When a driver notices something that may be important to their driving such as a road sign, hazard, or change in the traffic flow, central vision is used to discern detail, identify, and/or recognize what the driver is looking at.

DMV uses Snellen wall charts to screen driver’s license applicants for a far visual acuity of no worse than 20/40 in Snellen notation. Impaired visual acuity makes it harder to discriminate the fine differences that distinguish one letter from another. Passing this test means that the applicant can read the letters at 20 feet or more that are large enough for a young and healthy observer to read at 40 feet.

Visual acuity impaired by one or more vision conditions can result in the driver failing to read signs and/or recognize hazards in a timely manner.

Normally when you look at something, you center the visual image in the central portion of your point of view. Peripheral or side vision is the field of view that surrounds the central portion of the vision field.

In driving, peripheral vision is used in part to detect information that may be important for safe driving. This kind of information includes road signs, appearances of hazards, and changes in the flow of traffic. When a healthy driver notices something important, head and eye movements are used to move the visual image into the central portion of the visual field. In other words, the driver moves their head and eyes to look at the object or event of interest.

Peripheral vision is also used in controlling the vehicle. When the driver looks in the rear view mirror, peripheral vision is used to monitor traffic in front of the vehicle. In keeping the vehicle centered in the lane, peripheral vision is used to monitor the lane boundaries.

Peripheral vision impaired by one or more vision conditions can result in the driver failing to react to a hazard coming from the driver’s far left or far right, failing to heed a stop light suspended over an intersection, weaving while negotiating a curve, and/or driving too close to parked cars.

A driver’s vision can be good in the daylight, but very different at night. Driving safely at night requires seeing well not only under low light, but also requires drivers to see low contrast objects. Someone wearing dark clothes and crossing the street in front of the driver is much harder to detect at night than during the day because there is much less contrast at night between darkly clothed pedestrians and a dark background.

Night vision impaired by one or more vision conditions can result in the driver failing to react to hazards located directly in front of the vehicle, tailgating, and/or failing to steer when necessary because the driver is unable to see low contrast features of the roadway such as its edges and irregularities in the road surface.

Glare is the disruption of vision due to a bright light (such as the light from the headlights of oncoming traffic at night) so that you can’t see very well what is in front of you (such as the outline of the car ahead of you).

Glare resistance is the extent to which the driver can still see critical objects and events while facing a steady source of glare such as the setting sun or the light from the headlights on a steady stream of oncoming traffic at night.

Glare recovery is how fast the driver’s vision function returns to what it was before the glare was encountered.

Glare resistance/glare recovery impaired by one or more vision conditions can result in the driver being blinded by a glare source and consequently missing curves in the road, striking unobserved pedestrians, and/or crashing into the rear of slow-moving, stalled, or stopped vehicles.

Judgment of distance impaired by one or more vision conditions can result in the driver stopping too short of the limit line or inside the intersection, turning too wide or too short, and/or failing to maintain speed and/or following distance appropriate for prevailing driving conditions.

Eye movements impaired by one or more vision conditions can result in visual scanning deficiencies. There may be uncontrolled up and down or random scanning rather than side-to-side scanning. There could also be a tendency to look at a specific object too long or continuously look straight ahead. Consequently, the driver may fail to react to hazards and fail to heed traffic signs and signals. Changing lanes could be especially hazardous if the driver spends an excessive amount of time looking to the rear of the vehicle.

Visual perception is how your brain processes what you see in front of you. Visual perception impaired by one or more vision conditions can result in difficulties with performing several visual tasks at the same time. The driver may have impaired ability to:

• Switch attention to important events without blurred or clustered vision.
• Distinguish foreground from background.
• Determine the position of other vehicles, signs, and pedestrians relative to themselves and to each other.

Consequently, the driver may brake and/or stop unexpectedly, maintain inordinately long following distance (to keep from having to react quickly), fail to react to hazards, and/or fail to heed traffic signs and signals.