How a structural solution to a real-world social dilemma failed: A field experiment on the first carpool lane in Europe

In the current field experiment we evaluate a structural solution to a real-life social dilemma by examining the effects of a carpool priority lane on judgments and preferences concerning the decision to commute by carpool (i.e., the presumed cooperative option) or driving alone (i.e., the presumed noncooperative option). Our general hypothesis was that this intervention would evoke a process of self-justification in solo drivers, arising from feelings of relative deprivation and/or cognitive dissonance. Consistent with predictions, we found that in comparison with judgments made before the implementation of the carpool lane, solo drivers tended to decrease the importance of an attribute inherently linked to carpooling (i.e., low travel costs) and to increase the importance of an attribute inherently linked to driving alone (i.e., flexibility). Moreover, solo drivers exhibited a weaker preference for carpooling after the establishment of the carpool lane. This finding suggests that the negative side effects of this structural measure were more pronounced than the intended carpool-promoting effects.