Nearly everyone in the Bay Area is sticking close to home during the coronavirus shelter in place, which has left the region’s roadways unusually empty and sped up typically congested commutes for the relatively few people still on the road.

Caltrans is also taking advantage of the empty lanes, accelerating everything from guardrail installation and pothole plugging to big projects that typically tie up traffic for weeks or months. BART, which is stopping its trains hours early each night, is also taking advantage of the shelter-in-place slowdown by speeding up work to revitalize the region’s rail system.

The goal for these agencies is to get construction work done before the Bay Area reopens and congestion starts to return.

“This is getting a lot of talk in Caltrans and BART and other agencies as well,” said Randy Rentschler, spokesman for the Metropolitan Transportation Commission. “They want to get as much work done as possible.”

The biggest impact is being felt in San Francisco, where the Alemany deck replacement project at the Highway 101-Interstate 280 interchange had transportation officials previously wringing their hands and predicting a “Carmeggedon.”

But traffic is a lot lighter now — Caltrans reports Bay Area freeway traffic is down as much as 70% — so the state agency and its contractor will start work Friday at 6 a.m. to replace 800 feet of decaying concrete deck slabs that were installed a half century ago.

At least three other Bay Area projects, including extensive paving on a new stretch of the Marin-Sonoma Narrows project north of Novato on Highway 101, are also being expedited.

“If we can get this work done now, it won’t impact people nearly as much, and it will be done when people return to work,” said Bart Ney, a Caltrans spokesman.

The projects include:

• Marin-Sonoma Narrows:

Contractors started paving work last week on the years-long collection of projects to widen Highway 101 between Novato and Santa Rosa. For the next three weeks, crews will pave a 1-mile stretch of Highway 101 in Sonoma County in each direction starting at the Petaluma Boulevard South/Kastania Road junction, before moving south to within 2 miles of the Marin County border. Two lanes of traffic will be closed in each direction between 7 a.m. and 8 p.m.

The work had been scheduled to take place between June and August at night but was moved ahead — and into mostly daylight hours — because of the light traffic, Caltrans spokesman Jeff Weiss said.

• Highway 1 in San Francisco between Lincoln Way and Lake Street:

Crews started grinding down the roadway and laying new pavement last week along the stretches also known as Park Presidio Boulevard in the Richmond District and Crossover Drive through Golden Gate Park. The work will continue through early May.

Lighter traffic is allowing the construction to be done mostly during daylight hours — 9 a.m. to 9 p.m. daily — when it’s safer, takes less time and costs less, Caltrans officials said. Crews are working in two of the three lanes, leaving at least one open in each direction. Despite reduced traffic, delays are anticipated. The work could be suspended, Ney said, if traffic avoiding the Alemany project overwhelms Highway 1.

• Highway 29 roundabouts in Napa:

Caltrans and Napa are building a series of roundabouts to relieve traffic congestion and make it easier to get downtown from the freeway. Work on three roundabouts near Highway 29 and First and Second streets, as well as California Boulevard, should be completed earlier than the original schedule of late 2020, but a specific date is not available.

Crews are also taking advantage of the traffic breaks to complete maintenance and other freeway work around the state.

“But Alemany is by far the big daddy in the state,” Ney said. “Moving that project forward is the most important thing we can do.”

Officials with the Metropolitan Transportation Commission say they’re working with Caltrans and speeding construction on a project to improve the Bay Bridge metering lights, improve access to the Richmond-San Rafael Bridge and advance work on the Interstate 680 southbound express lanes.

BART is now replacing 500 feet of rail each night, which is five times more than before hours changed. The agency also has started work on electrical cable replacement in San Francisco, advanced work on the 19th Street and El Cerrito Del Norte stations modernization projects, and begun work to protect the Transbay Tube against corrosion.

“The addition of just a few hours a night of wrench time is having a dramatic impact on infrastructure replacement projects,” said BART spokesman Jim Allison.

However, it’s not possible to ramp up every project by simply sliding the schedule ahead a few months on the calendar. Contractors across the region may already be working on other jobs or have difficulty assembling the needed number of workers, especially with school and day care closures. Or contractors might be unable to obtain the concrete, steel, and other materials and equipment required for the project.

“Just because it makes sense to move a project forward doesn’t mean it can be,” Ney said.

It’s unclear if the shutdown has benefited major projects in the heart of San Francisco, such as the Van Ness Improvement Project and the Central Subway Project.

Erica Kato, a spokeswoman with the San Francisco Municipal Transportation Agency, said “work has been productive during the emergency,” but she did not offer details.

“Some teams are able to allocate more resources, and traffic has been light allowing us to expand some work areas,” she said.

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan