As coronavirus lockdowns loosen around the world, city leaders are scrambling to address a new problem: the prospect of gridlock worse than before the pandemic. From Shenzhen to Milan to Austin, officials are trying to coax people back onto buses and subways and reclaim road space for cyclists and pedestrians.

In many cities, officials worry that people will avoid public transit for fear of catching the virus, and decide to drive instead, which will push vehicle traffic higher than ever. Staving off a surge of cars on city streets is important not only to avoid congestion delays, accidents and higher air pollution, which kills an estimated four million people worldwide each year. It’s impossible to stop global warming unless cities sharply reduce pollution from cars, trucks and motorcycles.

"Cities have a window of opportunity to make changes and keep the cleaner air they saw during the lockdowns," said Corinne Le Quéré, a climate scientist at the University of East Anglia who has tracked global carbon dioxide emissions during the pandemic. "But if they don’t pay attention to this issue, emissions could rebound back to where they were before or even go higher."

Transportation accounts for a quarter of the world’s carbon dioxide emissions, and emissions from road vehicles in particular have grown sharply in the last 50 years.

There are already warning signs: More than 30 large cities coming out of lockdown, including Hong Kong, Shenzhen, Oslo and Geneva, recorded more congestion on their roads in mid-June compared with the same period last year, according to data from TomTom, a navigation company. Other early evidence suggests that driving is increasing faster than public transit use as people step out of confinement and move around again.

Many city leaders are trying to fix that, in some cases leveraging lessons learned from earlier pandemics in Asia. Here’s a look at some of what they’re trying.

The pandemic has given leverage to city officials to do things that had been politically contentious in the past, like taking space from cars.

San Francisco, where bus ridership declined by around 80 percent between early March and late May, has opened up 24 miles of car-free corridors for walkers and bicyclists to get around; another 10 miles are in the works, and most of these corridors span several city blocks.

Bogotá, Colombia, which had in the past carved out bike lanes on sidewalks, has now set aside 52 miles of road space for cyclists. It was intended as a temporary measure, said Nicolás Estupiñán, the city’s transportation secretary, but public support has emboldened the city to make it permanent.

Mr. Estupiñán said Bogotá was also staggering work hours for different industries — a 10 a.m. start for construction, 12 p.m. for retail, and so on — in order to make the roads less congested.

Milan has also made its pandemic-era network of bike lanes permanent. "The physical distancing requirements of Covid gives us huge leverage," said Maria Vittoria Beria, a spokeswoman in the Milan mayor’s office. "What did we have in the drawers that could help social distancing? Bike lanes."

They are being used — at least for now, when the weather is mild. Data from the city suggests that bike sharing and electric scooter use rose sharply in May, while traffic congestion remained well below 2019 levels.

In other cities emerging from lockdowns, including Berlin, London and Paris, data collected from bicycle counters indicated that cycling had become more popular than it was before the pandemic, according to Felix Creutzig, a transportation specialist at the Mercator Research Institute on Global Commons and Climate Change, a think tank in Berlin.

But as cities reclaim streets from cars, they are also struggling with deep inequities in access to transportation. New York City, for instance, has historically built fewer bike lanes and bike-share docks in neighborhoods that are home to large shares of essential workers, the majority of whom are people of color.

Some cities have been trying to dissuade drivers from bringing older, more polluting vehicles into city centers, mainly by imposing levies to enter congested areas during rush hour.

In May, as its lockdown loosened, London began reinstating low-emissions zones around the city, which impose fees on older cars, trucks and vans that don’t meet air pollution standards. The city also recently raised its congestion charge by 30 percent, requiring many drivers to pay $18 per day to enter the busiest parts of central London.

In Pôrto Alegre, Brazil, where bus ridership has plunged 60 percent during the pandemic, city officials are worried about a death spiral for the system. They have proposed both a congestion tax on private vehicles entering the city as well as a per-mile tax on ride-hailing services like Uber, with the goal of plowing that money into the bus network to reduce fares.

Still, officials concede that targeting private cars can be difficult in a struggling economy. New York City had planned to become the first American city to impose a congestion tax at the end of 2020, but the measure’s fate is now unclear. Last week, as New York allowed more nonessential businesses to reopen, the Department of Transportation warned in a sign on the Robert F. Kennedy Bridge: "Anticipate traffic."

While ridership on buses and subways has cratered during the pandemic, public transit remains critical for essential workers and those who don’t have a car. One recent study in New York City found that subway ridership fell less sharply during the lockdown in neighborhoods with more low-income and nonwhite residents.

To make public transportation safe, many cities have focused on mask-wearing and constant cleaning. In Seoul, masks are required on mass transit and because talking can spread the virus, noisy conversations inside subway cars can prompt complaints to the authorities. Taipei has begun temperature checks at train stations.

Some are using more high-tech solutions to keep passengers at a safe distance: Beijing’s transit agency now allows essential workers to reserve bus seats by mobile app and provides custom routes to transport these workers, allowing for space between seats. Denmark’s rail company, DSB, introduced an app showing which cars have the most space available, which helped increase transit ridership as lockdowns eased.

Many transit officials remain optimistic that bus and train ridership will eventually return, citing early evidence that few people have caught the virus in large, crowded transit systems like Tokyo’s, as long as people wear masks and keep to themselves. But in the meantime, many cities are facing severe financial crunches as revenue falls and budgets are strained.

"Without help, some systems may not survive, and others may have to reduce their service or hike fares," said Paul Skoutelas, president of the American Public Transportation Association, which has called on Congress to provide additional aid to help transit agencies weather the storm.

Even amid the crisis, some transit agencies are reimagining public transportation altogether.

In Austin, Tex., the city has expanded its system of public shuttles that can be reserved through a mobile app by riders who aren’t well served by existing bus lines. Officials are also drawing up plans to better integrate existing bus and rail lines with the city’s bike-share system by offering unified ticketing and apps. They also plan to eventually replace the city’s 1,000 shared bikes with electric versions that make travel easier in the sweltering Texas heat.

"The pandemic has really pushed us to think more creatively," said Randy Clarke, president of Capital Metro, the Austin public transportation system. "How do we make a system that’s more equitable and sustainable, and give people more options besides cars?"