Explanation

"Always start the Mirrors - Signal - Manoeuvre (MSM) routine well before you intend to change lanes. On motorways, vehicles are travelling at much higher speeds, so they may come up behind you very quickly. By checking the mirrors early, you'll have a chance to plan and give any necessary signals in plenty of time, so that other drivers have advance warning of your intended actions. In turn, they'll have more time to expect a change in traffic pattern and be able to prepare and act accordingly."