Operation of buses is prohibited if:

Explanation

When white reflectors are fitted at the rear of the bus.

The colour rule for lighting devices and reflectors has a simple, exceptionless logic: white at the front of the vehicle, red at the rear. On that basis, fitting a white reflector at the rear is an offence and prohibits operation of the vehicle, whereas a red reflector at the rear is exactly what the standard requires.

The safety reason for this rule is orientation in darkness. Another driver who at night sees only two points of light ahead of him must work out in a fraction of a second whether he is facing the rear of a vehicle travelling the same way or the front of one coming towards him. Colour is the only information that makes this judgement possible: red means "you are behind it, it is moving in your direction", white means "it is facing you". If the rear of a bus reflects white, a driver approaching from behind misreads the distance and direction, and on an intercity road that error leads straight to a rear-end collision.

Apply the same rule to all similar questions: red at the front prohibited, white at the rear prohibited. The only recognised exceptions are the reversing lamp and the number-plate lamp, which are white; and if an option does not concern those, apply the rule without hesitation.

• A red reflector at the rear is the standard and required condition and creates no prohibition whatever. • Since a red reflector at the rear is entirely permitted, "in both cases" cannot be correct.

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