Why is there a warning ‘Reduce speed now’?

Explanation: You'll see a sign warning you to slow down when you're approaching a part of the road where you need to be especially careful. For example, as you approach a roundabout, a crossroads or a bend.
If you're travelling on a major road and coming up to a junction with a minor road, you should slow down - even if you don't want to turn. This is because another driver at the junction may not be able to see clearly and they could pull out in front of you.
Check your speed as you approach a junction or a bend and be prepared to slow down or stop.

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References: The Official DVSA Guide to Driving: Section 8, Highway Code: Rule 127