• Dirty, overcrowded metro systems deter riders and pose safety risks, underscoring the need for smarter cleaning technologies and behavioral interventions that foster shared responsibility and boost ridership.
• Deploying AI-powered autonomous cleaners allows metros to maintain hygiene without shutdowns, addressing the challenge of limited cleaning windows while reducing labor costs and encouraging cleaner passenger behavior.
• Leveraging subtle social and visual cues—like descriptive norms, “watching eyes,” and home-like design—encourages passengers to take responsibility, reducing litter and fostering shared ownership of metro cleanliness.
• Making waste disposal effortless and rewarding—through strategic bin placement, visual cues, and gamified incentives—reduces friction, motivates cleaner behavior, and fosters shared pride in maintaining metro cleanliness.

Public transit systems are a powerful solution for some of our society's most pressing modern issues. Effective metro systems help to reduce the number of cars and vehicles on the road, lowering vehicle operating costs, relieving congestion, and reducing carbon emissions that are harming our health and our planet. According to a 2024 World Bank analysis exploring 192 cities with a population of more than 500,000, suggests that, on average, cities with subways produce roughly half the carbon emissions of comparable cities without them.¹ 

Despite their benefits, ridership levels on metros can be lower than expected in some metropolitan cities. While there are multiple challenges to increasing ridership, one of the deterrents is that metro systems are considered among the dirtiest places in our cities. Grimy high-touch surfaces, litter on train and platform floors, and stains on seats are a familiar sight for individuals who use the metro system. Beyond making it an unappealing form of transportation, it can also lead to risks: trash can cause track fires and lead to delays, while germs can accelerate the spread of viruses and disease, as we experienced during the COVID-19 pandemic.

With the amount of traffic that metro systems experience during peak hours, it is difficult for cleaning teams alone to help reduce the mess. Riders must do their part in keeping the metro system clean, but diffusion of responsibility paired with a busy, chaotic environment presents a challenge. To encourage cleanliness in metro systems, we must match cleaning technologies and strategies to the dynamic environment and develop interventions that target the behavioral drivers of disorder. 

Thousands or even millions of passengers will filter through metro systems in busy metropolitan cities on a daily basis, bringing with them dirt from their shoes, moisture from wet umbrellas, and germs that are left behind on seats, doors, and handrails. Although passengers can do their part to help keep metro systems clean, any environment used by a large volume of people will naturally experience a build-up of grime and dirt. The issue is that it’s often difficult for transit operators to schedule deep cleans without affecting the quality of service for passengers.

When passengers are actually riding the metro system, it’s very difficult for operators to effectively clean. Some metro systems—such as New York City, Copenhagen, and Chicago—are in operation 24/7, meaning there is no downtime for cleaners or maintenance to come in and clean or fix things. Even for systems that have some nighttime shut-downs or where cleaning can be done during off-peak hours, these windows are rare and short. 

That’s why many underground systems only undergo a full clean every couple of months. The Société de transport de Montréal reports that its metro system only receives a deep clean every five to six weeks, citing the fact that it negatively impacts passengers if metro systems are closed down regularly for cleaning.² 

Beyond being an inconvenience for public transit riders, it’s very costly for metro operators to close down their operations and halt trains to complete a deep clean. During the COVID-19 pandemic, the New York City subway system shut down every night from 1:00 am to 5:00 am—the first time in the system's 116-year history that it facilitated planned shutdowns—to be cleaned as a public health precaution. It is believed that this regimen costs the Metropolitan Transportation Authority $500 million that year.³ Shutting down the system also represents an opportunity cost: no fares are collected during that time period. While this may have been a necessary cost during a pandemic to help prevent the spread of the virus, it’s unlikely that most transport authorities can take on this extra burden. According to a study conducted on cleaning trends from 30 metros before the pandemic, on average, only 5% of a metro’s operating cost was spent on cleaning.⁴ 

If regular deep cleanings are too much of a financial burden on metro operators and negatively impact passengers by affecting when and how they can access public transit, then we must find opportunities for quick, micro-cleaning regimens and invest in materials that are more resistant to germs to help avoid the build-up of mess.

To avoid regular system shutdowns, operators must find ways to complete more frequent and effective quick cleans during operations. One of the key factors to enable these microcleans is flexibility. Metro systems are dynamic environments, where passenger flow varies from line to line and hour to hour. While it would be difficult to have cleaning teams on standby at every station, technology can be leveraged to facilitate quick cleans. 

Autonomous floor scrubbers—also known as robot cleaners—may be part of the answer. The robotics company Avidbots began to develop autonomous floor scrubbers, powered by artificial intelligence, in 2014, specifically targeting busy environments with high foot traffic like airports and transit facilities. Their bots—Neo and Kas—have dynamic planning capabilities and are able to adapt to changes in the environment.⁵ Maps can be synced with the robots to allow them to navigate effectively, and obstacle-avoidance technology ensures they avoid interfering with passengers. These robot cleaners could be stationed at metro train terminals, waiting for rush hours to end, to conduct a thorough cleaning of floors. Sensors could be placed throughout the system to signal to the robots when a particular area is in need of cleaning. While investing in autonomous floor cleaners might present a high initial expense, it can help to reduce personnel costs. 

Moreover, having these robots present could remind passengers that operators are doing their part to keep metro systems clean. Humans are reciprocal beings—when we feel that effort is being put into keeping an environment clean, we are more likely to also contribute toward maintaining these spaces.

Singapore Changi Airport invested in Avidbots in its terminals to improve the productivity of cleaning teams and reduce costs. Twenty-two Neo Avidbots were deployed across the airport, with different functions such as floor mopping or waste collection. Reports suggested that the robots allowed Changi Airport to save 20% in cleaning operations costs.⁶ Travelers are also able to interact with the robots by asking questions, and can even ask the robots to sing for them.⁷ This helps to humanize the cleaning technology, making it more likely that people will behave in a reciprocal manner and keep the airport clean. 

Transit systems could similarly deploy robots, even giving them unique names and personalities. These cleaners could act as a kind of mascot, which has been shown to foster behavior change and collective action.⁸ It could provide passengers with a sense of pride in their metro system and city, thereby motivating them to behave in a pro-social manner while using the systems and improve their own treatment of the environment. 

While limited space in transit terminals and aboard metros may make it difficult for cleaning robots to be effective, as technology continues to evolve, we may see smaller, more compact designs that would better serve subway systems. 

People will often look to others and their environments for cues on how to behave or understand what is acceptable in a given space. As we’ve explored, it’s difficult for transit operators to consistently maintain a clean metro system, and when the system appears dirty, it can quickly become a self-reinforcing cycle. Cleaning teams typically operate during off-peak hours or during shutdowns, making them invisible to most passengers. When passengers enter a dirty space, and seemingly no one is cleaning up, cleanliness is seen as a lower priority. 

Research has demonstrated that descriptive norms (what we see most other people actually doing) can influence our behavior more than injunctive norms (what we believe we should ethically or morally do). In 1990, a series of experiments revealed that people were far more likely to litter in environments where people had already thrown papers on the floor. In a fully littered condition, where participants entered a dirty environment, 27% threw a piece of paper on the floor compared to 11% in a clean environment.⁹

Consider how many passengers pass through a busy metro system, such as New York City, where 1.757 billion people ride the subway annually. If each person leaves behind just a small amount of trash, it’s no wonder that our metros become so dirty. In New York, cleanup crews reported having removed 2.3 million pounds of trash over the course of a few months in 2017.¹⁰ 

Even if passengers are not actively contributing to the mess, how many of us can say that we’ve reported a spill or mess to a transit staff member? Put simply, it just doesn’t feel like our job. When we enter a shared, public space, we experience a diffusion of responsibility. When everyone is responsible for keeping a shared space clean, no one feels personally accountable. Passengers may also assume that keeping a metro system clean is the city or transport authority’s job. That can lead passengers to litter, put their dirty shoes up on a seat, or not clean up a spill—behaviors they would never exhibit in more private spaces.

While some transit operators have attempted to combat people’s behavior on metro systems through anti-littering campaigns, such as Toronto’s 2006 “You wouldn’t do this at home” campaign¹¹ and New York’s 2023 “If you litter, you’re garbage” campaign,¹² the ads often use injunctive claims, trying to guilt passengers into doing the right thing, a tactic that is often ineffective.

Rather than relying solely on guilt-based messaging, interventions that leverage descriptive norms—showing passengers what most others actually do—and small, visible cues that make responsible behavior easier and more immediate could shift social behavior. By addressing the gaps in responsibility and perception, transit authorities can reduce littering and spills, creating a cleaner, more inviting environment that passengers are motivated to maintain.

To get metro passengers to do their part in keeping the system clean, interventions must be created that align with what motivates and drives behavior. When we consider the public transit system in Japan, which is often praised for its condition, it becomes clear why increasing a sense of responsibility in passengers is key to improving cleanliness in metro systems. In Japan, strong social norms around mutual respect and cleanliness reinforce collective responsibility in public spaces. Fostering group cohesion over individual interest also makes keeping public spaces clean a priority.¹³ 

However, since cultural norms vary from country to country, we must find other ways to inspire a sense of responsibility in passengers. Instead of guilt-based messaging, descriptive-norm messages that play to people’s desire to exhibit socially acceptable behavior are more likely to promote cleanliness. For example, campaigns should leverage descriptive messages that tell riders what most other people do, such as “8 out of 10 riders help keep the metro clean,” or “9 out of 10 commuters put their trash in the garbage can.” 

Research has shown that descriptive norms are more influential for promoting pro-environmental behavior, and a 2024 study in Singapore found that descriptive norms were especially influential in causing people to feel a sense of responsibility to do their part if their environment seemed dirty. That’s because the descriptive norm, combined with a dirty environment, acted as a signal that help is needed. As it’s difficult to ensure metros are clean at all times, it’s likely riders will see some litter on trains or in stations. This is where descriptive norms can be effective, as they counter the sense of diffusion of responsibility and inspire people to take action.¹⁴ 

Another way to dissuade passengers from littering is by using images that imply that their behavior doesn’t go unnoticed. Research has shown that having images of “watching eyes” promotes pro-social behavior, including increasing charitable donations, being more honest, reducing theft, and finally, reducing littering.¹⁵ Imposing that idea of the watching eyes on metro trains or stations could therefore act as a cue that passengers are being observed, thereby increasing a sense of responsibility to keep the train clean.

A creative way to shift people’s behavior could also be to create cues that evoke the context of a home or other private space, where people usually feel inclined to take care of their environment. This acts as a form of norm priming, where exposure to a stimulus that reminds people of their home influences how they behave on the train. This approach was used in Germany in the early 2000s, where a marketing campaign included a large mat that mimicked a doormat with text that read “Welcome. We do everything we can to keep the station clean. And we go pretty far in doing this.”¹¹ This implicitly signals to passengers that they should behave as they would in their most valued spaces. 

These examples of using subtle visual cues to increase a sense of responsibility and shift societal norms to be pro-environmental are simple, cost-effective ways that transit authorities can nudge people to keep their metro systems clean, or at the very least, not contribute more to the mess. 

Even well-intentioned passengers may find themselves leaving litter or waste behind if they can’t easily find a way to dispose of their items. If garbage bins are scarce, difficult to locate, or inconvenient, these frictions contribute to the likelihood of littering. 

One of the three laws of human behavior proposed by behavioral scientist Aline Holzwarth is that behavior tends to follow the status quo unless it is acted upon by an increase in fuel or a decrease in friction. Although we would hope that the status quo is to avoid littering, increased frictions in metro systems shift passenger behavior. There are rarely any garbage bins on the trains themselves, and stations tend to be very busy, especially during peak hours, making it difficult to find or get to a garbage bin. This creates friction that goes beyond what people experience in other environments, such as public buildings or even streets, where there are typically frequent waste bins that are easy to access. At the same time, there is very little “fuel”: no real incentive to not litter beyond being a good person. 

A survey conducted in New York in 2025 demonstrates that doing the right thing is not enough to prevent people from littering. According to the survey, 83% of New Yorkers acknowledged that littering is a problem, but only 29% of people said they had never littered. Two of the most common reasons respondents gave for littering were that there was a lack of trash cans nearby or that they were in a hurry.¹⁶

Metro systems can be very complex and confusing. These are often chaotic environments—crowded, noisy, and time-pressured. This places a heavy cognitive load on riders, and they are more likely to focus on navigating the station and ensuring they get on the right subway line than trying to find a bin. They simply do not have the time or the cognitive capacity to properly dispose of their waste.

While in an ideal world, people would behave in pro-social ways out of the goodness of their hearts, or their care for public infrastructure, the reality is that even a small amount of friction can cause people to abandon these behaviors and contribute to a messy metro system. 

The challenge of littering on public transit isn’t driven by apathy—it’s driven by friction and a lack of fuel. To encourage people to properly dispose of their trash, we must make it easier and more appealing. 

Reducing Friction

It’s not simply about adding more waste bins in metro systems—research has shown that there is only a weak correlation between the number of receptacles and the amount of litter.¹⁷ Waste bins must be strategically placed, with consideration both to where people typically litter (by escalators, train doors, or vending machines) and on high-traffic paths that passengers already take, as most people will be dissuaded by an extra detour. By placing bins at the point of decision, the physical effort of disposal is eliminated, and it becomes the easy, obvious choice.

To reduce the cognitive effort of finding a bin, visual cues can be used. Bright colors help to grab attention, especially in busy environments, and wayfinding symbols can lead riders to disposals. These symbols should be common and consistent across stations within a city, to help people navigate garbage disposal more intuitively, even if a particular station is unfamiliar to them.¹⁸ Black or brown arrows, which are commonly associated with general waste disposal, can be used as a priming tactic to make it clear that the arrow is informing them of where to dispose of trash. Again, it is important to place these wayfinding symbols at decision points. The most optimal place to position wayfinding symbols may be as someone emerges from the platform, either via stairs or escalators, as they are likely already looking for directions for where to exit. 

Increasing Fuel 

Getting a reward for exhibiting a behavior motivates people to repeat the behavior, while suffering a negative consequence for exhibiting a behavior dissuades them As we’ve explored, although there are usually fines in place for littering on public transit, as they are rarely given out, it’s not enough of a disincentive to counter other potential negative consequences (e.g. being late as a result of searching for a waste disposal). 

The potential consequences of littering must therefore be made more visible and tangible. While increasing the number of security officers at stations could help reduce littering (as few people are bold enough to litter in front of a figure of authority), this would be a costly expense for transit authorities. However, even the idea of being watched can cause people to consider the potential consequences. As we’ve previously explored, images and messaging can help promote pro-social behaviors, like proper waste disposal. Messages could also be placed throughout metro systems that make the consequences more tangible. For example, a poster that reinforces these ideas, like “Harry was fined $200 for littering,” may discourage people from repeating the behavior. 

On the other side of the coin is finding ways to incentivize people via rewards. Gamification is a powerful method through which to motivate behavior. People chase behaviors that give them a sense of accomplishment, which means turning cleanliness into a game acts as fuel. Cities could establish visible leaderboards that demonstrate which of their stations have experienced the least amount of litter or that are cleanest, making it a competition for riders who frequent each station. This can also help people feel a sense of pride for their station, which can foster a sense of shared ownership over keeping it clean.

While behavioral insights can help to encourage cleaner metro systems, they are a small piece of a larger puzzle. Even if riders do their part to keep the system clean by not littering, any environment where thousands of people pass through daily is naturally going to get dirty. While technology may allow more frequent and targeted cleaning, it will require significant investment from authorities that already face significant budgetary and operational constraints. 

Additionally, there is no one-size-fits-all solution for encouraging cleanliness. Due to differences in culture, passenger volume, and system size, what works in one city may not work in another. However, by understanding what encourages people to behave in environmentally friendly ways, small design changes and campaigns can help nudge people in a cleaner direction. 

Metro systems offer a city so much potential—to improve connectivity, reduce the environmental toll of our reliance on personal vehicles, and limit congestion. However, for this potential to be realized, public transit must be perceived as an appealing alternative—and one way to improve perception is to make metro systems cleaner.

The messiness of metro systems isn't due to malice, but from natural human tendencies. Cognitive overload, diffusion of responsibility, and friction can cause people to leave their trash behind. However, small visible cues and nudges can help encourage cleanliness. Using descriptive norm messaging to make people feel like others are doing their part in keeping the system clean, as well as visual representations that they are being seen, can make littering seem more objectionable. Reducing friction by ensuring that waste disposal is easy to find and access, as well as highlighting the negative consequences of littering, can both push people to properly throw away their trash. 

The busy environment of metro systems and heavy foot traffic makes it incredibly difficult for cleaners to frequently and sufficiently clean up after passengers. However, innovative cleaning technologies like autonomous floor cleaners can help to address some of the issues created by infrastructure design and busy schedules, by allowing for quicker, targeted cleaning.

Here at The Decision Lab, we specialize in people-centered innovation and design, and leveraging behavioral science to achieve operational excellence.  Partner with us to harness cutting-edge technology and evidence-based insights to design cleaner, smarter, and more human-friendly systems across sectors.

TDL Brief: Public Transit in Japan

Public transit in Japan is often referred to as a gold standard for cleanliness, reliability, and efficiency. This creates a more pleasant passenger experience, which encourages ridership. In this article, Dan Pilat and Dr. Sekoul Krastev explore what makes Japan's public transit system so successful.

The Uber-fication of Public Transit with Remi Desa

Ride-sharing apps, like Uber, are often used as an alternative to public transit. One of the draws of these apps is that they offer riders on-demand transportation, whereas public transit systems use schedules and routes. In this podcast, we speak to Remi Desa, the CEO and co-founder of Pantonium, a company focused on improving urban mobility, about how some of the appealing characteristics of Uber could be applied to public transit. 

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