Dread Risk

What it is

The claim has two parts. The first is about perception: people fear low-probability, high-consequence events more than their expected harm would justify, and they fear them most when the deaths arrive together and spectacularly. The second is about behaviour. A fear that leads people to avoid one danger can push them towards another that is statistically larger, so the response to the first event causes harm of its own.

Gigerenzer set the argument out in a 2004 paper in Psychological Science titled "Dread risk, September 11, and fatal traffic accidents". His hypothesis was that after the attacks Americans avoided the risk of flying and drove instead, which would raise traffic deaths. Using United States Department of Transportation data for the three months after 11 September, he found that the number of people killed on the road while avoiding flying exceeded the total number of passengers killed on the four fatal flights. He concluded that educating the public about psychological research on dread risks could save lives.

In a 2006 follow-up in Risk Analysis, "Out of the frying pan into the fire: Behavioral reactions to terrorist attacks", he reported three findings. Air travel fell after the attacks. Interstate highway travel rose for about a year, which suggests that some people drove instead of flying. Fatal highway crashes exceeded the baseline of previous years in each month of that year. He estimated that about 1,500 Americans died on the roads while trying to avoid the fate of the flight passengers.

This vault has not traced who first coined the term dread risk, and the origin is left open here.

In effect

The headline numbers differ, and the difference is part of the story. Gigerenzer's 2004 paper estimated 353 additional road deaths in October to December 2001, according to a later re-analysis. That re-analysis, by Michael Sivak and Michael Flannagan in Transportation Research Part F, used January to August trends in 2000 and 2001 as its baseline and found 1,018 additional road fatalities for the same three months. It also found that most of the increase fell on local roads, that driver fatalities did not rise more than passenger fatalities, and that pedestrians and cyclists accounted for a disproportionate share. Those details argue against a simple swap of flights for car journeys over the same distances.

A 2012 analysis by Wolfgang Gaissmaier and Gigerenzer, summarised in an Association for Psychological Science release, used monthly miles driven and traffic fatalities for all 50 states and the District of Columbia. It reported about 1,600 more traffic deaths than expected in the year after the attacks and an average monthly increase of 27.2 miles driven per inhabitant over the three months after them. Residents closer to New York City, the authors' proxy for fear, showed only a slight increase in driving. Increased driving was strongly associated with driving opportunity, measured by the length of the National Highway System and the number of car registrations per inhabitant. The authors concluded that understanding fear is not enough: behaviour changes only where the environment lets fear turn into action, and they proposed changing minds or changing environments as the two routes.

The same logic was tested in London. Peter Ayton, Samantha Murray and James Hampton, in Judgment and Decision Making in 2019, studied the bombings of 7 July 2005 on public transport. Underground journeys ran 7.1 per cent below prediction, 55.6 million fewer than expected. Bicycle journeys ran 10.6 per cent above prediction. Bicycle casualties were 15.4 per cent above prediction in the second half of 2005, an estimated 214 additional casualties, while car casualties showed no significant increase. The substitute was the one the city made available.

For anyone who sells transport, insurance or safety products, the practical reading is narrow. Demand moves after a salient event, the direction of the substitution depends on what alternatives exist, and the side effects can fall on a different group from the one that was frightened.

What it does not say

It does not say that availability is the cause. Dread and availability are different accounts. One concerns the emotional weight of a kind of hazard, the other the ease with which examples come to mind, and the studies above were not designed to separate them. In the first days after a vivid attack the two are bound to move together.

It does not say how large the effect was. Three months of data gave 353 in one estimate and 1,018 in another. Twelve months gave about 1,500 and about 1,600 in two others. The periods, baselines and methods differ, and the figures should not be read as one number measured four times.

It does not say that fear was measured. The London authors state that their evidence is correlational and cannot confirm that dread caused the switch to bicycles. The 2012 analysis used distance from New York City as a proxy for fear, and found that the proxy predicted little.

It does not say that every frightened person changes behaviour. The research points to environment, meaning whether a person has a car, a road and an alternative, as the gate between fear and action.

It does not trace the idea to a named originator in this vault's notes, and none is claimed.

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