Authors: Tom Vanderbilt
All this eye movement and the relative motion of the objects we are seeing, as confusing as it seems, help us judge how far away things are from us. As Mark Nawrot, a psychologist at North Dakota State University and an expert in motion parallax, describes it, this is why film directors like Peter Jackson like to move the camera around a lot. Because we are sitting, stationary, in a theater, and thus cannot get the sort of depth cues our eyes give us when we move, Jackson moves the camera instead, to make the film appear more realistic. But the price we pay for the depth cues that motion parallax provides us is the occasional illusion that we may or may not consciously notice. In traffic, motion parallax may trick us into thinking that an object is far and stationary when, in reality, it is
near and moving.
The mind can play tricks on what we see, but motion parallax reminds us that what we see while driving plays tricks on our minds. Sense and perception are connected by a quite busy two-way street. The white stripes on the highway and the distance between them are designed precisely as an illusion, to make these high speeds seem comfortable. If both the stripes and the distance between them were short, the experience might feel nauseating. In fact, in some places, engineers have tried to exploit this by employing “illusory pavement markings” to make drivers think they are going faster than they are. In one trial, a series of arrowlike chevrons were painted, ever closer together, on a highway exit ramp. The theory was that as the drivers began to pass more chevrons for each moment they drove, it would appear as if they were going faster than they really were, and would thus slow down. That study did find that drivers reduced their speed, but in other trials the results have been mixed. Drivers may slow once or twice simply because there are strange markings on the pavement, but they may also quickly acclimate to the markings.
These experiments have been focused on exit ramps because they are a statistically dangerous part of the highway. One crucial reason involves a particular illusion we face in traffic: “speed adaptation.” Have you ever noticed, when driving from a rural highway onto a village road with a lower speed limit, how absolutely slow it feels? When you again leave that town to rejoin the rural highway and its higher speed, does the disparity seem as noticeable? The longer we drive at high speeds, the harder it is for us to slow down. Studies have shown that drivers who drove for at least a few minutes at 70 miles per hour drove up to 15 miles per hour faster when they hit a 30-miles-per-hour zone than drivers who had not previously been traveling at the higher speed.
The reason, as Robert Gray, a cognitive psychologist at the University of Arizona, explained to me, is something that might be called the “treadmill effect.” After running on a treadmill for a while, you may have noticed that the moment you stop you may briefly experience the sensation of moving backward. As Gray describes it with driving, neurons in the brain that track forward movement begin to become fatigued as a person looking ahead drives at the same speed for a time. The fatigued neurons begin to produce, in essence, a negative “output.” When a person stops (or slows), the neurons that track backward motion are still effectively dormant, but the negative output of the forward neurons fools you into thinking you’re moving backward—or, if you’re changing from high speeds to lower speeds, it can fool you into thinking you have slowed more than you actually have. The illusion cuts both ways, studies suggest: We
underestimate
our speed when asked to slow down and
overestimate
our speed when asked to speed up. This helps explain why we often go too fast coming off a highway (and hence the chevron patterns); it might also explain why drivers entering a highway frequently fail to reach the speed of traffic by the time they’re merging (frustrating those in the right-hand lane who are forced to slow).
We misjudge speed in all kinds of ways. Our general perception of how fast and in what direction we are moving—indeed that we are moving at all—comes largely, it is thought, from what has been called “global optical flow.” When we drive (or walk), we orient ourselves via a fixed point on the horizon, our “target.” As we move, we try to align that target so that it is always the so-called focus of expansion, the nonmoving point from which the visual scenes seem to flow, approaching us in a kind of radial pattern—think of the moment in
Star Wars
when the
Millennium Falcon
goes into warp speed and the stars blur into a set of lines streaming away from the center of the ship’s trajectory. The “locomotor flow line”—or what you and I would call the road—is the most crucial part of the optic field in driving, and the “textural density” of what passes by us influences our sense of speed. Things like roadside trees or walls affect the texture as well, which is why drivers overestimate their speed on tree-lined roads, and why traffic tends to slow between noise-barrier “tunnels” on the highway. The finer the texture, the faster your speed will seem.
The fineness of the road texture is itself affected by the height at which it is viewed. We sense more of the road’s optical flow the closer we are to it. When the Boeing 747 was first introduced, as the psychologist Christopher Wickens has noted, pilots seemed to be taxiing too fast, on several occasions even damaging the landing gear. Why? The new cockpit was twice as high as the old one, meaning that the pilots were getting half the optical flow at the same speed. They were going faster than they thought they were. This phenomenon occurs on the road as well. Studies have shown that drivers seated at higher eye heights but not shown a speedometer will drive faster than those at lower heights. Drivers in SUVs and pickups, already at a higher risk for rollovers, may put themselves at further risk by going faster than they intend to. Studies have shown, perhaps not surprisingly, that SUV and pickup drivers speed more than others.
The reason we have speedometers, and why you should pay attention to yours, is that drivers often do not have a clue about how fast they’re actually traveling—even when they think they do. A study in New Zealand measured the speed of drivers as they passed children playing with a ball and waiting to cross the street. When questioned, drivers thought they were going at least 20 kilometers per hour (or about 12 miles per hour) more slowly than they really were (i.e., they thought they were going 18 to 25 miles per hour when they were really doing 31 to 37). Sometimes it seems as if we need someone standing on the side of the road, actually reminding us how fast we are really going. This is why we see “speed trailers,” those electronic signs posted by the road that flash your speed. These plaintive appeals to conscience are usually effective, at least in the immediate vicinity, at getting drivers to slow down slightly—but whether drivers want to keep slowing down, day after day, is another issue. The speed trailers work, when they do, because they give us crucial feedback—which, as mentioned in the previous chapter, we so often lack on the road. Some highway agencies, responding to rising numbers of often-fatal rear-end crashes, have tried to put feedback of sorts right on the road, in the form of painted dots that inform drivers of the proper following distances (in one case, someone responded by painting a dot-eating Pac-Man on the highway). Drivers’ following distances have tended to increase after dots are put down. Noise also gives feedback: We know we are going faster when the amount of road and wind noise picks up. The faster we go, the louder it gets. But have you ever found yourself listening to the radio at a high volume and then suddenly noticed you were speeding? A variety of studies have shown that when drivers lose auditory cues, they lose track of how fast they’re going.
The robot car Junior, as you will recall, did not need to be able to “see” brake lights because he knew exactly how far the car ahead of him was, to within a few meters. For humans, however, distance, like speed, is something we often judge rather imperfectly (hence the Pac-Man dots). Unfortunately for us, driving is really all about distance and speed. Consider a common and hazardous maneuver in driving: overtaking a car on a two-lane road as another approaches in the oncoming lane. When objects like cars are within twenty or thirty feet, we’re good at estimating how far away they are, thanks to our binocular vision (and the brain’s ability to construct a single 3-D image from the differing 2-D views each eye provides). Beyond that distance, both eyes are seeing the
same
view in parallel, and so things get a bit hazy. The farther out we go, the worse it gets: For a car that is twenty feet away, we might be accurate to within a few feet, but when it is three hundred yards away, we might be off by a hundred yards. Considering that it takes about 279 feet for a car traveling at 55 miles per hour to stop (assuming an ideal average reaction time of 1.5 seconds), you can appreciate the problem of overestimating how far away an approaching car is—especially when they’re approaching
you
at 55 miles per hour.
Since we cannot tell exactly how far away the approaching car might be, we guess using spatial cues, like its position relative to a roadside building or the car in front of us. We can also use the size of the oncoming car itself as a guide. We know it is approaching because its size is expanding, or “looming,” on our retina.
But there are a few problems with this. The first is that viewing objects straight-on, as with an approaching car, does not provide us with a lot of information. Think of an outfielder catching a fly ball—a seemingly simple act, but one whose exact mechanics still elude scientists (and the occasional outfielder). One thing that’s generally agreed upon, as University of Missouri psychology professor Mike Stadler notes, is that balls are harder to catch when they are hit directly at a fielder. Fielders often have trouble gauging distance and trajectory, and they find they need to move back or forth a bit to get a better picture; studies have shown that fielders have a harder time judging which balls can or cannot be caught when they are asked to stand still. Viewing a car head-on or directly from behind, as we almost universally do, is like viewing a baseball hit right at you: It doesn’t give us a lot to go on.
Another problem is that the image of that car, when it does begin to expand in our eyes, does not do so in a linear, or continuous, way. The book
Forensic Aspects of Driver Perception and Response
gives this example: A parked car that an approaching driver sees 1,000 feet away will double on the retina by the time the driver is 500 feet away. Sounds about right, no? But it will double again in the next 250 feet, and again in the last 250 feet. It is nonlinear. To put it another way, we can tell the car is getting closer—although this itself may take as much as several seconds—but we have no idea of the rate at which it is getting closer. This difficulty in judging closing distance also makes passing the lead car a problem; studies have shown that it is struck in about 10 percent of overtaking crashes. Another way to think about this is to imagine what happens to skydivers. For much of their fall, they have little sense, looking downward, of how fast they are falling—or even that they’re falling at all. But suddenly, as the distance to the ground begins to come within the limits of human perception, they experience what is called “ground rush,” with the terrain suddenly exploding into their range of view.
If all this was not enough to worry about, there’s also the problem of the oncoming car’s
speed.
A car in the distance approaching at 20 miles per hour makes passing easy, but what if it is doing 80 miles per hour? The problem is this: We cannot really tell the difference. Until, that is, the car gets much closer—by which time it might be too late to act on the information. One study that looked at how and when cars decided to pass other cars on two-lane highways found that they were as likely to attempt a pass when an oncoming car was approaching at 60 miles per hour as when it was coming at 30 miles per hour. Why? Because when the passing maneuver began, the cars were about 1,000 feet apart—too far to tell the speed of the opposing car. At those distances we are not even really sure if the car is coming toward us or not; the fact that it’s in the opposite lane, or that we can see its headlights, might be the only giveaway.
So at the crucial distance where one must make a decision, the driver has no idea of a key variable: the “closing rate” of the other car. This is why you may have been forced to rather suddenly abandon your attempted passing and make either a voluntary or a forced return to your own lane. We “cheat” like this regularly, relying on a car’s perceived distance without taking into account its speed. One study, looking at drivers’ left turns across oncoming traffic, found that when the speed of approaching cars was doubled, drivers’ estimates of the safe “gap” in which they could cross, which you would guess should have also doubled, went up by only 30 percent. These small discrepancies are the stuff of crashes.
Evidence suggests that we are sometimes fooled into thinking things are not as far away as they appear (and not only the approaching objects in our mirrors!). Studies have shown that people think small cars are farther away than they really are, either because we maintain a mental image of a larger car or because there is less of the car to actually see. Large objects, though, also create problems. Researchers have long been puzzled about the relatively high number of drivers killed while crossing railroad tracks—often when visibility was clear and warning signals were in place. It raises an obvious question: How could a driver not see something as large (and as loud) as a train? One answer is that a driver may have crossed the same set of tracks three hundred times in the last year without ever seeing a train, even when the signals were flashing. Did they simply not expect it on the 301st trip across the tracks? Did they “look but not see”? The influential psychologist and vision expert H. W. Leibowitz, in what has become known as the “Leibowitz hypothesis,” offered another possible explanation: biases in the drivers’ perceptual systems.