Since cities came to exist 5,000 years ago, epidemics have shaped their fate.

Plagues weakened the Roman Empire and may have helped bring it down. The sewers that cleaned up a filthy London in the 19th century were built in direct response to a cholera outbreak. Many of the great urban parks, including Central Park in New York City, were similarly planned after epidemics, to provide more open space.

Today, the coronavirus pandemic, in all its horror, opens the prospect of sweeping urban change. Cities suddenly see the possibility of correcting their greatest mistake of the 20th century, the surrender of too much public space to the automobile.

Cities need to seize this moment and move at lightning speed. We need to find a better balance between the cars on our streets and the bicyclists and pedestrians who have, for decades, been neglected and pushed to the margins.

All over the world, forward-looking cities large and small have already jumped into action. In Medellin, the innovative Colombian city nestled in the Andes, workers are seizing traffic lanes and slapping down yellow paint to signify a change: Cars have been evicted and the lanes are now reserved for bicyclists. In Kampala, the capital of Uganda, the authorities have closed streets, encouraged cycling, and sped the construction of new bike lanes and walkways. In European cities, “corona cycleways” have become the new norm.

In New York, the city has responded to community demands by pledging to set aside 100 miles of roads in the next few weeks for people on foot or bike, largely closing the streets to traffic during daylight hours. Letting people dine at tables in the middle of the road may help in the salvation of New York restaurants. Across the country in Oakland, Calif., the city has decided to close nearly 10 percent of its streets. And in the middle of the country, Kansas City, Mo., was one of the first to limit traffic and turn parking spots into mini-parks to extend restaurant service.

This is a golden moment for the movement known as tactical urbanism. More than 200 cities have already announced road closings in response to the coronavirus pandemic. Thousands of cities have yet to act in any bold way, however. If they do not, they may miss what could be a once-in-a-lifetime opportunity.

The circumstances that give rise to this situation are lamentable, of course, just as were the cholera epidemics that altered cities in the 19th century. Bicycling is booming — bike stores are reporting record sales and order backlogs — as people look for easier means to get around and find streets with reduced traffic to be safer and more congenial. Cities are finding they can make bold moves to accommodate all the new bikers and walkers because the drivers who would normally object to street closings are hunkered down in their homes.

The suppression of automotive traffic is giving us a vivid illustration of the potential future benefits of cleaning up our cities. Air pollution, which kills millions of people every year, is down nearly everywhere. In Mexico City, measurements of the smallest, deadliest particles have fallen by about half. The Indian government has publicly reported that several pollution measures are down as much as 70 percent in New Delhi; in some cities, Indian children are able to see distant mountains for the first time in their lives.

Most of the road closures announced so far have been billed as temporary, meant to last until the pandemic loosens its grip. The willingness of drivers to leave their cars parked is certainly not going to last. What can cities do to make sure they hold on to the recent gains as the economy reopens?

To answer that, we return to a phrase we used earlier: tactical urbanism. For the last couple of decades, this movement has been seizing moments of opportunity to improve urban life.

Sometimes a city government is the instigator, as in 2009, when New York closed several blocks of Broadway, one of the busiest streets in the city, to traffic. Sometimes citizens employ guerrilla tactics — converting a vacant lot into a miniature park or garden, for instance, or throwing up orange traffic cones in the middle of the night to create a bike lane.

The basic idea is to show people the benefits of a change, however temporary, in order to shift the political dynamic in favor of a more permanent alteration. You can bet that parents whose bored children are suddenly able to ride their bikes in the Oakland streets are seeing this whole set of issues with new eyes.

When Broadway was closed, thousands of New Yorkers flooded the street, delightedly plopping down in cheap lawn chairs the city had set out on the pavement. From that moment, the vision of a Broadway for people took hold, and the blocks of Broadway through Times Square have been closed to traffic for a decade.

Similarly, tactical urbanist projects all over the world have led to closed streets, new parks and many other amenities. A large majority of these projects entail reclaiming public space from the automobile. A third or more of the space in any city is devoted to streets, and in the middle of the last century, much of that was converted to traffic lanes and free parking spaces.

Today, we have been thrust into perhaps the greatest opportunity ever for tactical urbanism. With traffic missing from the streets, people are sensing how completely cars dominate them in normal times, endangering the lives of the pedestrians and cyclists squeezed into tiny strips along the margins. This situation was never sensible or moral, but until now, fixing it was politically impossible in many cities.

A viral twist of fate has given us a chance to alter the balance, creating streets that work for everyone. Cities that were thinking about lane changes or street closures before the pandemic should move quickly to try them out, and the most popular should be made permanent. Government leaders must pay particular attention to poor neighborhoods, which tend to be forgotten but whose people have just as much right to bike and walk as anyone else. Those neighborhoods are often deprived of parks or sports fields, so a street with few or no cars can be a godsend for children.

In the end, reclaiming streets will not be enough to lock in improved air quality and other benefits. Every city needs a comprehensive program of car control. Some, like London, are already banning the most polluting vehicles, and a few have gone so far as to declare they will no longer allow fuel-burning engines after 2030 or 2035. In those towns, you will drive an electric car if you drive at all.

Cities need to follow the lead of London, Singapore and more recently New York in enacting stiff congestion charges that discourage unnecessary driving, with the money plowed into mass transit, as well as more protected lanes for walking and cycling.

Cities need to be designed for the well-being and health of people, not for cars. We don’t have time to wait. Now is the moment for cities to imagine that future and start willing it into being.

Heather Thompson (@hfthompson_) is the chief executive officer of the Institute for Transportation & Development Policy, which promotes sustainable and equitable transport worldwide.