Highway signs showing traffic deaths don’t reduce crashes

Several years ago, Joshua Madsen was driving on an Illinois freeway when he saw something strange: an electronic sign displaying the number of traffic deaths in the state. “I just kind of freaked out. I had never seen anything like that before,” Madsen, a behavioral economist at the University of Minnesota, Twin Cities, says of the incident. It was a troubling experience to have while driving, he says. “My mind started racing.”

In the past decade, at least 28 U.S. states have started to display traffic fatality numbers to scare motorists into safer driving. But a new analysis of Texas car crashes co-authored by Madsen suggests such signs may actually be associated with more crashes, not fewer.

“It’s counterintuitive, but the analysis is solid,” says Gerald Ullman, a transportation engineer at Texas A&M University, College Station, who wasn’t involved with the study, but wrote a perspective published alongside the study today in Science. Dmitry Taubinsky, a behavioral economist at the University of California, Berkeley, says the new paper illustrates an important lesson: that people have limited cognitive capacity.

Most states show year-to-date traffic deaths on electronic signs above or alongside highways, meant to act like a warning label on cigarettes—grabbing a driver’s attention and prompting them to reconsider their risky behavior. In Illinois, the signs won unanimous approval from the State Police, Department of Transportation, and Department of Public Health. Texas, which consistently leads the nation in yearly traffic deaths, adopted them in 2012. But their efficacy has never been closely studied.

“Is this helping?” Madsen wondered. So he teamed up with Jonathan Hall, who studies transportation economics at the University of Toronto, to investigate what effect the signs had on crash numbers.

Each state has a different policy on whether and how often to display death numbers. Many states only show the stats during “safer” times—during a midday traffic lull, for example, and not during rush hour, when other traffic messages have to be displayed.

The researchers focused on Texas, which consistently displayed the messages for 1 week every month on 880 signs across the state’s highways. Madsen and Hall gathered data on all traffic crashes that happened on affected roads between 2010 and 2017. They compared crashes that occurred in weeks when fatality stats were displayed with those that happened during the rest of the month, taking care to compare only the accidents that happened at the same hour and on the same day of the week. They also controlled for weather and for holidays, which can independently affect the number of accidents.

The analysis of 844,939 accidents showed that in the 10 kilometers downroad of the signs, crashes increased by 1.35% when the numbers were displayed, the researchers report today in Science. Madsen and Hall propose that the fatality stats are so in your face that they grab too much of the driver’s attention, causing a crash. The data support this explanation, they say: Crashes increased when the death numbers displayed on the signs were higher.

Ullman isn’t totally convinced. The effect of higher fatality numbers doesn’t seem plausible to him because he doubts drivers are really processing larger and smaller death rates differently. He’d like to see more research on the cause of the increase. Madsen agrees, but says that, at the very least, the new research shows the fatality messages don’t do what they’re supposed to do: Reduce crashes. Proponents of the signs adopt a “it could possibly help, and it couldn’t hurt,” attitude about the messages, which isn’t born out in the data, he says.

More research into which types of messaging really do inspire safer driving will help save lives, agrees Gilles Duranton, a transportation economist at the University of Pennsylvania who wasn’t involved with the study. “Between 30,000 and 40,000 people die on American roads every year,” he says. “We should do much better than that.”