Traffic on motorways usually travels faster than on other roads, so you have less time to react. It is especially important to use your mirrors earlier and look much further ahead than you would on other roads.
Advice, not law. This rule is not written in capitals, so breaking it is not an offence in itself — but it can still be used in evidence in court to establish liability. The rules that say MUST or MUST NOT are the ones backed by law.
5 theory test questions on this rule
From the DVSA revision question bank. These are not the questions asked in the real theory test. Each one shows the answer, the explanation, and our own note on why every other option is not it.