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It’s what researchers call a “roadway illusion.”

Details are still emerging about the tragic car crash that killed John Nash, the mathematician celebrated in A Beautiful Mind, and his wife, Alicia, but early reports suggest their (inexperienced) cab driver lost control while passing another vehicle. It’s possible the lane maneuver was justified. But whether or not that’s the case, the news offers a fitting, if terribly unfortunate chance to remember the following persistent traffic myth: the lane next to yours isn’t actually moving faster.

The fact that changing lanes is such a regular part of the daily traffic grind—on average, we tend to do it once every mile and a quarter—inures us to the risky nature of these moves. When you’re changing lanes you lose sight of the cars in front of you, make braking demands of the cars behind you, and often end up in the blind spot of the new car beside you. You make very quick calculations about spacing and velocity that soak up some cognitive attention you might otherwise be using to monitor the road.

And you often go through all that effort for no gain. That’s the key takeaway from high-profile work on lane-changing behavior from Donald Redelmeier and Robert Tibshirani published back in 1999. They conclude a visual “illusion” tricks us into thinking the other lane is moving faster, even when in actuality both lanes are going the same average speed. Via Nature:

This occurs because vehicles spread out when moving quickly and pack together when moving slowly. A driver can therefore overtake many vehicles in a brief time interval, but it takes much longer for the driver to be overtaken by the same vehicles.

Redelmeier and Tibshirani came at this finding two ways. The first was via a computer simulation of 10 minutes of traffic in two adjacent highway lanes. They stacked the traffic flows in each lane, tracked the velocity and location of a target vehicle (modeled to have the driving performance of a Honda Accord), and observed how many times the target either passed a neighboring car (known as a “skip”) or was passed (called a “slip”).

Of the 600 individual seconds that made up the 10-minute drive, the target car spent more time being passed than it spent passing other cars—creating the impression that it was losing ground to traffic at large. But one second of skipping can overtake a whole bunch of cars jammed together, while one second of being slipped involves a single car pulling away. So because each “skip” involves more cars than each “slip,” the total number of cars you’re passing and being passed by balances out.

In a longer version of their research paper, published in a 2000 issue of Chance, Redelmeier and Tibshirani report that the target car skipped as many total cars as it was slipped by: 46.

Because cars spend more time being passed than passing, it can seem that they’re losing ground to traffic at large, even when both lanes are going the same speed. (Nature)

The researchers got further evidence of this lane-changing illusion by videotaping traffic sequences with a mounted camera on a Canadian highway. They clipped four minutes of traffic with both slips and skips then showed it to 120 people. About 70 percent look at the scene and thought the adjacent lane was moving faster, and 65 percent said in that situation they’d make a lane change. In fact, write the researchers, the adjacent lane had a lower average speed of traffic. They conclude in Chance:

Together, these findings suggest a roadway illusion — namely, that the next lane on a congested roadway appears to be moving faster than the driver’s current lane even if both lanes have the same average speed.

So what’s behind this illusion? There’s a basic directional element: since you tend to look forward while driving, you spend more time watching your car be passed by that jerk beside you than you do watching those suckers you’re passing in the rearview. Traffic itself plays a self-serving role; congestion intensifies the illusion, even as frequent lane-changing increases congestion. People also have a general aversion to loss compared with gains. That means every time you get passed hurts proportionately more than the joy you derive from passing someone else.

Here’s the worst of it: drivers that tend to tailgate or frequently glance to the lane beside them are “more prone” to the illusion than others, according to Redelmeier and Tibshirani, and thus more inclined to think they’re losing ground. So the very drivers already engaging in risky road behaviors have a more intense perception that they need to engage in this risky road behavior.

Obviously there are times when changing lanes on a highway is appropriate. Avoiding an obstacle or lane closure is the clearest example. And, for sure, sometimes the lane next to you is actually moving faster. But the point is that these cases are happening far less often than you think, and they largely fall outside our powers of detection. As Redelmeier and Tibshirani put it, “the best way to arrive five minutes sooner is to start five minutes earlier.” The risks that come with trying to make up that time mid-trip by zigging and zagging out of a lane are not worth the small reward.

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