Late at night, a dark road (and if they're anything like the back roads around here it was DARK), he's riding all over the road, and you're staying back to make sure you don't hit him. Many different things could have happened, without resorting to alternate universes bleeding over into this one. Perhaps he swerved to the left while you moved to the right, and he drove off the road into a culvert without you noticing. Or he went to the side and stopped, while you weren't looking right at him. Almost anything could have happened to cause him to seem to disappear.
One thing we have to remember is that, if this other universe is mingling with ours it would have physical, measurable effects on our universe. People have been seeing ghosts from the beginning of human history, yet no one has ever been able to actually measure one. Or even capture one! I went on a "ghost" hunt once. Middle of the night, a dark field, half a dozen of us looking for ghosts. Everyone else saw them, or claimed to. All I saw was patches of mist drifting over the field. Because the weather was cold, and the air was damp. Water vapor, not ghosts.
People have a tendency to see patterns in random images, like seeing a face in the swirls of paint on a wall. When they see a vague shape, their brains tend to equate that shape with something familiar. Doesn't make what they see any less real, only brings into question their interpretation of what they saw.