA. Standard measures of visual acuity do not predict drivers' recognition performance under day or night conditions. Optom Vis Sci. 2005 Aug;82(8):698-705. doi: 10.1097/01.opx.0000175562.27101.51. PMID: 16127335.
National Highway Traffic Safety Administration. Addressing the safety issues related to younger and older drivers—a report to Congress. Washington DC; US Department of Transportation; 1993.
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