When the pandemic emptied New York of its usual traffic, the city’s streets transformed into an open speedway where drivers drag raced down major roads, racked up thousands of tickets and in some cases left fatal wreckage in their wake.

At the time, city officials saw the rash of reckless driving as an aberration that would vanish when the city’s usual traffic reappeared.

But as restrictions lifted this summer and traffic crept back toward pre-pandemic levels, the spate of speeding — and fatal collisions — did not end.

Now, alarmed by the sustained rise in fatalities and bracing for the possibility of a second lockdown that could worsen the current speeding crisis, city officials are reducing speed limits by five miles per hour on nine of the most dangerous streets across the five boroughs.

On Tuesday, officials will announce the speed limit will be lowered to 25 miles per hour — the standard limit on most the city’s roadways — on eight of those streets, including parts of Riverside Drive in Manhattan, Flatbush Avenue in Brooklyn, Northern Boulevard in Queens and Bruckner Boulevard in the Bronx.

The limit will also drop to 25 miles per hour on Shore Parkway Service Road and Dahlgren Place in Brooklyn, Webster Avenue in the Bronx and Targee Street in Staten Island.

On the ninth, Rockaway Boulevard in Queens, the limit will drop from 40 miles per hour to 35.

“People got in the habit of driving too fast and too recklessly when roads were more open, and unfortunately, we’re still seeing that behavior,” Polly Trottenberg, the city’s transportation commissioner, said in an interview. “We’re starting to get almost back to normal, but there are still times and places in the city where traffic levels are lower and drivers are able to get up to higher speeds.”

Already this year, more passengers, drivers and motorcyclists have been killed in car crashes than all of last year: 28 drivers, 16 passengers and 26 motorcyclists have died, according to city data.

In June, when traffic in New York City returned to around 80 percent of pre-pandemic levels, the number of passengers and drivers killed in collisions jumped 22 percent compared with the same month last year, according to data from the city and INRIX, a data collection firm. In July, things got much worse: Those deaths spiked 300 percent compared with last year.

Since Mayor Bill de Blasio vowed to eliminate all traffic deaths six years ago, his administration has lowered speed limits and enforced them with automated speed cameras, bringing traffic deaths to their lowest level in a century in 2018.

City officials have tried to crack down on reckless driving by installing 60 new automated speed cameras every month since the beginning of the year, bringing the total to nearly 1,000. The police department also increased speed-radar enforcement along some highways and deployed hundreds of officers to locations with many speeding drivers at the height of the lockdown.

But with the city’s sprawling subway system facing looming cuts and New Yorkers buying bicycles, scooters and cars in record numbers, many transit experts say that Mr. de Blasio needs to take more drastic action — like accelerating the creation of new busways and protected bike lanes, and restricting traffic into Manhattan during rush hours — to ensure streets are safe and functional.

“New York City is facing four existential challenges: the death spiral of public transit, ballooning car ownership, an increase in traffic deaths and serious injuries and the lack of a plan for addressing these from the mayor,” said Danny Harris, executive director of Transportation Alternatives, an advocacy group.

“The decisions we’re making now about street infrastructure will affect us for decades,” Mr. Harris added. “We should be taking decisive action now.”

Across the country, after the pandemic hit and traffic levels dropped more than 90 percent in some large cities, speeding — and the death rate from car crashes — surged.

In March, the rate of fatalities nationwide from crashes rose 12 percent, in May it jumped 34 percent and in June — the latest month when statistics are available — it rose 23 percent compared with the same months last year, according to the National Safety Council, an advocacy group.

“I think folks started to feel like the roads are emptier and it’s an open speedway for them,” said Lorraine M. Martin, president of the council. The empty roads may have also lured some drivers into a false sense of security, she added, leading them to ignore laws that mandate wearing a seatbelt and not driving impaired.

But in New York, the sustained rise in fatalities suggests that drivers who picked up reckless behavior during the lockdown have maintained it since — and may continue to imperil street safety.