If someone behaves courteously towards you, they have demonstrated their awareness of your existence and intended actions. If they're wrong about your intended actions, you can indicate (electrically, facially or handwise) what you wish to do.I take the view that courtesy has no relevance to road safety.
This is particularly noticeable on High St., Morley, where there is often parking on both sides. People courteously flashing you thro' make your decision making safer