San Francisco’s car ban on Market Street — introduced a month ago after 10 years of handwringing — has barely affected motorists, with almost no spillover traffic on side streets. But it’s a huge improvement for buses, streetcars and bicycles.

Congestion increased only marginally on nearby roads, according to new data from the traffic analytics firm Inrix. It shows that the biggest slowdown occurred on Mission Street, where southbound vehicle speeds decreased by 4% — from 10.3 miles per hour to 9.9 miles per hour — during the 8 a.m. commute. On other adjacent streets, car speeds declined by an average of 1%.

On the flip side, transit riders on Market Street benefited significantly from the removal of cars. Muni lines are running 6% faster on average, said Erica Kato, an agency spokeswoman. Some bus lines shaved 12% travel time, which means rides are two minutes shorter.

Cyclists are thriving. Since the city banished private automobiles from its main spine on Jan. 29, the number of bicycles increased by 25%.

These outcomes mirror those of the 14th Street busway in New York, a major east-west Manhattan thoroughfare that officials closed to private automobiles in October. It’s had “very little negative impact” to drivers on surrounding roadways, said Trevor Reed, a transportation analyst at Inrix.

In downtown Seattle, officials bar cars from driving on Third Avenue between 6 a.m. and 7 p.m., a change that took effect in August 2018. Every weekday the buses on Third move 52,000 people — “as many people as all the other avenues combined,” Reed said.

His conclusion: When cities shut streets off to cars, people drive less. The myth of a “carmageddon”-style traffic jam is apparently overblown.

“People’s choice to drive is a lot more flexible than we typically think,” Reed said, noting that motorists adapt by not using their cars for shorter trips.

Some may even switch to buses, now that Muni is moving faster.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan