2026年4月30日 / Last updated : 2026年4月30日 桑子 研 Science

Frozen in Sight, Racing to Impact: The Terror of the Collision Course

I’m Kuwako Ken, your science trainer. Every day is an experiment.

A clear, open intersection. “Wait… is that car stopped?” — just as you think that, you suddenly realize it’s actually moving, and your heart skips a beat. Maybe you’ve even heard of accidents happening this way. This strange and slightly frightening phenomenon actually has a name: the “collision course phenomenon.” The word “collision” means exactly what it sounds like — a crash. In other words, it describes a situation where you’re on a collision path, but it’s surprisingly hard to notice.

What is the collision course phenomenon?

This phenomenon occurs when two vehicles are moving in such a way that they will reach an intersection at almost exactly the same time. For example, imagine you are driving north while another car is heading east.

Normally, since both vehicles are moving, you would expect to notice that they are getting closer. But under these conditions, something surprising happens: the other car barely changes its position in your field of view, making it look as if it’s standing still.

By the time you realize what’s really happening, the distance between you has already become dangerously small… and that’s what makes this phenomenon so risky. This video gives a very clear demonstration of how it works.

The key: relative velocity

The key to understanding this phenomenon is relative velocity. If you think about the other car’s motion as “how it appears from your point of view,” everything starts to make sense. Here’s what the actual motion looks like. Ships and cars behave in the same way.

When you combine your velocity with the other vehicle’s velocity, the result is that the other car appears to be moving straight toward you. That’s why, even though it is moving sideways in reality, it seems to stay in the same position in your view — almost as if it were not moving at all.

For a deeper explanation, check out this video:

Illusions hiding in everyday life

This kind of visual illusion based on relative motion actually shows up in everyday life. For example, have you ever been on a bullet train and thought, “Oh, we’ve started moving!” only to realize that it was actually the train next to you that moved? That’s a perfect example of how your perception of motion depends on your frame of reference.

The human brain judges motion based on relationships with surrounding objects. Because of this, it can be easily tricked under the right conditions. This isn’t unusual at all — it’s simply how our perception works.

So what can we do about it? The key is simple: never assume that “it looks still” means “it’s safe.” Open intersections may feel safe because you can see clearly, but they are exactly where this phenomenon is more likely to occur. Take a moment, pause, and ask yourself, “Is it really not moving?” That small habit can make a big difference in preventing accidents.

Physics isn’t just something for exams. It plays an important role in keeping us safe in everyday life. By understanding science, the way we see the world changes — and it can even help protect lives.

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