Vanishing Motorcyclists

I was sitting as a passenger in a car which was driving along a curved road. I lazily noticed there were two motorcyclists riding about 50 meters ahead us. At times they were visible, at times they vanished behind the forest besides the curving road. I was not giving too much attention at them, but unconsciously I had been thinking they would proceed in an even pace, so I would see them again after the next curve. But suddenly, they were not there, they had vanished. Maybe they just speeded up, or then they really vanished in the thin air.