The Orange County Transportation Authority has secured a $629-million federal loan for its 405 Freeway expansion project, the agency said.

The funds from the U.S. Department of Transportation will pay a major portion of the $1.9-billion effort to improve a nearly 14-mile stretch of the 405 between the 73 Freeway in Costa Mesa and 605 Freeway near Rossmoor.

When the project is complete, OCTA officials say, both directions of the 405 will have an additional general-purpose lane between Euclid Street and the 605.

There also will be a new “express lane” toll option on the 405 between the 73 and 605. In both directions, OCTA plans to build a new express lane and convert the existing carpool lane into an express lane.

Construction is expected to start in 2018 and last through 2023.

OCTA officials said the loan has a low interest rate that’s expected to save taxpayers about $300 million over its 35-year life, compared with traditional bond financing. The loan will be repaid using revenue from the new toll lanes, the agency said.

The project also is being funded with county Measure M funds as well as money from various state and federal sources.

For the first 3½ years after the 405 express lanes open, single drivers using them will pay a toll at all times, OCTA said. Carpools of two people will be able to use the lanes for free during non-peak hours, and carpools with three or more people will be free all day.

Starting in mid-2026, two-person carpools will pay a toll at all hours.

bradley.zint@latimes.com

Twitter: @BradleyZint