At a Certain Dam

My dad's hobby has always been riding motorcycles, and he apparently used to travel to various places on his bike before he got married. One day, he was out on a touring trip as usual. I can't say the exact location, but as he was passing by a certain dam and entering a tunnel, he heard another motorcycle sound from behind. However, there were no other motorcycles or even cars around; he was completely alone. As the motorcycle sound grew louder, my dad, though scared internally, sped up, determined not to be overtaken. But then, the sound suddenly vanished. He figured the bike must have passed him. The road he was on is known for being winding and having frequent accidents. It's likely the rider of that mysterious bike is no longer of this world.