The goal of this study was to nudge commuters toward a public transport subscription ticket and thus to make commuting more sustainable on a voluntary basis. It was first investigated if the theory of planned behavior, extended by environmental concern, could predict the subscription decision regarding the ticket. Second, a default nudge and a social nudge were tested to determine their effectiveness regarding increasing subscription numbers, with these nudges ultimately being integrated into the aforementioned model. An experiment with four nudge conditions and a questionnaire revealed that the theory is well suited to predict the decision, but surprisingly, environmental concern did not add direct predictive value.
Interestingly, more than half of the participants (57.4%) purchased the Job ticket, which is significantly more than the true number of tickets purchased (16% of employees purchased a ticket in 2018). This effect could be partly explained by the “nudge” of confronting everyone with the ticket, even in the control condition. This would explain why even in the subsample which excluded those who had not previously owned such a ticket, purchase numbers were quite high (38.7%).
5.1 Extended theory of planned behavior
The extended theory of planned behavior  predicted the purchasing decision well. It did, however, not prove to be of higher utility than the original theory of planned behavior by Ajzen . Commonly, subjective norm is described as the weakest predictor of behavior [2, 38]. In this experiment, however, subjective norm was the strongest predictor of the subscription decision. If a subject believed others appreciated the ticket (injunctive) and would purchase it as well (descriptive), he/she was likely to purchase the ticket, too. As expected, attitude toward purchasing a ticket predicted the subscription decision as well. Perceived behavioral control, however, had no statistically significant influence on purchasing decision. This is likely due to the questionnaire items revealing slight skewness and kurtosis, meaning that most participants found it similarly easy to purchase the ticket in this study. After all, it took only a simple mouse click to decide for or against the ticket. Undoubtedly, perceived behavioral control still plays a significant role when it comes to using public transport, as shown in several studies (e.g. Donald [18, 24, 25]), however, this was not investigated in the present study.
Lastly, environmental concern had an indirect effect on behavior which was mediated by each of the other predictors of the TPB, but had no direct effect on the subscription decision. On the one hand, this finding is in line with Heimlich and Ardoin , who summarize that pro-environmental attitudes rarely lead to actual behavioral changes. In the traffic sector (and numerous others), behavior change is often induced by marketing/advertising for the environmental benefit or sustainability of a transport mode. However, according to our results, sustainability concerns did not drive the decision to buy the ticket on its own, so it might be worthwhile to interlock environmentally-focused marketing with subjective norms. On the other hand, the lack of a direct effect on behavior could, in part, be explained by the use of environmental concern as opposed to other constructs measuring environmental attitudes. For example, personal (environmental) norm, as found in Schwartz’s  norm-activation theory, targets self-expectations based on internalized values and might have yielded different results.
5.2 Effectiveness of nudges
We investigated whether the effectiveness of nudges stretches to the transport sector. While trends were observed, statistically significant results were not obtained, which provides an important understanding of the limits of the effectiveness of nudges in the transport sector.
The results presented here fall in line with the summarized findings of Byerly et al. , i.e., that in changing the environmentally relevant behavior of transportation choice, defaults and norms (as well as education) have no effect whatsoever—even though it was established with the structural equation modelling, that subjective norms do have a strong effect on the purchase decision. It seems, thus, that it is very hard for interventions to influence this predictor. Interventions targeting sustainable transportation using commitments, salience, and finances did previously show mixed to promising effects. Still, it is claimed that “the nudging of travelers could be one of the most promising approaches to deal with the need for a radical and urgent behavioral change” (, p. 15). If this is the case, social nudges and defaults, as designed in the presented experiment, do not seem to be the appropriate choice.
Even though subjective norm was the strongest predictor in the model, the social nudge was not at all effective, producing even less subscriptions than the control condition. There has previously been evidence that people display reactance to social nudges . Considering that the social nudge used here was quite obvious (a bright yellow banner), and mentioned a high number of purchased tickets compared to the real numbers (73% vs. 16%), reactance might be an explanation. Since social nudges were not effective in other transport studies as of yet, one can assume reactance cannot be the sole factor. Social nudges do work, on the contrary, in the sector of reducing waste  and water use . These studies focused on refraining from “bad” behavior (using less water, reducing paper waste), while this experiment focused on incentivizing “good” behavior (buying a ticket, using public transport). The social nudge used here further targeted highly habituated behavior and involved monetary costs, which probably increased the cost of changing behavior.
The involvement of habituated behavior and monetary costs might also explain the results of the default nudge. Even though there was a trend following our expectations, the result was not robust. First, commuting is highly habituated. Commuters have traveled this route countless times and changing the mode or route is thus connected to possibly uncomfortable alterations and mental workload. The targeted behavior in successful nudging studies using the default (e.g. choosing an energy contract, Momsen and Stoerk  is typically not routine behavior. Why should subjects invest in changing something that already works, and potentially lose money while doing so?
These significant monetary costs could have led to bigger resistance toward purchasing the ticket. Losses weigh higher than gains , which might have activated conscious thinking (system 1 of the dual process theory), and nudges are believed to attack system 2 (i.e. unconscious processing). On top of that, opportunity and comfort costs may arise. If various leisure activities are planned for after work, a car is a symbol of freedom, it provides flexibility and spontaneity which people might think public transport cannot offer. Our default might have been designed too softly to consider all factors.
In addition, the effort to not subscribe was kept very low (compared to e.g. the opt-out of organ donation, which is a lot of paperwork) on purpose to keep it realistic. This can be seen clearly in the results of the perceived behavioral control items: participants found it similarly easy to purchase the ticket. Defaults might need to connect the undesired behavior with more hurdles than the design of this study provided.
As previous studies found, nudges can be effective. In this experiment, where there was monetary cost involved, switching the default was simple, the social nudge was quite obvious, and the targeted behavior was very habituated, nudging seemed to hit its limit, and these limits still require systematic analysis. In future studies, it seems worthwhile to investigate exactly how harsh nudges should be designed to ensure their impact in the transport sector.
Since the nudges themselves were not effective, they could not be tied to the predictors of the model. We therefore urge that connecting nudges to model predictors should be retested with effective nudges to facilitate the search for a comprehensive framework regarding nudging theory. However, in light of these results, it is debatable if there is a good enough reason to keep using the general term of nudging or if it is more promising for future research to revert back to looking at interventions and their mechanisms individually.
When interpreting the above-mentioned results of the structural equation model, it is important to remember that the questionnaire was targeted at simply the purchase of the ticket, in line with Paul et al. , who targeted the purchase of green products. However, the purchasing decision of a public transport ticket would naturally be influenced by attitude, social norm, and self-efficacy toward using public transport, as well. To get a more holistic picture of the issue, it would be valuable to consider this in future studies.
Another suggestion for future studies is taking the experiment out “on the road”. Participants were asked to imagine the scenario and to act accordingly. While this procedure offers great feasibility and is fairly common, it can obviously not be guaranteed that it would produce the same results as in a field study.
While promoting public transport seems like a good idea to increase sustainability, it entails an important drawback: instead of attracting car drivers, promotions tend to entice walkers and bikers, who are already moving sustainably. In the city of Hasselt (Belgium), 16% of public transport users stated that they had left a car at home, while 21% would have walked or biked instead . To avoid this, ways to target such strategies especially and directly at car drivers need to be found.
While the theory of planned behavior could predict the purchasing decision toward the Job ticket well, environmental concern did not directly affect it. This is an important finding regarding the advertisement of sustainable transport while also implying considerations regarding the measurement of environmental attitudes. The lack of effectivity of the nudges provides insights into the limitations of nudging theory for the transport sector. Here, nudging is different to—and apparently more difficult than in—the fields of behavioral economics, in which the concept initially boomed. Rather than a single or once in a lifetime decision, transport mode choice is habituated behavior, enforced daily, and monetary costs are involved. For policy makers, it is important to consider that social nudges could potentially trigger reactance in people, and that softly designed nudges might only have very slight effects in the transport sector, which needs to be weighed against the (typically low) cost of implementing them.