Behavioural “nudges” are ineffective at encouraging commuters to carpool or take the bus

You'll probably be familiar with the idea of behavioural "nudges" — interventions that encourage people to make better choices, without changing the actual options available. As a concept, nudging has become hugely popular, with at least 200 "nudge units" in governments and institutions around the world.

We've certainly reported on a few studies finding that simple nudges encourage people to give more to charity, and help people to make healthier soft drink choices from fast food menus, for example. You might be forgiven for thinking, then, that there are no limits to what nudging can do….

Well, a recent set of studies designed to "nudge" commuters' behaviour, published in Nature Human Behaviour and involving a total of almost 69,000 people, has found that there definitely are limits. "The failure of these well-powered experiments … highlights both the difficulty of changing commuter behaviour and the importance of publishing null results to build cumulative knowledge about how to encourage sustainable travel," write Ariella S. Kristal and Ashley V. Whillans, of Harvard Business School.

The participants were employees of a major European airport. About half had reported driving by themselves to and from work. But almost 60% of this group said they would consider carpooling, and 41% said that finding a "matched" person with a similar commute to carpool with was a key reason why they weren't already doing it.

Many employees also reported that if they were given discounts on public transport, they'd be more likely to use it. The airport did already have a carpool service, and was also well connected to subway, rail and bus infrastructure. It seemed, then, like the ideal place to nudge people towards commuter behaviours that would benefit their own health and pocket, as well as, of course, the environment.

In the first experiment, Kristal and Whillans tested a letter and email campaign designed to encourage employees to sign up for carpooling by highlighting the benefits and providing testimonials from carpoolers. Though this intervention did increase registrants, this increase was tiny, and, statistically, it made absolutely no difference to the numbers who actually used the service. (For this study, the researchers sent out 15,000 letters. Only 33 employees registered for the service as a result. One month later, only three who'd received the initial letter were active carpoolers.)

For the second study, they gave employees details of good personal carpooling matches, and reminded them that carpooling would save them money. (Remember, a lack of knowledge about someone suitable to carpool with was reported in initial surveys as a key barrier to doing it.) But  again, this intervention did not increase carpooling behaviour.

Next, they tried giving a different group of more than 7,000 employees a free one-week bus trial. But this intervention failed to subsequently increase registration for discounted bus passes.

In the fourth experiment, the pair provided just over 1,000 employees with comprehensive personalised travel plans. These plans included information about carpooling matches, bus and train routes and times, transit pass discounts and bike routes. These employees were also offered a one-on-one session with the airport commuter team to talk this through. Only 21 people signed up for these sessions. And, overall, the intervention made no difference whatsoever to the participants' commuter behaviour.

There is one significant limitation to the study that the researchers themselves point out: this particular airport offers employees a free parking space. Perhaps these attempted nudges would have had more impact on people who have to pay for their parking.

However, they do draw a few cautionary conclusions from their findings. One: the work "suggests that people do not always reveal their true intentions or barriers to sustainable commuting." Though many had initially said that they would carpool or use public transport if they had discounted travel, this isn't what transpired.

Further work will be needed to investigate exactly why this is (maybe they don't really want to talk to co-workers during the commute, for example). But it does seem that nudges aren't enough to change commuter behaviours. Heavier-handed policies, such as congestion charges and parking bans, may be necessary to bring about shifts in this type of behaviour, the researchers write.

More broadly, though, they think the findings also suggest that nudges might not be that effective at changing habitual behaviours, in general.

An accompanying editorial in the journal welcomes the publication of this paper. Though the majority of studies reporting nudge interventions have found them to be successful, given the known problem of bias towards publishing positive results, "it's possible that many more unsuccessful interventions exist but have never been reported," the editorial observes. And clearly, this is an example of a case where null results have policy implications.

Finally, the editorial notes, "the study points to the limits of nudges and reminds us that they are not a panacea. While they can be a low-cost and simple way to shift many behaviours, it's important to remember that not all behaviours are equally easy to shift."

Further reading

– What we can learn from five naturalistic field experiments that failed to shift commuter behaviour

About the author

Emma Young (@EmmaELYoung) is a staff writer at BPS Research Digest