When I was a kid, I felt like I ran faster after dark. Summer nights were the best because I could stay out late and chase the fireflies. When I ran, I felt as if I was flying over the grass in my backyard. Unfortunately, when the sun came out the next morning, I returned to my normal, slow, plodding self. I always suspected there was something odd with my perception of running faster at night. Even as a kid, I didn't think I turned into the Flash or Superman after dark (although that would have been really cool).

Recently I've been reminded of running fast after dark. I biked home a little late one evening last week. Luckily, I have lights on my bicycle because twilight was upon me by the time I finished my ride. I felt that I was riding really fast that night - until I glanced at my speedometer and saw that I was actually going a little slower than usual.

Similarly I've been paying attention to how fast it feels like I'm driving on my regular trips between Bellingham and Seattle. I generally go the same speed because I rely on cruise control. Nonetheless, sometimes on the dark stretches of the freeway when no one else is around, I have to check my speedometer to make sure that I haven't set the cruise control incorrectly. I haven't, but it can feel like I'm driving incredibly fast.

In thinking about my illusions of speed, I've been considering how we know that we are moving. Visual information contributes strongly to the perception of self-motion. When all the objects and background appear to move past our visual reference point that specifies that our own motion is the cause. JJ Gibson described to this as self perception through optic flow. The visual impression is that the world flows past us as we move forward. In contrast, if only a few objects are changing their positions while the rest of the world is visually holding still, that specifies that those few objects are moving. I wrote about times when people get confused about perceived self motion in an earlier post (Is It Me or Is the Room Moving?).

So we know that we are moving because of optic flow information. But when we are moving, not all things flow past us at the same rate. Things that are close appear to move past us quickly whereas things that are farther away appear to move more slowly. When I drive on Interstate-5, I fly over the white dashes and road turtles. The trees beside the road appear to move more slowly and the mountains move past at their own geological pace. The differential rate at which things appear to move is called motion parallax - it specifies which things are close and which are far. Optic flow and motion parallax contribute to speed perception.

During the day when I run, I can see the ground under my feet zipping past. I can also see the nearby bushes and trees moving past more slowly. At the rate I run, the houses and buildings hardly appear to move at all. Motion parallax at work. My feeling of speed is based on some average impression of how quickly I am passing everything in my visual field.

When I run at night, however, the optic flow information is limited. I see what is near me and those things flow past rapidly. I can't see the slowly moving things that are farther away. At night my perception of speed is the average of what I see - only the fast moving close things. Thus I feel like I am running fast after dark.

Biking and driving should work the same way. At night, the optic flow information that contributes to my speed impression is limited. I only see the closer, faster flowing information. Moving at the same speed I drive during the day, I nonetheless perceive myself moving faster at night.

As a child, I flew over the ground during the day and at night. After dark, I could only see that I was flying over the grass and I felt fast. During the day I could also see that I moved past the rest of the world ever so slowly. During the day, I accurately perceived that I ran slowly. From now on, I'll only run at night so that I can feel like Superman.

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