MILES AND FEET

Evening Post, Volume CXXXII, Issue 2, 2 July 1941, Page 11

MILES AND FEET

SOME NEW ANGLES

SPEED AND VISIBILITY

When members of the Wellington branch of the New Zealand Institution of Engineers met last week to listen to an address by Mr. Justice Blair they had no inkling of the subject but none could have been more appropriate than that chosen, "Some Aspects of Speed and Visibility."

"The speedometer of a car is marked in miles per hour, but what does this convey to the driver?" said Mr. Justice Blair. "All it says is that if the driver continues for an hour, he will have covered a certain distance in miles; but ask him how long it will take him to cross a road intersection, and he is usually at a loss. It is no use approaching the problem from the speedometer angle of miles, because the road intersection is only a measure of feet. What he really needs to know is the speed he is travelling in feet per second.

"Few individuals can visualise a distance of a mile, or, indeed, a much shorter distance; but the majority know that an average street is 66 feet wide. What is of great importance to the motorist is to know how many seconds he will take to cover a distance, or some other distance within the range of his visibility. A motorist approaches an intersection at 30 miles per hour, and a child steps out on the other side to cross the road. If the driver continues at the same speed, he will cover 44 feet in the next second, while the child, moving at 4 miles per hour, will have covered about 6 feet. If it runs, it will cover about 12 feet. In one second and a half, ifi the child walks, the car and the child arrive at the same point in the road. "If it is contended that the thing, for the motorist to do is to apply his brakes, then he is faced with another fundamental fact—there is a time delay between the moment he realises the danger and the moment his consequent actions cause the brake to function. A very good driver has a time delay of half a second; the average driver's time delay is probably nearer a full second. He may, therefore, have travelled another 44 feet, and it is unlikely that, under the best conditions, he will travel less than 20 feet." Here was an aspect of speed which the speaker developed and he contended that a much more sensible method of marking a speedometer was in feet per second. LIGHT AND DARKNESS. Dealing with visibility, Mr. Justice Blair made many interesting references to vision and the actions and reactions of the eyes. He dealt particularly with the iris. The iris governed the quantity of light admitted to the retina. When light was abundant the iris closed; when light faded it opened. The speed of closing was greater than the speed of opening. If one emerged from the dark into light, the measure of closing could be indicated by the figure one; but on proceeding from i light into dark, the comparative figure; was nearer ten. j

The motorist, therefore, who passed in daylight along a road in which tiie intensity of light varied should realise that when he passed from sunlight into shade his vision was temporarily impaired, and following that, if he proceeded at his previous even speed, he was for a short while suffering under a physical disability which might result in accident.

If he was pounding along at 60 miles an hour, he was covering 88 feet per second, and 44 feet in half a second, and much might happen in a distance of 44 feet. If he was driving at night through city lights he passed from the bright patch of the street light to the comparatively dark patch of the space between lights. His iris was continually performing a quick close down and a slow opening when he had passed the light. He would be well advised to so limit his speed in feet per second as would afford him ample time to control his car in an emergency, having regard to the cumulative delay made up by the lag between mind and muscle and by the lag in the opening of the iris.

Mr. Justice Blair's address was listened to with great interest by several of his brother Judges, and by a large audience of engineers, who expressed their appreciation of his address and his interest in the work they are doing.