Displaying the highway death toll on USA message boards is a common awareness campaign, but research has shown that this tactic actually leads to more crashes.
This latest study evaluated the effect of displaying accident death totals on highway message boards (eg., ‘1,669 deaths this year on Texas roads’). Versions of highway fatality messages have been displayed in at least 27 US states.
The findings show that there were more accidents during the week with fatality messaging compared with the weeks without any messages.
The research was carried out by University of Toronto assistant professor Jonathan Hall and University of Minnesota Carlson School of Management assistant professor Joshua Madsen. Their study focused on Texas, where officials display these messages for only one week each month.
The researchers compared crash data from before the campaign (January 2010-July 2012) to after it started (August 2012-December 2017), and examined the weekly differences within each month during the campaign. They found there were more accidents during the week with fatality messaging compared with weeks without.
Displaying a fatality message increased by 4.5% the number of crashes over the 10km after the message boards. This increase is comparable to raising the speed limit 4km/h-8km/h or reducing highway troopers by 6%-14%, according to previous research.
Their findings suggest fatality messages cause an additional 2,600 crashes and 16 deaths per year in Texas, costing $377m annually. The researchers suggest this “in-your-face” messaging approach weighs down drivers’ “cognitive loads”, temporarily affecting their ability to respond to changes in traffic conditions.
“Driving on a busy highway [and] having to navigate lane changes is more cognitively demanding than driving down a straight stretch of empty highway,” said Madsen. “People have limited attention. When a driver’s cognitive load is already maxed out, adding on an attention-grabbing, sobering reminder of highway deaths [can] become a dangerous distraction.”
The researchers, whose study was published in the journal Science, found the bigger the number in the fatality message, the more harmful the effects. The number of additional accidents each month increased as the death toll rose throughout the year, with the most additional crashes occurring in January when the message stated the annual total. They also found that accidents increased in areas where drivers experienced higher cognitive loads, such as heavy traffic or driving past multiple message boards.
“The messages also increased the number of multi-vehicle pile-ups, but not single-vehicle crashes,” said Hall. “This is in line with drivers with increased cognitive loads making smaller errors due to distraction, like drifting out of a lane, rather than driving off the road.”
However, the researchers found there was a reduction in crashes when the displayed death tolls were low and when the message appeared where the highways were less complex. Madsen says this suggests that at times the messaging was not as taxing on drivers’ attention.
While the use of highway fatality messaging varies by state, Madsen says agencies should consider alternative ways to raise awareness.
“Distracted driving is dangerous driving,” he said. “Perhaps these campaigns can be reimagined to reach drivers in a safer way, such as when they have stopped at an intersection, so that their attention, while driving, remains focused on the roads.”
Study details
Can behavioural interventions be too salient? Evidence from traffic safety messages
Jonathan Hall and Joshua Madsen.
Published in Science on 22 April 2022
Abstract
Introduction
Policy-makers are increasingly turning to behavioural interventions such as nudges and informational campaigns to address a variety of issues. Guidebooks say that these interventions should “seize people’s attention” at a time when they can take the desired action, but little consideration has been given to the costs of seizing one’s attention and to the possibility that these interventions may crowd out other, more important, considerations. We estimated these costs in the context of a widespread, seemingly innocuous behavioural campaign with the stated objective of reducing traffic crashes. This campaign displays the year-to-date number of statewide roadside fatalities (fatality messages) on previously installed highway dynamic message signs (DMSs) and has been implemented in 28 US states.
Rationale
We estimated the impact of displaying fatality messages using data from Texas. Texas provides an ideal setting because the Texas Department of Transportation (TxDOT) decided to show fatality messages starting in August 2012 for 1 week each month: the week before TxDOT’s monthly board meeting (campaign weeks). This allows us to measure the impact of the intervention, holding fixed the road segment, year, month, day of week, and time of day. 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. We estimated how the intervention affects crashes near DMSs as well as statewide. As placebo tests, we estimated whether the chosen weeks inherently differ using data from before TxDOT started displaying fatality messages and data from upstream of DMSs.
Results
Contrary to policy-makers’ expectations, we found that displaying fatality messages increases the number of traffic crashes. Campaign weeks realise a 1.52% increase in crashes within 5 km of DMSs, slightly diminishing to a 1.35% increase over the 10km after DMSs. We used instrumental variables to recover the effect of displaying a fatality message and document a significant 4.5% increase in the number of crashes over 10 km. 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%. We also found that the total number of statewide on-highway crashes is higher during campaign weeks. The social costs of these fatality messages are large: Back-of-the-envelope calculations suggest that this campaign causes an additional 2,600 crashes and 16 fatalities per year in Texas alone, with a social cost of $377m per year.
Our proposed explanation for this surprising finding is that these “in-your-face,” “sobering,” negatively framed messages seize too much attention (i.e., are too salient), interfering with drivers’ ability to respond to changes in traffic conditions. Supporting this explanation, we found that displaying a higher fatality count (i.e., a plausibly more attention-grabbing statistic) causes more crashes than displaying a small one, that fatality messages are more harmful when displayed on more complex road segments, that fatality messages increase multi-vehicle crashes (but not single-vehicle crashes), and that the impact is largest close to DMSs and decreases over longer distances.
We discuss seven alternative hypotheses, including the possibilities that treated weeks are inherently more dangerous and that fatality messages help in the long run. We provide evidence inconsistent with each alternative hypothesis.
Conclusion
Our study highlights five key lessons. First, and most directly, fatality message campaigns increase the number of crashes, so ceasing these campaigns is a low-cost way to improve traffic safety. Second, behavioural interventions can be too salient, crowding out more essential considerations and causing the intervention to backfire with costly consequences. Thus the message, delivery, and timing of behavioural interventions should be carefully designed so they are not too salient relative to individuals’ cognitive loads when the intervention occurs. Third, individuals don’t necessarily habituate to behavioural interventions, even after years of treatment. Fourth, the effects of interventions do not necessarily persist after treatment stops.
Finally, it is important to measure an intervention’s effect, even for simple interventions, because good intentions do not necessarily imply good outcomes.
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