The Liberty Tunnel

The Liberty Tunnel

Doug Kerr / Flickr

The science of why drivers slow down for Pittsburgh tunnels

I brake before tunnels.

This is an admission that, as a person new to Pittsburgh, I did not think would cause a scandal. But in a “city of tunnels,” confessing that you are guilty of this particular vehicular sin elicits sighs, groans and exclamations of outrage from people who just moments before were kindly offering you recommendations for good pierogies.

That’s not to say that braking for tunnels isn’t a weird point of civic hate-pride: There are Reddit threads. There’s a T-shirt. There’s a song.

There are also scientific reasons that drivers do this, which do not include a tunnel monster.

In an effort to defend myself, I contacted Roberta Klatzky. She’s a psychology professor at Carnegie Mellon University and is on the faculty of the Center for the Neural Basis of Cognition and the Human-Computer Interaction Institute.

Klatzky studies how “people perceive and how it relates to how they act,” she told me by phone this week.

Slowing down in tunnels? “This certainly is the sweet spot of that,” she said.

Below is part of our conversation, which has been condensed and edited for clarity, with sections bolded for emphasis (mine, not hers).


I recently moved to Pittsburgh. I lived in D.C. before this, and I slow down in the tunnels. My question is: Why?

I have to say, I think there are multiple reasons why people slow down in tunnels, but some of them certainly relate to the processes of perceiving the visual world.

The brain doesn’t really have direct access to speed. Now, when you walk, you probably have, by years of calibration, a sense of your walking speed and how long it’ll take you to get from A to B.

But a car is capable of taking on many different speeds, and you can’t calibrate a car, except by looking at a speedometer, perhaps.

And so your visual system offers you information about speed, but it doesn’t really give you speed directly. The brain simply does not have a computer that perceives speed in a direct way. 

So what your visual system does have at any moment in time is where things are, especially relative to the back of your eye. So you take multiple snapshots of the world — and of course we take continuous snapshots of the world. You can see how things have changed, and that gives you a cue to how they’re moving.

The problem is that this is imperfect.

If you are going through a tunnel and … [see] the tiles pass by, if you knew perfectly how spaced they are, and you knew how distant the wall was from you, then you could actually use the rate of change to compute speed.

But you would do that on paper and pencil, rather than in your brain. Because your brain doesn’t do that. It comes up with good approximations, but it makes errors.

Fort Pitt Tunnel

Fort Pitt Tunnel

WestonStudioLLC

This whole process of reading how the points in the world pass by as you move is called optical flow. From this theory you can compute speed.

My belief is that in a tunnel, you have such dense information from all those tiles, and you’re pounding by and you don’t really know how well they’re spaced and you’re not really sure how far you are from the wall, what your brain does is make the error of telling you you’re going faster than you really are.

So optical flow, I think, is one of the key signals that you’re getting, and my belief is that when I’m in the tunnel I think … that I’m going too fast.

Another interesting cue is the perspective cue of how the lines in the road seem to come together as it goes off in the distance. The walls are converging. The road is converging. The ceiling is converging. And that’s telling you, “Wow, it’s a long way to the end of the tunnel.”

So suppose you get into the tunnel and you think, “That’s a long way away.” And then you’re driving along and self-correct and realize it wasn’t quite as far away as you thought. The only way your brain can understand that is that you must have been moving pretty darn fast to get that close to the end of the tunnel in such a short period of time. So that’s another way that you’re brain could miscompute your speed.

These are like the standard visual illusions that everybody wants to see on the web, or you know you look up in books, right? The brain doesn’t have perfect information. It has to use rules of thumb, so to speak. And there are lots of little computational algorithms the brain actually executes. And they’re prone to these kind of errors.

I think that errors in visual perception tend to communicate to you in tunnels that you’re going faster than you really are, and that’s why people slow.

There’s always just … very little error tolerance in the tunnel. You don’t have any side to maneuver in. Someone’s coming up on the other side. So probably that makes a lot of people more cautious. And so that is a non-perceptual reason, but certainly a psychological reason why people might want to slow down in the tunnels.

I think actually the basic structure of the tunnel presents a problem to the brain, which makes it extremely hard to calculate speed and probably misreading it that you’re going faster than you are.

So would length of time that you’ve lived somewhere, could that change your visual perception? We were talking about the “snapshots.” If you’ve seen something every day for 20 years of your life, will you go faster in the tunnel?

I think it does, and I think it’s a great example of this that we all know. In fact, the visual system is always recalibrating, especially relative to your movement.

Think of those situations when you’re on a people mover in the airport. You’re walking on a belt that’s actually speeding you up. And as you walk along you’re getting all the optical — the walls of the airport, posters are going by, things like that, right?

As you’re going along in the airport [on the people mover] … [you’re recalibrating] how much every footstep is worth. You don’t do this consciously; your brain’s actually thinking, “Wow, the optical flow field is telling me I’m going quite a distance for each footstep, so I’m a super stepper.

So when you get off the end of the people mover, all of the sudden you feel like you’re molasses, right? You’re walking super slow, and that’s because it recalibrates. You got off the darn thing, and now you’re just a normal human being again. Because you thought you were a super stepper, you’re expecting a huge flow in the visual world and it’s not there. What your brain says is, “Oh my god, I’m moving so slow.”

It self-corrects again very quickly. You’re a super walker, then you’re a sludge turtle walker, then all of the sudden it recalibrates and you’re a normal walker again. That’s what happens.

So the next time somebody yells at me for slowing down in the tunnels, can I say, “Don’t blame me. Blame, you know, my …”

Blame my brain!

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