Functional assessment involves assessment of a patient's visual, physical, and cognitive abilities. (See the National Highway Traffic Safety Administration web site for an Assessment of Driving-Related Skills [ADReS].) Adequate function in these areas is required to drive safely. Much of the assessment can be done by primary health care practitioners, but specialists (eg, ophthalmologists, occupational and physical therapists, driving rehabilitation specialists) may need to be consulted. Identified deficits may require interventions (see The Older Driver: Interventions with the Older Driver), including driving rehabilitation, assistive devices, driving cessation, and reporting to the state Department of Motor Vehicles. Some complicated cases may be referred to state medical advisory boards.

Visual function

Visual function is vital to safe driving. Age-related and pathologic changes in vision are common and can contribute to driving impairment. Changes with aging include

• Decreased retinal illuminance (amount of light reaching the retina), visual acuity, and peripheral vision
• Presbyopia (decreased ability to accommodate), which impairs depth perception
• Decreased ability to adapt to changes in light and heightened sensitivity to glare, which impair night driving

In many states, central visual acuity and peripheral vision are routinely tested by the Department of Motor Vehicles when a license is renewed. Most states require 20/40 visual acuity in at least one eye for unrestricted licensing (glasses or contacts are allowed). However, in some states, practitioners can extend the requirement pending medical justification. Additionally, some states have approved use of bioptics (a lens system with a telescope attached to a pair of glasses) for people with severely reduced vision. For horizontal peripheral vision, safe driving thresholds vary widely among states from no requirement to about 140°.

Tests of useful field of view (spatial area from which visual stimuli can be acquired during a single fixed glance) provide integrated measures of visual performance (eg, visual-processing speeds, visual-spatial attention, visual memory) and can be used to predict a higher risk of crash. For example, the incidence of crashes is 6 times higher when useful field of view is reduced by > 40%. However, field-of-view tests are not yet widely available.

Older drivers often require referral to an ophthalmologist for comprehensive testing.

Physical function

Various parameters of physical function can be assessed in the office.

• Motor speed, balance, and coordination can be assessed with the rapid-pace walk test. The patient is asked to walk a 3-m (10-ft) path, turn around, and walk back to the starting point as quickly as possible. If the patient normally walks with a walker or cane, it should be used during the test. A time of > 9 sec may indicate an increased risk of a motor vehicle crash.
• Range of motion should be tested in the cervical region and in all joints of the upper and lower extremities. Decreased cervical range of motion impairs ability to turn the head and scan for traffic (particularly in the blind spot). Decreased range of motion in the extremities impairs ability to operate car controls.
• Strength in upper and lower extremities should be assessed qualitatively (in terms of meeting the needs of driving a vehicle). Grip strength should be measured with a dynamometer. A grip strength of < 16 kg for men or < 14 kg for women may reflect decreased ability to manipulate the steering wheel.
• Proprioception and peripheral sensory function should be tested. Decreased peripheral sensation can impair ability to modulate pressure on foot controls.

Physical and occupational therapists who specialize in driving rehabilitation can provide comprehensive testing of physical function related to driving ability. A rehabilitation driving assessment sometimes also involves the specialist going out in a car with the patient to evaluate actual driving skills. The car used during the evaluation should be equipped with features that allow the specialist to maintain safe control (eg, passenger-side brake). Driving rehabilitation specialists can be located by contacting local rehabilitation facilities or going to www.driver-ed.org. However, in most states, the cost of a rehabilitation driving assessment is not covered by insurance (Medicare or private).

Cognitive function

Cognition is moderately impaired in about 3% of community-dwelling people aged 65 to 74, 14% of those aged 75 to 84, and > 20% of those > 85. People with cognitive impairment often do not recognize their limitations and are at higher risk of crashes; risk increases with severity of impairment. Tests that may predict driving performance include the following:

• The Clock Drawing Test: This test assesses long-term memory, short-term memory, visual perception, visuospatial skills, selective attention, and executive skills.
• The Trail Making Test (Part B): This easily given test assesses divided attention and visual scanning and can be found at the National Highway Traffic Safety Administration web site. Drivers with an abnormal score on this test (eg, > 180 sec) may be candidates for more specialized testing by a driving rehabilitation specialist.
• The Mini-Mental State Examination: Examination of mental status (see Approach to the Neurologic Patient: Examination of Mental Status) screens for cognitive impairments.

Drivers with mild cognitive impairment should be considered for referral for more precise neuropsychologic testing.

Last full review/revision October 2009 by David B. Carr, MD; Peggy P. Barco, MS, BSW, OT/L