U.S. Transportation Secretary Anthony Foxx on Thursday announced a $275 million loan to the state to help build the Northwest Corridor project.

The loan will cover more than a quarter of the $833 million project that will add nearly 30 miles of reversible toll lanes along I-75 and I-575 in Cobb and Cherokee counties. It is the most expensive road project in state history.

The project is expected to be completed in 2018.

The loan is part of the federal Transportation Infrastructure Finance Innovation Act. The money must be paid back over 35 years and carries a 3.79 percent interest rate. The state does not have to make any payments for five years.