This time it was Barcelona. An ordinary van was transformed into a deadly and indiscriminate weapon.

It seemed to be yet another blow to trust in a basic social compact: that people are essentially safe when they walk down the street, relying on drivers to at least try to follow the rules. That accidents would be impersonal and random, and that everyone would try to avoid them.

Even though the automotive terrorist attacks of the past two years are far rarer than accidents, they are a warning that a driver can wield the ordinary car as a weapon. If anything — even something as ubiquitous as a car — can be a weapon, that adds a sense of menace to daily urban life.

Years of research has found that fear can eventually divide and poison societies, hardening people against perceived outsiders, even causing them to abandon key values. This kind of attack, using one of the most ordinary objects of daily life, could heighten that effect.

The political scientists Marc Hetherington and Elizabeth Suhay, for instance, have found that when people who are usually open and trusting toward outsiders feel they are at risk of a terrorist attack, they become more likely to support harsh, authoritarian policies and more willing to sacrifice civil liberties in exchange for perceived safety.