Can behavioral interventions be too salient? Evidence from traffic safety messages

https://www.science.org/doi/abs/10.1126/science.abm3427

Contrary to policy-makers’ expectations, we found that displaying fatality messages increases the number of traffic crashes. Campaign weeks realize a 1.52% increase in crashes within 5 km of DMSs, slightly diminishing to a 1.35% increase over the 10 km 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 2600 crashes and 16 fatalities per year in Texas alone, with a social cost of $377 million 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.

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, behavioral 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 behavioral 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 behavioral 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.