Q: Why are the coloured lights in traffic signals universally arranged red over amber over green, as opposed to the universal practice of railway signals which have green over amber over red (for a three-aspect signal)?
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A: Gerald Dorey (28 January) is only partly correct in his historical explanation for the order of railway signal lights. Indeed, he overlooks the large parts of the country where lower quadrant semaphore signals (in which horizontal means danger and 45掳 down means clear) were used. In these signals the red light was therefore at the top.
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The main reason for the red light being at the bottom in modern British signalling installations is the weather. To ensure visibility in bright sunshine, each colour light has a long cowl or hood above it. In the winter, however, snow can build up on these cowls and obscure the light above. Being at the bottom, the most safety critical red light has no other light below it and therefore no cowl, so snow cannot build up to cover a red light.
A: There are two kinds of mechanical, or semaphore, signals. In the older lower quadrant type, the arm slopes downwards from the pivot to show clear or green, is returned to the horizontal by a counterweight, and the lamp glasses are red above green. In the newer upper quadrant type, the arm slopes up for clear, returns by its own weight (as in the scene in the classic film The Lady Killers) and the lamp glasses are in fact side-by-side. Red is nearer the pivot and green is to its right on the outside.
In both, the horizontal arm means stop but this is not synonymous with down which means opposite things in the two cases. Red arms are always used in stop signals but distants (meaning warning) operate similarly. However, on distants the arm and lamp glass are yellow, not red, and these mean pass with caution.
The arrangement of multiple-aspect, coloured light signals has nothing to do with that of arm position. The red is at the bottom simply because it is the position nearest the driver鈥檚 eyes; yellow is above, then green and, in four-aspect signals, the second yellow is top-most, above the green.
A: Road users do not have to pass a colour vision test, and therefore the position of the red, amber and green lights must always be the same, so the light illuminated can be recognised by position as well as colour. Such signals are usually placed at sites where a speed limit applies, and, because of the higher braking coefficient of rubber tyres, the driver can still stop safely even after identifying a red indication only by its position.
The train driver, whose colour vision is checked regularly, has to act on signals at a far greater distance to ensure the train can be stopped in time. On main lines the indication has to be identified accurately at long range, when it is impossible for the driver to see its position and has to rely solely on its colour.
The original question was, in fact, incorrect, because there is no universal railway layout of green over yellow over red (in railway parlance, the caution indication is referred to as yellow, not amber). In the past, some signals only had a single lens the different indications being given by interposing coloured filters over the beam. The only fixed rule with the layout of signals with multiple lenses is that the one which exhibits the red indication is mounted nearest to the line of the driver鈥檚 eyes. In some places, therefore, it may be at the top, as it is with a road traffic signal.
On high-speed lines, it is necessary to have a double-yellow. indication, which is exhibited by the signal before the one showing a single yellow for caution. That, in turn, will be a further three-quarters of a mile or so before the one showing red for stop. This gives a run of two signals warning of a stop signal ahead. Such double-yellow signals normally have the two yellow lights separated by the green one, to maximise their visual separation when viewed from a distance.