Studies may have overestimated cellphone crash risk

Increased risk of having a car crash attributed to cellphone use may have been overestimated in some past studies, a new analysis suggests.

So-called "distracted driving" has become a big public health issue in recent years. The majority of U.S. states now ban texting behind the wheel, while a handful prohibit drivers from using handheld cellphones at all (though many more ban "novice" drivers from doing so).

But studies have reached different conclusions about how much of an added crash risk there is with cellphone use.

In the new report, Richard A. Young of Wayne State University School of Medicine in Detroit finds that two influential studies on the subject might have overestimated the risk.

The problem has to do with the studies' methods, according to Young. Both studies - a 1997 study from Canada, and one done in Australia in 2005 - were "case-crossover" studies.

The researchers recruited people who had been in a crash, and then used their billing records to compare their cellphone use around the time of the crash with their cell use during the same time period the week before (called a "control window").

But the issue with that, Young writes in the journal Epidemiology, is that people may not have been driving during that entire control window.

Such "part-time" driving, he says, would necessarily cut the odds of having a crash (and possibly reduce people's cell use) during the control window - and make it seem like cellphone use is a bigger crash risk than it is.

The two studies in question asked people whether they had been driving during the control windows, but they did not account for part-time driving, Young says.

So for his study, Young used GPS data to track day-to-day driving consistency for 439 drivers over 100 days.

He grouped the days into pairs: day one was akin to the "control" days used in the earlier studies, and day two was akin to the "crash" day.

Overall, Young found, there was little consistency between the two days when it came to driving time. When he looked at all control windows where a person did some driving, the total amount of time on the road was about one-fourth of what it was during the person's "crash" day.

If that information were applied to the two earlier studies, Young estimates, the crash risk tied to cellphone use would have been statistically insignificant.

That's far lower than the studies' original conclusions: that cellphone use while driving raises the risk of crashing four-fold.

And, Young says, the results might help explain why some other studies have not linked cell use to an increased crash risk.

A researcher not involved in the work said that the two earlier studies may well have overstated the crash risk from using a cellphone.