Do Electronic Signs Displaying Number of Traffic Fatalities Actually Cause More Crashes?

"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%."

From Jonathan Hall & Joshua Madsen, Can behavioral interventions be too salient? Evidence from traffic safety messages, in the journal Science:

Policy-makers are increasingly turning to behavioral interventions such as nudges and informational campaigns …. 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 … behavioral 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….

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….

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. {Because of imperfect compliance and competing demands, traffic engineers do not display fatality messages on all DMS hours during campaign weeks, implying that the effect of displaying a fatality message on a DMS is even larger.} 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%…. 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….

Thanks to Prof. Glenn Reynolds (InstaPundit) for the pointer.