One of the primary effects of speed is to decrease for objects in the foreground, which are rendered unclear because of the motion and vibration of the car. Investigation has shown that this range of sub standard foreground vision increases by approximately 20 feet for each increase of 10 miles per hour in speed. Thus at 20 miles per hour we cannot see details clearly within 40 feet ahead of the car; at 60 miles per hour, not within 120 feet or more.
Let us imagine a driver with 20/20 visual acuity driving at 60 miles per hour. There is a highway sign ahead with three lines of writing on it. This driver does not know the area – his memory cannot be of any help – so he must read the sign. We have seen earlier that this driver can read 5 inch letters from 280 feet. We now know that when his car reaches a point 120 feet from the sign, he cannot clearly see the details. He has only 160 feet, or 1.8 seconds, to read the sign.
A driver with 20/60 vision cannot see that same sign before he reaches a point 93 feet from it – but his close up vision is affected the same as the first driver’s. Will this driver be able to read a three line message? The problem of reduced close up vision explains why some drivers make emergency stops or slow down abruptly at road junctions, especially at super highway exits. You must always be aware of this danger and be ready for any sudden action by other drivers. If you know the area you are driving in, a short glance at the sign will be sufficient – but remember that strangers who do not know their way will need longer periods of time.
Road departments, having studied this problem, use symbols as much as possible, or make the written message as brief as they can. Super highways always have signs with large and easily read letters.
Field of Vision
Your field of vision is reduced by concentration and speed. When you concentrate on one single object, the field of vision is nil – which is generally what happens with new drivers, because they tend to stare at what they believe to be important. Field of vision is also narrowed by speed. When stopped, a driver’s field of vision may be as high as 190 degrees, but for the same person, the angle will be narrowed to 40 degrees at 60 miles per hour. That is why, whenever possible, highway signs are positioned above the roadway.
Good distance judgment is dependent on good visual acuity. Since acuity is reduced by speed, distance judgment will also be adversely affected when you are driving fast.
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