Although in many states, visual acuity and visual field are the only screening tests performed when obtaining a driver’s license, there are many functional visual skills for driving that are important. Research evaluating driving often uses physical measures of driving behavior (e.g. speed, braking, latency, scanning behavior, lane positioning). Research shows in particular that cognitive impairment elevates crash risk and impairs driving performance.
Physical Vision and Driving
Patients with low vision conditions, such as diabetic eye disease, macular degeneration, albinism, and many more, may qualify to drive using a bioptic telescope (see bioptic telescope). These drivers have decreased acuity and sometimes central blur, normal side vision and are able to use a telescope to help improve their spotting acuity to that of a normal vision. Most people do not know that the central area which is responsible for the clearest and best acuity only makes up about 5 degrees of total visual field. This is the reason these drivers, when trained, are able to safely operate a vehicle using their intact side vision (the other 95%) which helps the detect moving cars and remain visually oriented although they may test 20/60 or worst on the traditional acuity chart. See how to bioptic driving (http://www.biopticdrivingusa.com/how-bioptics-work/). Every state has its own regulations for bioptic drivers.
Reduction of contrast sensitivity and glare also can make driving harder. These symptoms are common with advancing cataracts which cause clouding of a once clear lens over time. Patients often complain about reduced vision at night or at dusk or overcast conditions. These symptoms can sometimes be managed by filter lenses, come read our previous blog about filtered lenses.
Patients may also develop larger areas of visual field loss from stroke or brain injury which have damaged parts of their brain responsible for vision. Often these neurological conditions result in additional functional difficulties that need to be addressed.
Visual Processing and Driving
People can have functional vision problems despite having 20/20 acuity and no field loss.
They may complain about falls, bumping into objects or people, difficulty locating objects, memory difficulties. Driving complaints may include difficulty parking, cars appearing to move too fast, difficulties judging stopping distance to cars when parking or driving, difficulty maintaining lane alignment, excess braking.
Driving is a task that requires spatial orientation (for awareness of how the car is aligned in the lane, how to park, and where adjacent moving cars are relative to the driver), timing (to help integrate with traffic and understand how long it takes to stop a vehicle after pressing on the brakes), gross motor (to operate a motor vehicle), and isolation of eye-head movements. Visual skills for driving include smooth scanning of moving vehicles or road hazards, eye jumps for checking mirrors, eye focusing which allows for clear vision when viewing between distance and near objects such as the speedometer, adequate scanning and localization of signs, signals, and road hazards.
Cognitive and Motor Skills
Cognitive abilities enable one to safely analyze hazardous road conditions, ignore distractions when presented with complex stimuli, and shift attention quickly are also other visual skills for driving that are important. Driving requires one to plan in order to navigate to a particular destination and anticipate paths of moving pedestrians and adjacent cars to avoid collision. Physically, drivers need to be able to freely move their head and neck and have adequate motor coordination and timing of hands and feet to operate the vehicle efficiently.
Functional vision problems can be associated with stroke, brain injury, side effect of medications, decreased cognitive ability, health problems, or progressive age.
Assessing driving competency for those affected by functional vision problems requires a team of professionals which may include optometrists, occupational therapists, and driving instructors.