Motorists took nearly 1.9 million trips on Interstate 580’s new express lanes in Alameda County in the first four months the lanes opened to vehicle traffic earlier this year, according to a recent report.

The $345 million project opened on the notoriously congested Tri-Valley corridor in mid-February to solo drivers, who pay an extra toll to use the lane, and to carpoolers, who can drive for free.

The first full month of operation saw around 549,000 trips along the east- and westbound lanes, growing to 647,000 trips in May — an 18 percent increase from March — according to a report presented to the Alameda County Transportation Commission on Thursday.

Tess Lengyel, ACTC’s executive director of planning and policy, said motorists are already seeing the benefits. Average hourly speeds in the express lanes are estimated to be between 10 and 33 mph faster than the average hourly speeds in general purpose lanes during the morning rush-hour commute, according to the report.

So far, Lengyel said there’s been a bit of a learning curve to using the lanes correctly. The majority of motorists — or around 67 percent — were either carpool or HOV-qualifying vehicles with a valid FasTrak Flex tag, or single-occupant cars with either a flex or standard FasTrak tag, which is required to use the lane. The remaining 33 percent either had no tags or invalid tags, according to the report.

“It’s a new system, so people are just getting familiar with the lane,” she said, adding that the agency has been seeing more people adopting the correct toll tags.

Drivers paid an average toll of $1.55 to use the westbound lane and $2.40 to use the eastbound lane in May, and have generated over $2 million in total revenue since the lanes opened, the report said. It remains to be seen, however, whether that revenue will cover the costs of operating the lanes, Lengyel said.

“We actually are watching this very closely because we haven’t gotten in all the bills to understand how much this is costing compared to revenue,” Lengyel said.

It took several years for revenues to cover the costs of operating the express lane on southbound Interstate 680, the first express lane to open in Northern California, though Lengyel said the lane is now paying for itself.

Rentschler said there’s almost been too much demand for the lanes on Hwy 237, whereas demand for the lanes on I-680 has been inconsistent. He was hopeful though that I-580 will strike the right balance.

“You have sustained traffic on I-580,” Rentschler said. “Everyone is confident the express lanes will do what they are supposed to do.”

That means the express lanes should save time for its users, relieve some pressure on the general purpose lanes and pay for their operating costs, he said.

Joel Ramos, the regional planning director for transportation advocacy nonprofit TransForm, said the lanes could also be a boon for public transit. He’s hoping the express lanes on I-580 will encourage express bus service on the freeway, especially for residents in the Livermore area and beyond who have limited access to BART.

“From our perspective, we would like to see those lanes getting more transit quickly so people who are using that corridor could actually have a viable alternative to driving,” Ramos said.