Transport For London's brilliant "Do the Test" cycling safety video invites you to pay close attention to a video of some basketball players, then demonstrates just how little you really saw, ending with a voice-over that explains how easy it is to miss things you're not looking for, like cyclists:
This phenomenon is known as "change blindness" - only a tiny fraction of all the information going into your brain enters your consciousness. People often fail to see a change in their surroundings because their attention is elsewhere.Link (Thanks, Mr Jalopy!)
Even stranger, if you are concentrating on something, you can become blind to other events that you would normally notice. This "inattention blindness" is possibly the reason why motorists collide with cyclists.
Just as it is important for road users to keep an eye out for cyclists, cyclists must also take steps to ensure they are seen by motorists.