The default speed limit on D.C. streets will drop to 20 mph from 25 mph beginning Monday, Mayor Muriel E. Bowser (D) announced Friday, taking a bold step in implementing her centerpiece plan to reduce pedestrian deaths in the nation’s capital.

The Bowser administration, in its second term, has claimed that slower speeds can help improve safety. Pedestrians are less likely to die when hit by a car traveling at the lower speed, according to research cited by the city.

Even so, drivers in the District may need some time to embrace the change. The 25 mph speed limit has been in effect in the city for decades — at least since the ’70s. There have long been calls for speed reductions in many neighborhoods, and in recent years, they have grown in response to a rash of fatal high-profile crashes involving pedestrians. Law enforcement officials cite speed as a factor in many of the District’s deadly crashes.

Concern among officials in the District and other major cities has only grown in recent weeks. The stay-at-home orders to slow the spread of covid-19 may have decreased the amount of traffic on roads, but there also has been an increase in speeding — sometimes dangerous speeding — on highways and local roads.

“One thing that we have for sure learned with less traffic on the street is that people are driving faster and we see it all over,” Bowser said at a news conference. “So we are making the default speed limit on local roads 20 mph.”

And, she said, the new limit will be permanent.

“While it may seem like a small change, we know that surviving accidents is strongly correlated to speed, and lowering the speed limit will help us keep people safe,” she said.

In 2019, 12 pedestrians and two bicyclists were killed in traffic crashes, down from 15 pedestrians and three bicyclists in 2018, according to D.C. police data.

Bowser also announced a “slow streets” initiative through which some neighborhood roads will be restricted to local traffic only and have a posted 15 mph speed limit. The District Department of Transportation is identifying locations, she said.

While those roads will not be closed to other traffic, Bowser said, the city will place barriers and signage about the restrictions so that drivers know they should not cut through.

As the city begins to reopen after two months of coronavirus restrictions, Bowser said it is reimagining its roadways to support businesses. The city will issue permits to restaurants to use sidewalks, and possibly parking and travel lanes, to expand their dining space, officials said.

Businesses will be able to apply for a permit to expand their outdoor space starting Friday, officials said. Bowser said the city continues to take suggestions for roadway closures and wider sidewalks to support the use of public spaces post-pandemic.

Traffic safety advocates on Friday celebrated the lower speeds and safety initiatives, saying that 20 mph is enough on city streets.

Greg Billing, the executive director of the Washington Area Bicyclist Association, called the announcement a “great first step” in preparing the city for reopening.

“The transportation system must adapt to the current covid-19 public health emergency and economic crisis,” Billing said. “Space for social distancing will encourage those who can to walk and bike while keeping capacity for those who must drive or take public transit.”

But some drivers have expressed concern about the impact lower speeds could have as traffic returns.

Alcer Berhe, a cabdriver who works in downtown Washington, earlier this year said he was not pleased to hear about plans for a new speed limit. He said traffic congestion in the city already irritates passengers, who he said want to get to their destination fast.

“The speed limit at 20 is not enough,” Berhe said, noting the speed reduction would only delay trips in one of the most congested cities in the United States. “It’s bad.”

City workers will be replacing about 1,400 signs to alert drivers to the new default speed limit. The limit does not apply to major commuting corridors, such as Wisconsin and Georgia avenues, that have their own posted speed limits. It does, however, apply to about 60 percent of the city streets and smaller roadways that generally have parking on one or both sides of the road, DDOT spokeswoman Lauren Stephens said. And the vast majority of those roads don’t have painted centerlines, she said.

Stephens also noted that the city does not have speed cameras on those local streets.

Jurisdictions across the United States, including others in the Washington region, are embracing lower speed limits as the key to reversing a rise in traffic fatalities. Efforts include lowering default speed limits in major corridors and creating slow-driving zones in areas with heavy pedestrian traffic.

In December, Seattle announced it was lowering speed limits on all high-traveled arterials to 25 mph, after a deadly year on the streets. New York City dropped its default speed to 25 mph from 30 mph in 2014, also targeting a reduction in pedestrian deaths.

In the Washington region, Montgomery County last year lowered speed limits on some roads — including dropping the speed limit on portions of Georgia Avenue from 35 mph to 25 mph — in an effort to make the road safer for pedestrians. Alexandria lowered the default speed limit in some neighborhoods by 10 mph in 2016 to 25 mph.

Lower speed limits are part of Vision Zero, an initiative embraced by communities across the country to create safer streets for all. Bowser made Vision Zero one of her signature initiatives after taking office in 2015. The program has not led to significant reductions in road deaths. After years of traffic fatalities trending up, the city reported 27 deaths last year, a decline from 36 in 2018, but one more death than in 2016.

While the speed limit changes may be judged as a traffic-slowing inconvenience to some drivers, transportation experts and city officials say that for people who get struck, it could make a difference in how likely they are to survive.

Research by the United Kingdom Department of Transportation found that the chances for survival for someone struck by a vehicle driving at 20 mph is 90 percent. But that drops to 50 percent when the striking vehicle is traveling 10 miles faster. The fatality rate for older people getting struck at speeds above 20 mph is even higher, according to research.

Experts also say a driver going slower is able to react better when encountering pedestrians or cyclists on the road.

Still, some critics argue that lowering speeds too much can have significant impacts not only on travel times but also on safety. It could lead to unsafe lane changes and more crashes as motorists try to get around ­slower-moving vehicles, some say.

Berhe, the taxi driver, said people drive fast until they come across a speed camera and quickly and dangerously hit their brakes. He said he fears a lower speed limit would increase those incidents.