Electronic signs are a common sight on US highways. These dot-matrix displays date back to at least the 1950s and were first used to alert drivers to changing speed limits or hazards ahead; they now usually exhort us to drive safely.
But targeting drivers with safety messages when they’re driving may actually be counterproductive, according to a study published this week in Science. In fact, giving drivers an update on the current year’s road death total actually led to an increase in crashes.
Jonathan Hall and Joshua Madsen used Texas to study the impact of safety messages on highway safety, thanks to a unique feature of the state—it only displays the state-wide road death count on electronic highway signs in the week leading up to each month’s Department of Transportation meeting. That allowed the researchers to compare crashes downstream of an electronic sign during those weeks with the rest of the month, and to look back to crashes on the same stretch of road during the years before the safety campaign started in 2012.
The results are not encouraging. Crashes increased downstream of electronic signs during the weeks that the signs displayed updates on traffic deaths in Texas. And like chained-together combos in a macabre video game, the increased rate of crashes per hour remained elevated along stretches that had multiple electronic signs; otherwise, the effect diminished to background 4-6 miles (6-10 km) downstream of an electronic sign.
The overall increase in crashes was not large, but it was statistically significant: 2.7 percent over the first 0.6 miles (1 km), dropping to 1.8 percent at 4-6 miles downstream of a sign. Looking at historical data for the years before TxDOT started displaying the death toll one week a month showed that the effect wasn’t present before 2012, and there was no rise in traffic crashes in the week before the signs displayed the death toll. The effect was persistent across the five years (2012-2017) analyzed.
Hall and Madsen propose that the running total of traffic deaths increases anxiety and therefore cognitive load, robbing drivers of the mental bandwidth they need to pay attention and drive safely.
The researchers note that this effect increases or decreases in pace with the increase or decrease in road deaths and, in effect, resets each year when the total is reset by TxDOT between January and February. Bigger numbers weigh heavier on drivers’ minds.
A separate analysis showed that multi-vehicle crashes—but not single-vehicle crashes—increased in the week when TxDOT showed traffic death totals. “Because single-vehicle crashes are likely a result of large mistakes (e.g., driving off the road), the increase in multi-vehicle crashes suggests that more small driving mistakes occur when fatality messages are displayed that are plausibly related to distracted driving,” the researchers write.
In total, Hall and Madsen estimate that displaying road death information increases crashes by 4.5 percent over the following 6 miles of road. This has caused an extra 2,600 crashes a year in Texas since the practice began in 2012, at the cost of 16 deaths and perhaps $380 million to society.
The effect may not be entirely consistent across all US states that display cumulative road death statistics on highway signs, however. The authors note that how often the death toll gets displayed may have an effect, diminishing in impact with increased frequency, so drivers get desensitized in states like Illinois that display these numbers a lot (although they also point out that drivers do not get habituated). And they also note that the effect is magnified with a larger death toll and so may be more pronounced in Texas, a large state with a high annual death toll.