Is a Hands-Free Phone Safe While Driving? Crash Risk Was Still 3.8 Times Higher

Drivers who were talking on a mobile phone were about four times as likely to crash. On a hand-held phone the figure was 4.9 times, and with a hands-free device it was still 3.8 times. That is the result of an Australian study published in the BMJ in 2005.

In the Brain Mondo short video, I covered this study in about 40 seconds. In this post I go through the clever design the video had no room for, other studies that found similar results, and why “my hands are free, so I’m fine” is the wrong conclusion.

Video: Hands-free calls (Japanese)

The video is in Japanese.

What the study did

The study was run by McEvoy and colleagues, in Perth, Western Australia.

It looked at 456 drivers aged 17 or older who had been in a road crash serious enough to need hospital care between April 2002 and July 2004. All of them owned or used a mobile phone.

The researchers did not rely only on interviews with the drivers in hospital. They also checked the phone companies’ records of phone use. Instead of trusting people’s memories, they matched what drivers said against when the phone was actually used.

Comparing each driver with themselves

The study used a method called a case-crossover design. Instead of comparing people who crashed with people who didn’t, it compares the same person at the time of the crash and at a time when they did not crash.

Specifically, it compared phone use just before the estimated time of the crash with trips at the same time of day in the week before. Because it is the same person at the same time of day, things like driving skill, personality and usual routes are already matched before the comparison starts.

If you compare good drivers with bad drivers, it is hard to tell whether a difference comes from the phone or from how they drive anyway. Using each driver as their own comparison shrinks that problem.

The result: about four times, and 3.8 times hands-free

Using a phone up to 10 minutes before a crash was linked to about four times the likelihood of crashing. The exact odds ratio was 4.1, with a 95% confidence interval of 2.2 to 7.7.

For hand-held phones it was 4.9 times, and for hands-free devices it was 3.8 times. Both were clearly riskier than not using a phone. The paper’s conclusion is short: using a hands-free phone is not any safer.

The increase in risk was also similar for men and women, and for drivers aged 30 and over and under 30. Being young didn’t make it more dangerous, and being experienced didn’t make it safe.

Reading the gap between 4.9 and 3.8

On the numbers alone, hand-held at 4.9 looks more dangerous than hands-free at 3.8. But look at the confidence intervals: 1.6 to 15.5 for hand-held and 1.8 to 8.0 for hands-free. They overlap a lot.

According to the abstract, a third (21) of the calls made before crashes and on the trips in the previous week were on hand-held phones. With fewer hand-held calls, the estimate for them is less precise. What this study tells us is not which one is more dangerous. It tells us that both are.

What other research says

A similar result had appeared before this study. In 1997, Redelmeier and Tibshirani published a study in the New England Journal of Medicine.

It looked at 699 drivers who had mobile phones and had been in crashes that caused substantial property damage but no injuries. Using the same case-crossover method, they examined each driver’s calls on the day of the crash and during the previous week, using detailed billing records. Over the 14-month study period, 26,798 calls were made.

The risk of a crash while using a phone was about four times the risk when not using one (relative risk 4.3). Calls close to the crash were especially risky: 4.8 for calls placed within 5 minutes of the crash, compared with 1.3 for calls placed more than 15 minutes before. Units that left the hands free had a relative risk of 5.9 and hand-held units 3.9, so hands-free offered no safety advantage.

Crashes with injuries and without, two studies from different times and places: both found about four times the risk, and both found that hands-free was not safer.

So why are hands-free phones still dangerous? A 2006 study by Strayer and colleagues in Human Factors looked at this in a driving simulator. Whether on a hand-held or a hands-free phone, drivers in the middle of a conversation braked later and got into more accidents than when they weren’t talking. The authors concluded that when driving conditions and time on task are controlled, the impairment from using a cell phone while driving can be as profound as that from driving drunk.

Limits and common misreadings

The study has limits worth knowing.

First, it only included people whose crashes sent them to hospital. Minor bumps and near misses are not in it.

Second, the crash time was estimated. Whether someone was on the phone just before the crash was judged from call records and the driver’s own account.

Third, the data were collected from 2002 to 2004, and the study looked at calls. Using a smartphone the way many people do now, looking at the screen and tapping it, was not part of it.

Finally, “about four times” means that the likelihood of a crash while on the phone was about four times the likelihood when not on the phone. It does not mean that one call in four ends in a crash.

Using it in daily life

The key point of this study is that the danger is not in your hands but in your head. Even with both hands on the wheel, if your attention is on the conversation, you react more slowly to what is happening in front of you.

Here are three ways to apply this study to daily life.

• Don’t take calls while driving. If the phone rings, call back after you stop. The same goes if you have a hands-free device.
• Set things up so you don’t have to answer. Turn off notifications before you start driving. Even deciding whether to pick up uses attention.
• If you learn the other person is driving, end the call yourself. If a family member or coworker calls you from the car, say “I’ll call you back” and hang up.

Brain Mondo is a series of short daily videos about the brain, each based on a research paper. BrainDojo is an app for a few minutes a day of training in calculation, memory, attention and more. The Brain Dojo Check, once a day, gives you scores for processing speed, inhibition and short-term memory. Watching your own scores is one way to see that attention has limits.

References

• McEvoy SP, Stevenson MR, McCartt AT, et al. Role of mobile phones in motor vehicle crashes resulting in hospital attendance: a case-crossover study. BMJ. 2005;331(7514):428.
• Redelmeier DA, Tibshirani RJ. Association between cellular-telephone calls and motor vehicle collisions. N Engl J Med. 1997;336(7):453-458.
• Strayer DL, Drews FA, Crouch DJ. A comparison of the cell phone driver and the drunk driver. Hum Factors. 2006;48(2):381-391.