Starting Friday, eastbound commuters on the Richmond-San Rafael Bridge will have a third lane to drive on — but, for now, during their evening rides.

Caltrans officials are expected to flip a switch sometime Friday, illuminating an electronic “Right Lane Open” sign on the bridge’s Marin County side and 20 sets of overhead green arrows indicating that vehicles can pour into the new lane.

After Friday, the green arrows will show over the lane when it is open for traffic — typically between 2 and 7 p.m. — as well as over the two other lanes of traffic. Red X’s will be displayed above the new lane when it’s closed. Green arrows will appear over the two other lanes permanently, unless they’re closed by an accident, emergency or construction.

Caltrans crews will monitor the bridge by camera from their traffic command center in Oakland and can open the lane early, if warranted, to keep eastbound traffic flowing.

Commuters and transportation officials hope the additional lane will help ease the eastbound afternoon and evening peak-hour commute, which currently backs up through San Rafael and also along Sir Francis Drake Boulevard, where traffic trying to get to the bridge lines up past San Quentin prison and Larkspur Landing and beyond to the Highway 101 off-ramp.

The additional lane is part of a four-year experiment to see how effective the changes are in easing traffic.

“We hope to eliminate the backups on 580 itself as well as some of the traffic on Sir Francis Drake,” said John Goodwin, a spokesman for the Metropolitan Transportation Commission, which is overseeing the widening. “This isn’t going to eliminate the friction, if you will, on Sir Francis Drake, but it will improve it drastically — at least we hope so.”

The Richmond-San Rafael Bridge opened in 1956 with three lanes in each direction, but in 1977, the span was so lightly traveled that authorities decided use the lane for a pipeline to transport water to Marin County during a drought. The pipeline was removed in 1978, and the former lane became a shoulder.

But traffic has swelled in the past decade — about 13 percent over the past five years alone — and backups are routine, especially with eastbound traffic in the evening. Morning congestion, heading west, has also worsened.

Still, it took more than three years and $74 million to reopen the third lane. The commission, the Bay Area Toll Authority, Caltrans, and Marin and Contra Costa transportation agencies agreed to add the eastbound traffic lane while the westbound upper deck would get a bike and pedestrian lane protected by a movable barrier, which would also shield workers performing critical bridge maintenance.

Construction crews also needed to rebuild the San Quentin on-ramp to the bridge, push a tall retaining wall on the Richmond end of the bridge back 15 feet and expand an off-ramp to Richmond Parkway.

“To those who say, ‘You just have to get out there with a can of paint,’ I say, ‘Think again,’” Goodwin said.

Construction on the two-way bike and pedestrian path is expected to begin soon after a delay, and the path is scheduled to open in early 2019, despite a Marin County supervisor’s attempts to turn the westbound lane over to motor vehicles in the mornings.

Last month, the MTC agreed to study the feasibility of the proposed change, which would require work on the Richmond and Marin approaches to the bridge, and whether it would have much of an effect on morning backups.

Bicycling activists in the East Bay are distressed by what they see as a reconsideration. But Goodwin said the bike lane will be built, and the plan is to keep it open for only non-motorized commuters for the next four years.

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