Reaons for signalling when moving into a slower lane, include:
- the vehicle you've just overtaken might be contemplating speeding up
- a vehicle might be coming up a sliproad that you haven't noticed because of the lorries you've just overtaken
- a vehicle coming up behind you might be able to avoid a risky manouvre
- black BMWs that are 'undertaking' the vehicle you've just overtaken (particulaly on the A12 in East London)