The operation of vehicles is prohibited if:

Explanation

The external lighting devices and reflectors being dirty.

Dirt on a lamp or reflector looks at first like a cosmetic matter, but its optical effect is severe. A layer of mud and winter road salt can absorb a large part of a lamp's light output; that is, a lamp that is sound and does light up will in practice illuminate the road tens of metres less far. That is why cleaning the lamps is part of the pre-drive inspection, not a seasonal chore.

With reflectors the case is worse: a reflector produces no light of its own and its only job is to return the light of another vehicle's headlamps. A mud-covered reflector effectively does not exist, and a vehicle stopped on the verge becomes invisible at night. A collision with an unmarked stationary vehicle is one of the deadliest night-time crashes on intercity roads.

By contrast, fitting the external lamps of another make of vehicle to a model that is out of production is not an offence but a recognised exception to the rule. The logic is simple: original parts are no longer made for an old vehicle, and without this permission the driver would be forced to drive with a broken lamp or none at all, which is far more dangerous.

Worth memorising: in this family of questions, any option that places a "permitted exception" beside a genuine defect pulls you towards the "in both cases" answer. Weigh each option separately.

• This condition is an express permission of the law, not a defect: the external lamps of another make may be fitted to a vehicle that is out of production. • "In both cases" is correct only when both conditions are defects; here the second is permitted, which invalidates that option.

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