With age, many functions that affect driving ability may deteriorate, including muscle strength, reaction time, mobility, vision, and cognition (see TABLE 112-1).

Muscle Strength and Reaction Time
Decreased muscle strength, particularly decreased grip strength, can pose a problem. Dynamometric values of < 35 lb in the dominant hand should raise concern.
An increased reaction time also is a concern. Reaction time, which increases with the difficulty or number of choices, should be evaluated clinically. No established test exists.
Mobility
Formal evaluation of range of motion by an occupational or physical therapist using a goniometer is usually not required except in unusual circumstances. The neck is often a concern in patients with debilitating rheumatologic conditions because limited mobility may restrict the field of view, especially in critical traffic situations. Restricted mobility of the shoulder, wrist, or elbow can affect the ability to steer, though
power steering and adaptive devices often can compensate adequately. In many metropolitan areas, body shops can equip vehicles with these devices, but they may be expensive.
Although proprioception is important, measurement is often crude. It should be assessed clinically, particularly in patients with subacute combined degeneration caused by vitamin B12 deficiency. Patients with obvious deficits may warrant road testing or other focused evaluations.
Vision    /
Several age-related changes in vision can affect driving. Central visual acuity frequently declines because of physiologic or anatomic changes, such as increasing opacification of the lens, or medical conditions, such as diabetic retinopathy. Peripheral vision also declines with age, mostly through the same mechanisms that affect central vision.

The total horizontal peripheral visual field typically declines from 170° in a young adult to 140° by age 50. Because the peripheral retina is less sensitive to low levels of light, twilight can be the most difficult time for driving. Drivers with peripheral vision deficits have twice as many collisions as those with normal vision. Other age-related functional deficits include poor visual adaptation to light changes, increased sensitivity to glare, declining visual accommodation (ie, presbyopia), and diminishing depth perception.
In most states, central visual acuity and peripheral vision are routinely evaluated at the department of motor vehicles. The frequency of evaluations and the minimum acceptable, corrected, central visual acuity depend on state regulations. The most common requirements are visual acuity of 20/40 in the better eye and horizontal peripheral vision of 120°. Medical or surgical therapy, such as cataract surgery, may be helpful.
Cognition
About 3% of community-dwelling elders between ages 65 and 74, 14% between 75 and 84, and > 20% over 85 have moderate degrees of cognitive impairment. Those with such impairment may not fully recognize their limitations, and elderly drivers with mild to moderate dementia have a fivefold greater risk of collisions. To evaluate cognitive impairment, the physician can use the Mini-Mental State Examination. Persons who score less than 23 should probably stop driving pending further investigation (see Ch. 89). About 5% to 10% of demented outpatients have reversible components to their dementia.
Although patients with severe dementia should not drive, most older drivers with illness-related dementia do not have severe cognitive impairment. In these patients, attention deficits may play a role in motor vehicle collisions. The three major types of attention are selective, divided, and sustained. Selective attention, the ability to shift focus between competing stimuli, is evaluated using several neuropsychologic tests, such as the dichqtic listening test and the Stroop test. Selective attention is important in driving, where stimuli, such as a radio or a cellular phone, interfere with driving tasks. Divided attention, the ability to process two or more stimuli at once and make an appropriate response, is important when approaching intersections or merging onto freeways. Finally, sustained attention, an endurance in alertness, may be relevant to a driver with a chronic medical condition