DENVER – Sisters Trinity and Kiki Williams looked around the crowded bus stop as the #15 bus rumbled down Colfax Avenue toward them.

The bus looked to be about half full, the driver wearing a bandanna stretched across his nose and mouth to comply with government recommendations intended to help slow the spread of coronavirus. But among the awaiting passengers, only one wore a face covering.

"I'm damn nervous," said Kiki Williams, 19. "There's too many of us."

For protection, the women, who are African American, wore blue rubber gloves but no masks. "We forgot them at home," said Trinity Williams, 18.

Like millions of Americans, the Williams sisters depend on public transit at a time when health officials have told Americans to stay 6 feet apart and recommended that they wear face masks in public.

"It's the only transportation we've got right now," said Trinity Williams, whose car broke down and won't be repaired for weeks.

While transit ridership has dropped dramatically across the country during the coronavirus outbreak, millions of Americans are still riding public buses and trains, putting themselves and anyone they encounter at risk as they commute to work, go to the grocery store, visit the doctor, or, like the Williams sisters, travel to see family.

Experts say most of the people who have stopped riding are white-collar workers who can work from home and who tend to be white, leaving many of the country's poorest workers, who are disproportionately people of color, with no other choice but to pack into a small space designed to carry lots of people. In New York City, at least 41 transit workers have died from coronavirus infections, far more than police officers and firefighters.

"As always, higher-income households have more choices," said Evelyn Blumenberg, director of the Lewis Center for Regional Policy Studies and an urban planning professor at the University of California, Los Angeles' Luskin School of Public Affairswho studies how urban structures affect low-wage workers. "For low-income workers who have to take transit, they're in a confined place, in close proximity to other people. Their problems are compounded. They have no other option."

Statistics collected by the app developer Transit suggest white riders have largely abandoned buses and trains: A survey of 15,000 of the company's U.S. users revealed that only 22% of people using transit right now are white, compared to 40% normally. Transit is one of the most popular navigation apps for iPhones and Android phones, with millions of active users across the United States and Canada.

Preliminary data from states such as New York, Colorado and Michigan suggests African Americans and other minorities are dying from coronavirus-related complications at a much higher rate than their white counterparts, in part because of health care inequalities that can contribute to preexisting conditions, including asthma, diabetes and heart disease.

Transit agencies said they were working to provide additional safety measures for riders, including adding extra carriages to create more distance between riders and operators, and requiring more frequent deep cleaning of buses and trains.

But many transit agencies have also eliminated fares, which means in some cities, large numbers of people experiencing homelessness are now riding around all day without access to either hand sanitizer or soap and water, interacting with essential workers in critical jobs.

And despite recommendations that riders wear a mask or face covering when around others, few are. In some cases, public transit systems waited weeks after state stay-at-home restrictions went into place before announcing additional protections for passengers.

In New York City, the Metropolitan Transportation Authority began distributing N-95 masks to its workers on April 4, after weeks of following federal guidelines that said masks weren't needed. New York's statewide stay-at-home order went into effect March 22.

In Detroit, riders as of last week were being offered surgical masks following the coronavirus death of a bus driver who had complained of a rider coughing without covering her mouth. The state issued its first stay-at-home order March 23.

In Philadelphia, riders were ordered to wear face masks last week, but transit managers later said they wouldn't enforce it. Pennsylvania's stay-at-home order went into effect March 23.

"Every single agency was caught with their pants around their ankles," says John Samuelsen, 52, president of the Transit Workers Union International, which represents 150,000 bus drivers, railway employees and airline staff. "None of them did anything fast enough."

The union has been pushing agencies nationally to more aggressively protect transit workers, particularly bus drivers who are deemed critical workers but who are being sent out day after day without appropriate safety gear, including masks.

No one is certain why so many transit workers have died, but their repeated exposure to the public may have played at least some role, along with being a generally older workforce exposed to pollutants all shift long,Samuelsen said. Nevertheless, he added, drivers and other workers are committed to helping the rest of the country keep operating.

"Transit workers are absolutely proud of the essential role we play in delivering blue-collar workers to the front lines of this fight against COVID-19," he said.

Nationwide, transit ridership demand has dropped 75% compared to normal, according to statistics compiled by the Transit app company. In San Francisco, ridership demand has dropped by 85%. In Detroit, it's down about 67% and down 60% in Philadelphia, according to the company.

Local transit officials have also reported declines in ridership. In New York City, subway ridership is down 93%, according to the Metropolitan Transportation Authority, and in the Los Angeles area, Metro's ridership has dropped by roughly 60%, according to the agency. In the Washington, D.C., area, bus ridership has dropped by at least 60%, according to the Washington Metropolitan Area Transit Authority.

A Transit spokesman says San Francisco's sharp drop reflects the reality that white riders have largely abandoned the system, while low-income minority riders in places like Philadelphia and Detroit continue to board buses and trains. In response to the drop in ridership, San Francisco's transit system has redeployed buses to prioritize transport of low-income workers to hospitals and other critical facilities.

"The general trend we've seen is that as a share of ridership, the folks who are relying on transit to get to work are majority black or Hispanic, and younger," Transit spokesman Stephen Miller said.

In Denver, the Regional Transportation District is planning to slash service across its network because of falling ridership. Officials have limited ridership to 15 passengers per city bus and 30 per rail car. Security officers were posted at end-of-line stations to "prevent unsafe sheltering" by people experiencing homelessness. Like most transit agencies, it's not enforcing a mask requirement for riders.

"It really is tough. We've done everything we can to make it easy for people to be our partners in safety. But people have to participate in this to work," says RTD spokeswoman Pauletta Tonilas, referring to the the lack of mask wearing. "It's up to people's personal responsibility and choice. There's only so much of this that RTD can control."

RTD is providing about 120,000 trips each weekday, Tonilas said, down from about 390,000 on a normal workday before the outbreak. She said transit agencies across the country are struggling with how to best serve essential workers while keeping riders and drivers safe.

"We're all making this stuff up as we go. We are literally building the plane as we are flying it, and everybody is doing the best we can," she said.

At a bus stop on Denver's Montview Boulevard, Kevin Long, a dishwasher at Saint Joseph Hospital, said he's started walking a mile down the street to pick up a quieter line. His usual route is filled with people living on the street who are riding the now-free buses. Long, 44, worries some of them might be sick.

"There were just so many people on it," he said of his old route. "With this route, it's a little weird for sure. It's like it's always 3 a.m., it's empty and the only other person on there is drunk. And it takes like 20 minutes now, instead of 50, because it doesn't have to stop as much."

Long, who doesn't have a car, wasn't wearing a mask. He said riding the bus to work so the hospital can keep running is the right choice, despite the risk to himself and others.

At work, the hospital takes his temperature twice a day, and he constantly washes his hands and wears protective clothes while working in the kitchen.

"It feels like I'm making a difference, saving lives one dish at a time, so to speak," he said.