Why do they do it?
1) You are in the outside lane of a two lane dual carriageway. You pass a sign to say that the outside lane will close in 800 yards. You are closing up on a single vehicle in the inside lane, at 600 yards you are almost level with their rear quarter, and then they accelerate to stop you passing. Why?
2) You are on a motorway. it could be the M20 near Maidstone but it could be anywhere. It's after 11pm at night. It's dark. There is only one car on your carriageway ahead of you. You have your dipped beam on. As you overtake the car it puts its full headlamps on and then speeds up to pace you for a mile or so before dropping back. Why?