In 2021 more than 3,500 drivers in the U.S. alone died in traffic accidents linked to distracted driving. Using a cell phone is the primary source of distraction, but entering navigational information, trying to eat and performing other such activities can be just as risky. A new study in the Journal of Experimental Psychology: Applied suggests that distracted driving is even more unsafe than previously thought.

Multitasking has a hidden cost for drivers that past analyses have not taken into account. In two experiments, participants between the ages of 18 and 58 completed a driving-related activity while also performing a distracting task. Cognitive psychologists led by David Strayer of the University of Utah found that distraction depleted participants’ ability to pay attention to their driving for at least half a minute after the distraction ended. That extended effect implies that the number of traffic accidents caused by distracted driving could be substantially higher than current estimates indicate.

[Read more about how the brain works to suppress distractions]

In the first experiment, 32 participants began with a “baseline” phase, during which the researchers asked them to use a steering wheel to position a triangle over a dot that was moving horizontally across a computer screen. This simple activity captured a key aspect of driving, steering, while keeping participants in a safe laboratory environment. Simultaneously, people had to press a button on the steering wheel with their left forefinger each time a small device attached to their left collarbone vibrated. This extra step measured how much attention participants devoted to the primary activity, driving, as opposed to the secondary activity. Those who responded relatively slowly to the vibrations were assumed to be paying more attention to tracking the dot on the screen.

After three minutes in this baseline condition, participants transitioned to an “on-task” phase. During this period, they experienced a distraction designed to simulate the attention-demanding tasks that occur when people are keeping up a cell phone conversation or sending a text while driving. The “drivers” were presented witha random number and had to count aloud backward by either ones or threes. After 20 seconds of this challenging phase, there was a 30-second recovery period in which the backward counting task stopped, and participants only performed the driving and vibration-response activity.

Compared with baseline, the distracting on-task phase did not appear to affect the participants’ performance in tracking the moving dot—but they were slower and less accurate in their response to the vibrations. This finding indicates that more of the participants’ attention was occupied when they were trying to drive and count at the same time. More surprisingly, their performance remained impaired in the recovery period, too. In other words, even though they were no longer multitasking, people were still slower and less accurate in responding to the vibrations than in the baseline phase. This residual effect of multitasking was largest at the beginning of the recovery phase but still evident at the end of the 30 seconds.

A second experiment involving 47 participants had essentially the same design as the first, except that the primary task involved a realistic driving simulator. This approach gave the researchers a chance to observe responses in more true-to-life scenarios, including driving in light and heavy traffic. In addition, the researchers used special equipment to observe their participants’ eyes. When people are cognitively busy, their pupils expand, giving researchers an indicator of attentional engagement.

Once again, the distraction involved a backward-counting task, followed by a recovery period, now extended to 45 seconds. Just as in the first experiment, the researchers found that people performed worse throughout the recovery period than in the baseline phase. The driving task was especially difficult when the simulator put drivers in heavy traffic. For example, participants had more difficulty staying centered in their lane in the challenging driving simulator. Their pupils also dilated significantly during the distracting counting task and remained so throughout most of the recovery period. In fact, participants in the most challenging simulation showed dilation throughout the 45-second period. These findings provide additional evidence for the residual effect of multitasking.

What might explain these findings? When a person performs a cognitive task, they hold information from that task in their working memory: a “mental workspace” where details can be both stored and processed. Your working memory helps you with tasks such as doing arithmetic in your head and remembering the name of someone you’ve just met. Strayer and his colleagues propose that when a task is completed, this information isn’t purged from your working memory all at once. Rather it persists for some time, creating mental clutter that may divert attention away from subsequent tasks.

This work complements a large body of evidence that shows people tend to be bad at multitasking. (In fact, people are generally worse at juggling tasks simultaneously than they believe themselves to be.) If you’ve ever been working and gotten distracted by an incoming e-mail, you may have felt a “mental fog” when you switched back to your earlier task. That experience could have occurred because your mind was still holding details from reading your inbox even as you got back to work.

The findings also mean that drivers likely underestimate the true danger of distraction. If you send a text while driving, even though you may not miss your exit (or worse), you will be at a heightened risk of doing so down the road. Similarly, sending an e-mail while sitting at a traffic light means that once the light turns green, your mind will still be occupied by that message.

The new research highlights the need to strengthen laws that curb distracted driving. Legislation should define this concept broadly enough to include not only cell phone use but also other activities that can divert a driver’s attention away from the road. Of course, not all distractions come from technology, which is why all passengers should be aware of how hazardous distracting the person at the wheel can be. Parents, for instance, can set ground rules for kids in the car.

Finally, the findings point to a simple step that all drivers can take to make the roads safer. The next time you get behind the wheel, minimize the distraction you will experience by putting your cell phone in airplane mode, entering navigation information and finishing your lunch before you start the engine.

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