A street design revolution is under way in San Francisco’s downtown and waterfront neighborhoods, the bustling heart of the city and the site of many deadly car crashes.

New protected bike lanes wrap around the Caltrain station, with one under construction along Townsend Street between Fourth and Fifth streets, and another completed on Seventh Street, from Townsend to Sixteenth Street. On Sixth Street — where data released last year showed a pedestrian got hit by a car every 16 days — engineers are winnowing four traffic lanes down to three and putting up posts to prevent cars from parking right next to crosswalks.

And this week the board of directors for the San Francisco Municipal Transportation Agency unanimously approved another protected bike lane along Fifth Street, stretching from Caltrain to Market Street. On average one person gets injured each week in traffic collisions on Fifth Street, a disturbing trend that the SFMTA hopes to reverse, said agency planner Thalia Leng. The renovation would also widen sidewalks and change the geometry of the road, shaving it from four traffic lanes to three.

Crews will begin re-striping the street within six weeks, finish concrete work next winter and upgrade intersections in 2021.

Officials in City Hall characterized Fifth Street as the final piece in what’s now an intricate SoMa bicycle network, linking downtown to China Basin, the Mission District and Mission Bay. For neighborhoods that long served as a quick route to Interstates 280 and 101, it’s a dramatic transformation.

“I’m sure many of you are tired of the streets in SoMa being treated as freeways,” said Courtney McDonald, legislative aide to Supervisor Matt Haney, who represents the area.

“As it currently exists, Fifth Street is simply not designed to keep pedestrians and bicyclists safe,” Mayor London Breed said, noting that city surveys show that 45% of people feel safe walking along the corridor, 7% said they felt safe biking and only 25% said they can find reliable transit.

“That’s simply unacceptable,” she said. “This project will protect pedestrians and bicyclists, and our new quick-build policy will allow us to make immediate safety improvements while long-term changes are being made.”

The frenzy of quick-build projects represents a culture change at SFMTA, long known for its sluggish bureaucracy and inability to overcome political opposition.

In past years, infrastructure as simple as a raised crosswalk or a paint-and-post bike lane would run through a gantlet of community meetings, where residents and merchants would rail about lost parking or the narrowing of a road. Sometimes neighbors would kill a project all together. Often they would slow it down or significantly alter the plans.

Fifth Street was among the projects that appeared to be paralyzed, originally conceived as part of a 2009 bike plan that got shelved to accommodate construction of the Central Subway. In the intervening years, the artery became a “hellscape,” according to cyclists who spoke at the meeting. Charles Deffarges, senior community organizer at the San Francisco Bicycle Coalition, described scenes in which cyclists — some towing children in trailers — are sandwiched between trucks, buses and parked cars “with only scraps of paint encouraging them to be bold and ‘take the lane.’”

That all changed when the agency enacted a new quick-build policy in June, allowing planners to submit a list of urgent redesigns and get them approved all at once.

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