The Centers for Disease Control and Prevention recently released major guidelines on how U.S. offices should function as people return to work during the coronavirus pandemic — including advice that reverses years of public policy guidance on how people should commute to the office.
Instead of taking public transportation or carpooling, the CDC suggests people drive to work by themselves if feasible and advises corporations to provide incentives for employees to drive by themselves.
The new guidelines raised concerns over what could be unbearable traffic congestion and a surge in carbon emissions if people turn to cars in order to avoid exposure to the virus.
"Promoting private vehicle use as public health strategy is like prescribing sugar to reduce tooth decay," said University of British Columbia urban planning and public health professor Lawrence Frank.
The challenges will grow more acute if residents abandon cities for less densely populated suburbs, a trend that may be getting underway. Real estate service provider UrbanDigs recently analyzed new sales contracts divided by new listings to gauge relative demand, and found it was down in Manhattan but higher in Westchester County in New York, Greenwich, Connecticut, and Bergen and Monmouth counties in New Jersey.
"The level of vehicle dependence created by urban sprawl is a primary cause of [carbon] emissions and climate change, which has arguably even larger threats to life," he said. "Air pollution from car dependent development and commuting is a primary source of diabetes and heart disease."
Although it's unclear what commuting will look like as more people return to offices, there are already signs that people are turning to driving cars instead of using mass transit.
Data published by Apple Maps shows a nationwide surge in direction requests for people driving in cars over the last several months, while direction requests via mass transit have remained consistently low since plummeting at the start of the outbreak. During April and May in New York City, search demand for monthly parking in the city almost doubled on the parking app SpotHero.
And outside of the U.S., cities that have reopened in China and Europe had a surge in car traffic and higher than normal congestion levels during regular commuting hours.
Officials have criticized the CDC guidance as encouraging gridlock traffic in crowded cities, even if the advice may be more effective in rural areas.
Transportation experts warn that dense cities that have people commuting from outside suburbs cannot handle a sudden surge in cars on roads and bridges.
"Our roads cannot handle the increase in demand that will come from increased vehicle dependence. Congestion levels will likely become unbearable," Frank said.
For instance, almost half of New York City residents said in May that they won't take public transportation when the city reopens, according to a survey by research company Elucd.
Before the pandemic, more than half of New York City's population used the subway, but the city has since experienced a 90% decline in Metropolitan Transportation Authority ridership.
"Encouraging people, especially those without cars and in congested areas like New York, not to take public transit is misguided," MTA Chairman Patrick Foye said in a statement last week.
"Transit is, and has long been, the safest way to move around any city," he said. "Our transit and bus system is cleaner and safer than it has been in history, as we clean and disinfect around the clock."
Some transportation experts recommend that cities address problems of traffic gridlock by creating new bike lanes to handle an influx of commuters trying to avoid public transit. Some cities have seen an increase in memberships for bike-sharing programs during the pandemic.
Corporate strategies include dividing employee schedules from working remotely a few days a week while others come into the office, as well as staggering start and end times for businesses to avoid peak rush-hour traffic.
The shift away from mass transportation due to fears about contracting the virus could also create problems for efforts to combat climate change.
The transportation sector generates the largest share of greenhouse gas emissions in the country, with cars and trucks collectively accounting for roughly one-fifth of the country's emissions, according to the Environmental Protection Agency.
"If the virus decreases public transit use and increases single-passenger car trips, that change could be ingrained for years and would be devastating for climate action," said Rob Jackson, a professor of Earth system science at Stanford University and chair of the Global Carbon Project.
"Vehicle use is rising rapidly again," Jackson said. "We are inching towards 'normal' traffic again."
U.S. carbon dioxide emissions are projected to decrease 11% in 2020 because of the outbreak, according to the Energy Information Administration's May short-term energy outlook.
Carbon emissions are expected to rebound 5% next year as restrictions are lifted and the economy reboots, and vehicle traffic alone in half a dozen states has already returned to 2019 levels.
Zeke Hausfather, a climate scientist at the University of California, Berkeley, said that if some companies continue to allow employees to work from home and not return to the office, then the U.S. could potentially experience persistent reductions in transportation emissions after the pandemic subsides.
"There has been a lot of excitement about whether teleworking may persist after restrictions lift, but I think it's just as likely that ridership on many public transit systems will drop," said Steven Davis, an Earth scientist at the University of California, Irvine.
"There are probably limits to this kind of personal transportation rebound. How many people will go out and buy a car if they didn't already own one?" Davis said. "I suspect the answer to that question will determine whether health concerns drive an increase in car commuting or teleworking."
— Chart by CNBC's Nate Rattner.