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)?
A: The difference between road and rail usage derives from the history of railways and the primacy of safety. The old mechanical railway signalling arms were designed so that failure, which would be in the 鈥渄own鈥� position, meant stop. The illuminated part of the signal consisted of two coloured glass panels in the far end of the signal arm, beyond the pivot, which moved in front of a fixed lantern. Even though the higher of the two glass panels was the red panel, it showed when the signal was down and this meant stop. While railways retained mixed mechanical and electrical signalling the signals had to be compatible. Therefore, the new electrical signals showed red at the bottom so that train drivers always equated either signal in its down or bottom position with the order to stop.
Road signals had no mechanical forerunner and are designed so the most important light, the red, can be seen from the greatest distance. This means putting it as high as possible. Anyone who has used Junction 3 of the northbound M25 at night will appreciate this. Additionally, visibility for railway signals is not the same issue it is on the roads. Railway signal sites are carefully selected.
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