DENVER—What makes someone decide to take—or skip—the bus on a cold, rainy, hot, or smoky day?
New research led by the University of Colorado Denver finds that Denver-area bus riders change their commuting patterns when faced with cold or rain, but not as much for wildfire smoke or ozone warnings. But weather alone doesn't tell the whole story. Whether riders had to pay a fare, what time of day they traveled, and whether or not the bus stop is protected against the elements also play a role.
Published in Environmental Research , the article, “ Weathering the ride: Associations of heat, smoke, precipitation, and ozone with bus ridership in Colorado ” analyzed hourly bus ridership across the Denver metropolitan area from June 2022 through September 2023. Researchers examined nearly 44.3 million rider experiences from 7,283 Regional Transportation District (RTD) bus stops alongside data on heat and cold stress, rain, wildfire smoke, and ozone provided by NASA Research Opportunities in Space and Earth Science satellites.
When the Bus Is Free, Weather Matters Less
RTD's Zero Fare for Better Air program gave researchers another question to explore: Does weather still have the same influence when riders don't have to pay?
The program offered free transit during August 2022 and July and August 2023. During those periods, the researchers generally found less evidence that adverse weather discouraged bus use. Hot conditions were even associated with a short-term increase in ridership when fares were free. Because the program operated only during summer, the study cannot directly compare the effects of free fares with the large declines seen during winter cold.
For Elizabeth Dzwonczyk, PhD, who became involved in the project as a CU Denver doctoral student, examining these factors together was critical because that's how riders actually experience transportation.
“The true lived human experience is that you have to deal with taking the bus on a bad air pollution day, or when it's hot or cold. Your life responsibilities still exist,” Dzwonczyk said. “For that reason, it is really helpful that we were able to combine all of these elements and perspectives.”
Cold, Rain and Heat Affect Riders Differently
Instead of looking only at air temperature, Universal Thermal Climate Index (UTCI) data provided a more complete measure of how hot or cold conditions felt to people by accounting for temperature, humidity, wind, and sunlight.
Cold stood out as the weather condition with the strongest and most consistent connection to lower ridership. The decline began within hours and continued throughout the day. Rain also discouraged bus use, but its effects were concentrated in the first several hours, suggesting riders postponed or changed trips rather than skip them for the entire day.
Heat told a different story.
“On hot days, we expected to see less ridership, but it was more nuanced than that,” said Priyanka deSouza, lead author and assistant professor in CU Denver's Department of Urban and Regional Planning. “Over the course of very hot days, overall ridership is less. But when it's really hot, you actually see initial, immediate increase in the number of people who ride the bus.”
That short-term increase was most evident at stops without shelters. One possible explanation, researchers said, is that some people who might otherwise walk or bike choose an air-conditioned bus during hot conditions. However, the study did not track individual riders, or their reasons for choosing transit, just the impacts of weather, so this is just one potential reason for this increase to occur.
Building Transit for Changing Weather
These findings could help transit agencies, planners, and local governments think about weather resilience as more than a question of bus schedules and fare prices.
Rider health, comfort, and protection from the elements are all factors to be considered alongside fares and service. The relationship between heat and ridership changed depending on whether stops had shelters. Shelters are not evenly distributed across the system because they are planned for at the local, municipal level, rather than based on where there is the highest need within a community.
The research team identified several possible ideas, including real-time rider alerts and service adjustments during adverse weather, fare strategies that may help reduce weather-related barriers, and shaded, thermally protective bus shelters.
Denver as a Laboratory for Applied Research
To uncover meaningful insights into the experiences of bus riders, the researchers had to look beyond a single discipline. The project combined environmental data and urban planning with geography, mapping, sociology, statistics, and transportation expertise.
“Whenever you start to study a place, you must look broadly,” said Peter Anthamatten, an associate professor in CU Denver's Department of Geography and Environmental Sciences. “Bringing those perspectives together helps us move from understanding one piece of a problem to producing information that can inform public policy and improve people’s lives.”
That interdisciplinary approach also created opportunities for students. Dzwonczyk first encountered the RTD data while taking deSouza's urban data science class. Her classwork led to deeper involvement in the research and collaboration with faculty across many fields of study.
About the Research Team
The research team included Priyanka deSouza and Carrie Makarewicz as well as PhD student Ebru Altintas from CU Denver’s Department of Urban and Regional Planning; Adam M. Lippert from CU Denver’s Department of Sociology; and Peter Anthamatten from CU Denver’s Department of Geography and Environmental Sciences. Elizabeth A. Dzwonczyk, PhD, a graduate of CU Denver’s PhD in Geography, Planning, and Design program, is now assistant professor at the United States Military Academy. Additional collaborators included Peter C. Ibsen of the U.S. Geological Survey’s Geosciences and Environmental Change Science Center; Carl Green Jr. of the Regional Transportation District (RTD) at the time of the research; TC Chakraborty of Pacific Northwest National Laboratory; and Melissa R. McHale of the University of British Columbia’s Faculty of Forestry and Environmental Stewardship.
About the University of Colorado Denver
Millions of moments start at CU Denver, a place where innovation, research, and learning meet in the heart of a global city. We’re the state’s premier public urban research university with more than 100 in-demand, top-ranked bachelor’s, master’s, and doctoral degree programs. We partner with diverse learners—at any stage of their life and career—for transformative educational experiences. Across seven schools and colleges, our leading faculty inspires and works alongside students to solve complex challenges and produce impactful creative work. As part of the state’s largest university system, CU Denver is a major contributor to the Colorado economy, with 2,000 employees and an annual economic impact of $665 million. To learn more about how CU Denver helps learners meet their moment, visit ucdenver.edu.
Environmental Research
Observational study
People
“Weathering the ride: Associations of heat, smoke, precipitation, and ozone with bus ridership in Colorado
10-Jul-2026
None of the researchers have any conflict.