These highway safety signs cause more crashes than they prevent

The researchers analyzed 8 years of data to reach the surprising conclusion.
Grant Currin
A traffic fatality sign in Hawaii. HDOT

Every year, more than 1.35 million people are killed in car wrecks.

In an effort to bring those numbers down, some highway authorities are reminding drivers of the stakes. For a decade, nearly 900 digital signs on Texas highways have sent safety messages to drivers. Most of time, these signs convey a generic message, like “DON’T DRINK AND DRIVE.” For one week per month, they also include a morbid statistic: the number of deaths on Texas roads so far that year.

Most U.S. states have similar signs. A new analysis finds they may not have the intended effect. In a surprising paper published Thursday in the peer-reviewed journal Science, researchers find that signs communicating the number of on-the-road fatalities are associated with a slight increase in accidents — 1.35 percent — on stretches of highway downstream of the sign.

The researchers say the small percentage increase could have a big effect statewide. “Back-of-the-envelope calculations suggest that this campaign causes an additional 2600 crashes and 16 fatalities per year in Texas alone, with a social cost of $377 million per year,” they write.

Accident statistics contained some surprises

Behavioral economist Joshua Madsen, one of the co-authors, saw a fatality sign for the first time while driving down a freeway in Illinois. “I just kind of freaked out. I had never seen anything like that before,” he says. The gruesome statistic didn’t sit well with him. “My mind started racing,” he says.

The experience led him to team up with transportation economist Jonathan Hall to figure out what effect the signs were having on accidents. The researchers focused on Texas because of the state’s longstanding practice of displaying fatalities for one week each month. “We used data on 880 DMSs and all crashes occurring in Texas between 1 January 2010 and 31 December 2017 to investigate the effects of this safety campaign,” they write.

There were 1.52 percent more crashes on the three-mile (five km) stretch of road following the signs when they displayed the fatality number. That number fell slightly, to 1.35 percent, when considering the six miles (10 km) of road after the sign. A separate analysis found that statewide accident numbers are typically higher during the week when fatality numbers are on display. “The effect of displaying fatality messages is comparable to raising the speed limit by 3 to 5 miles per hour or reducing the number of highway troopers by 6 to 14 percent,” they write.

Experts can't agree on an explanation

What explains this counterintuitive finding? Hall and Madsen suggest that “these 'in-your-face' [and] negatively framed messages seize too much attention [and interfere] with drivers’ ability to respond to changes in traffic conditions.” While the data in this study can’t explain why this phenomenon is happening, some patterns support their idea. Crashes were more common when signs displayed higher numbers of fatalities, when signs were located on complex roads, and in the mile or two immediately following the sign (rather than several miles down the road).

Researchers who were not directly involved with the project say the problem may be information overload caused by the overall design of the safety messages. In their view, the fatality number is detrimental because it adds to the amount of information that drivers have to take in.

“Messages must be limited in length and formatted to ensure that motorists can quickly read and correctly process the information presented during limited viewing time,” they write. “It has commonly been assumed that drivers simply read and then quickly disregard messages that they deem unnecessary. However, the results of Hall and Madsen suggest that drivers may continue to try and assess how they are supposed to use that information for a much longer period of time after reading the message.”

Whatever psychology underpins these patterns, the data shows that this standard safety measure may be causing more deaths, not fewer of them.

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