The operation of vehicles is prohibited if:

Explanation

If the rear fog lamps are wired into the brake light circuit.

The rear fog lamp is the brightest red light on a vehicle and is deliberately several times brighter than an ordinary tail light so that it can be seen from a distance in thick fog. For exactly that reason it must have an independent switch and be lit only in very poor visibility. If it is wired into the brake light circuit, every press of the pedal sends an intense glare into the eyes of the driver behind.

Understand the safety reason through the mechanics of a collision: the driver behind you must, at the moment of braking, judge the distance and the rate of closure. Sudden glare destroys the eye's sensitivity for a few moments, and those few moments swallow the distance needed to react. The result is a rear-end collision in circumstances the light was meant to prevent.

The coolant temperature gauge is a different story. Its failure is bad for the engine — you may notice overheating too late — but it has no effect on the vehicle's behaviour in traffic or on the information you give others, and it is not on the list of prohibiting defects.

Build the practical test of this chapter here: ask yourself whether the fault impairs "control of the vehicle" or "informing others", or whether it relates only to mechanical health and comfort. The first category is prohibiting, the second is not.

• A faulty coolant temperature gauge is a problem for the engine, not for traffic safety; it has no place on the list of prohibiting defects. • "In both cases" is correct only when both items are prohibiting defects; the temperature gauge is not, so this option is ruled out.

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