Sometimes research findings fit together to produce a coherent set of knowledge, but other times they don't fit at all. This doesn't necessarily mean any of the findings are wrong. It might be a matter of uncovering new information to solve the puzzle so the pieces fit together.
This is the case when it comes to the hot-button topic of phoning while driving, a risky practice that has increased dramatically. A new survey indicates drivers use their phones not only on the open road but also in stop-and-go traffic. A quarter say they phone in fast, heavy traffic. Based on how much phoning while driving motorists admitted to surveyors and the estimated risk of driver phone use, an Institute analysis suggests this practice could account for 22 percent of all crashes, or about 1.3 million in 2008.
These numbers are so big that they would be expected to produce an increase in the total number of crashes, and the conundrum is that there's no such increase. Data from several sources reveal that crashes have been holding steady in recent years, even as cellphone use in general and driver use of phones in particular have proliferated.
"Don't take this to mean that phoning while driving isn't risky," cautions Institute president Adrian Lund. "It is. We just don't know yet why the risk isn't showing up in higher crash rates."
Crash patterns in federal data
Based on data from the federal Fatality Analysis Reporting System and General Estimates System, a total of about 5.8 million police-reported motor vehicle crashes occurred during 2008, the latest year for which data are available. This count doesn't differ much from the approximately 6 million crashes recorded annually during the early 1990s, when cellphones started getting popular, or from the 6.4 million crashes in 2000, when federal researchers began documenting the increase in phone use while driving.
Federal estimates of drivers using phones nearly tripled during 2000-08, from 4 to 11 percent. (The Institute's new survey pegs phone use at 7 percent, but this could be due to underreporting). Yet crashes didn't rise during these years. In fact, federal data indicate a slight decline.
All police-reported crashes, in millions, by year
Source: National Highway Traffic Safety Administration
Of course, the number of crashes over time is subject to multiple influences besides the increasing proportion of drivers using cellphones, and some of these other influences are likely to offset any increase in collisions associated with phone use. In particular, crashes are known to fluctuate along with economic conditions, and the struggling U.S. economy during recent years would be expected to suppress both miles driven and the number of crashes.
"Still the increase in driver phone use is so dramatic and the risk associated with it is so substantial that we expected to see an uptick in total crashes, but we haven't," Lund points out.
Insurance data show similar patterns
Another source of information on crash trends is the Highway Loss Data Institute, which collects and analyzes insurance claims and coverage information. This group, affiliated with the Insurance Institute for Highway Safety, reports no increase in the frequency of insurance claims for crash damage filed under collision coverage during 1998-2008, as driver phone use escalated.
Yet numerous studies establish a definite increase in crash risk associated with phone use (see "Cellphones and driving: Do bans improve safety?" Oct. 13, 2009). For example, 2 studies that examined the cellphone billing records of crash-involved drivers peg the increase at 4-fold. That is, the risk of a crash involving injury or property damage is 4 times higher during a phone conversation.
It doesn't matter whether a driver uses a phone that is hand-held or hands-free because the estimated risk is about the same, regardless of phone type (see "Phoning while driving increases crash risk," July 16, 2005). The risk also is about the same for men and women and for motorists young and old.
Collision claims per 100 insured vehicle years, by calendar year, 4 most recent model years
Source: Highway Loss Data Institute
One reason the trends in phone use and crashes are out of sync may be that phoning isn't a unique risk. It is distracting, but then again drivers always have been distracted by eating, fiddling with radios, tending to children, and so on. These distractions could be just as risky as phoning.
"If so, then laws banning driver phone use won't have much effect on safety. They might curb phoning among drivers, but any benefit of this might be offset by driver engagement in other distractions," Lund says.
Technology to block driver cellphone use might work to reduce phoning while driving, but the safety payoff is unknown. Crash avoidance features like lane departure warning and forward collision warning seem more promising (see Status Report special issue: crash avoidance features, April 17, 2008). These address all kinds of distractions, not just cellphones, by bringing drivers' attention back to the road.