On a frosty morning a driver clears only a hand-sized patch of the windscreen and drives off. Why is this unsafe and likely unlawful?

Why B is correct

Driving through a slot turns every hazard outside it into something that arrives without warning: the pedestrian stepping out on your left, the cyclist hugging the kerb, the car emerging from a side road. Vision also needs context, and staring through a tunnel strips the peripheral motion that tells you something is approaching before you have identified what it is. Vehicles are equipped to deal with frost precisely because this is predictable: demisters, wipers, washer fluid and a scraper cost nothing except about two minutes, and the rule existed long before heated screens were invented.

Why the other options are wrong

A. It is acceptable provided the side windows are clear

Clear side windows help with lane changes and do nothing about what is straight ahead. Forward vision through the windscreen is the requirement being breached.

C. It is acceptable as long as the driver keeps their head aligned with the clear area

Aligning your head with the clear patch makes the peripheral loss permanent rather than brief. Hazards do not politely arrive through the slot you happened to scrape.

D. It is acceptable below 25 mph

There is no speed at which limited visibility becomes lawful. At twenty-five the same pedestrian crosses the same road, and the driver has no more chance of seeing them.

On the road

Two minutes with a scraper, or twenty metres with the screen still frosted. The driver choosing the second meets a pedestrian stepping off the kerb on the near side, exactly where the frost still is. Every winter morning offers this choice, and almost nobody taking the second option thinks of it as a gamble. The stake belongs to whoever is crossing.

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