What Does the Doctor Think for July?
How not to crash at a road junction!
Did you ever “just not see it?” It must have happened to you at some time or another. When driving, you pull up at a junction, look right and left, then right again and pull out onto the main road, only to hear a blast on a horn as a car flashes in front of you. I expect you could not understand how you missed seeing the car and were thoroughly rattled by the experience. Well, don't feel too bad about it because, for small periods of time, we are unable to see anything at all.
Our forward vision covers roughly 220 degrees, of which 10 to 15 degrees is central vision and the remaining 205 – 210 degrees is peripheral vision. When we look directly at something, we see it with our central vision, which is very high resolution and is the image picked up from light rays that fall on the fovea, a small part of the retina. The large area of peripheral vision, formed from the light rays hitting the remainder of the retina, is only 10% as sharp as the central vision.
HOMEWORK: Look at a car number plate from10 metres away - the number is easy to read. Now swivel your eyes slightly sideways to look at the rear light of the car and try to read the number plate with your peripheral vision. I'll bet you cannot read it – this demonstrates what a narrow band of light rays generates the sharp image (central vision). The peripheral vision is used by the brain to complete the picture. Whilst your peripheral vision might make you aware of a bus or a lorry approaching you, you might miss a cyclist.
When we move our eyes from right to left to scan what is in front of us, it is not a smooth movement. To prevent blurring, the brain moves our eyes in a series of jumps (saccades) with very short pauses in between (fixations). It is only during the fixation phase that the brain processes the image. During the saccade, while the eyes are moving, the brain actually blocks the image. The brain fills in the picture using peripheral vision and an assumption that what is in the gaps must be the same as what is seen during the pauses. Hence, the possibility of missing something. Unless the eyes are tracking a moving object, the human eyes are incapable of moving smoothly across a scene. They move in jumps and pauses, taking snapshots during the pauses.
DRIVING: So, approaching a junction and looking from side to side, it is possible for our eyes to jump over an oncoming vehicle, especially if it is small or narrow, during a saccade. The faster you move your eyes from side to side, the longer the saccades and the shorter the fixations, so it is easier to miss things and you will detect less in your peripheral vision. When sailing, if another vessel is approaching, we line it up with our eyes against a stanchion on the side rail of the boat; if the other vessel does not change its position relative to the stanchion, we are on a collision course and action must be taken. Similarly, any vehicle on a direct collision course with our car will have no relative movement against our car and our peripheral vision will not be alerted. And, of course, the door pillar blocks out some of your field of vision.
WHAT TO DO: When approaching a junction:
a)Slow down. The alteration in speed will reveal to the peripheral vision anything which was previously on a collision course and producing no relative movement to alert the peripheral vision.
b)Look right and left, slowly, methodically focussing on three points to the right and three to the left, searching close, middle and far-distance. DO THIS TWICE, actually LOOKING with your central vision. By doing this, you are over-riding the natural limitations of the eye and the brain. Fighter pilots call this a “Lookout Scan”. It seems a slow process but it becomes more efficient with practice.
c)Lean forward to look around the windscreen pillars.
REMEMBER, JUST 10 DEGREES AWAY FROM YOUR CENTRAL SIGHT LINE**, YOUR VISUAL ACUITY IS ONLY** ONE TENTH AS GOOD! Peripheral vision (only 10% as sharp as central vision) is useful but is not detailed enough to rely upon when driving.
I am grateful to RAF pilot, John Sullivan, for his help with this article.
I was so upset by reading about the adverse effects of alcohol that I have had to give up reading!
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A charity called and asked me to donate some of my clothes for the starving people. I was happy to help but I am sure that no-one who fits into my clothes could be starving.
Tommy Cooper: 2 blondes walk into a building – You'd think one of them would have seen it! Police arrested two kids yesterday.One was drinking battery acid and the other was eating fireworks. They charged one and let the other one off. A man took his large cross-eyed rotweiler to the vet, who picked up the dog to examine him. After a moment, he said “I'm going to have to put him down” “Why?” asked the owner with tears in his eyes. “Because he's really, really heavy and my arms are sore”.
Best wishes to you all Ian Nisbet