The effect of speed on vision when driving

Visual acuity

One of the main effects of speed is the decrease of objects in the foreground, which become confusing due to the movement and vibration of the car. Research has shown that this sub-standard close-up viewing range increases by about 20 feet for every 10 mph increase in speed. Therefore, at 20 miles per hour we cannot see details clearly within 40 feet in front of the car; at 60 miles per hour, not at 120 feet or more.

Imagine a driver with 20/20 visual acuity driving 60 miles per hour. There is a road sign ahead with three lines written on it. This driver does not know the area, his memory cannot be of any help, so he must read the sign. We’ve seen before that this controller can read 5-inch letters from 280 feet. We now know that when your car reaches a point 120 feet from the sign, you cannot see details clearly. You only have 160 feet, or 1.8 seconds, to read the sign.

A driver with 20/60 vision cannot see the same signal before reaching a point 93 feet away, but his near vision is affected in the same way as that of the first driver. Will this controller be able to read a three line message? The problem of reduced near vision explains why some drivers make emergency stops or slow down abruptly at road junctions, especially at superhighway exits. You should always be aware of this danger and be prepared for any sudden action by other drivers. If you know where you are driving, a glance at the sign will suffice, but remember that strangers who don’t know the way will need longer periods of time.

Highway departments, having studied this problem, use symbols as much as possible or keep the written message as short as possible. Superhighways always have large, easy-to-read signs.

Visual field

His field of vision is reduced by concentration and speed. When concentrating on a single object, the field of view is zero, which is generally the case with new drivers, because they tend to stare at what they think is important. The field of view also reduces with speed. When stopped, a driver’s field of view can be up to 190 degrees, but for the same person, the angle will drop to 40 degrees at 60 miles per hour. That is why, whenever possible, road signs are placed above the roadway.

Judgment distance

Good distance judgment depends on good visual acuity. Since acuity decreases with speed, judgment of distance will also be adversely affected when driving fast.

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