A white rectangular sign bearing black lettering is posted at the roadside. What is this sign doing?

Why D is correct

Colour carries the category before a single word is read, and black on white means regulation. These are the signs that impose a legal duty or prohibition rather than merely advising: the speed limit, no turn on red, one way, do not enter, keep right. Warning signs use yellow instead precisely so that a driver can tell at a glance whether the sign ahead describes a hazard or states a rule. Mixing the two categories up has consequences, because ignoring one is careless driving while ignoring the other is a defined offence. Treat any black-on-white sign as an instruction rather than as advice, because that is exactly what it is.

Why the other options are wrong

A. Warning of a hazard ahead that calls for caution

Hazard warnings use yellow backgrounds, not white, and that split is entirely deliberate. Yellow prepares a driver for something unusual ahead, whereas white states the terms on which they may use this road at all. Treating the second merely as cautious advice means disregarding a lawful direction.

B. Showing destinations, distances and directions

Destination guidance uses green, or blue for motorist services. Guidance signs tell you where places are; regulatory signs tell you what you may and may not do.

C. Marking a temporary detour around construction

Temporary diversion signage uses orange so drivers can separate it from permanent control devices. Nothing temporary is ever printed black on white, precisely so that a driver can distinguish something standing for a fortnight from something standing for thirty years. White signals permanence rather than a detour.

On the road

A driver sails past a black-on-white no-turn-on-red sign, having mentally filed it under suggestions. Yellow would have meant use caution; white meant do not do this. The difference arrives in the post, along with the insurance excess, usually together. Two families of similar-looking signs carry quite different weight, and conflating them is one of the commonest quiet errors on the road.

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