Metro plans to ramp up bus service on 23 special corridors to shuttle riders downtown on Inauguration Day, helping take pressure off what officials expect to be a packed rail system.

The transit service also will offer a special all-day bus ticket for $5 that riders can buy at the farebox as they get on board, giving them another option besides paying for individual fares or the more expensive unlimited bus/rail passes.

Officials expect between one million and five million people to converge on downtown Washington when Barack Obama is sworn in as president. Federal and regional officials are encouraging people to take public transit or walk on Jan. 20, rather than driving a car.

But Metro trains are limited in how many people they can carry. At most, the transit agency expects to be able to carry one million trips on its rails that day, despite offering 17 hours of rush-hour service.

Officials are especially concerned that rail cars will fill up in the suburbs, leaving people inside the city stranded. Metro plans to run some trains, called gap trains, to serve those areas, but they worry that won’t be enough.

 “Taking Metrobus is a good option for people who are located beyond a two-to-three mile walking distance to the National Mall,” General Manager John Catoe said in a written statement.

The transit agency’s buses can help carry an additional 600,000 trips, according to Metro Planning Manager James Hamre.

Most buses will run on a modified Saturday schedule that day. But the new special corridor buses would run at least every 10 minutes from 4 a.m. to 9 p.m., according to Hamre, matching the special rush-hour service offered on the rail system.

All but one of the special corridors will use existing bus routes. Metro also is creating a special route from RFK Stadium, where many charter buses will park.

Yet the entire bus system will be challenged by road closures that day. Although buses have special access to many bridges, a large swath of downtown will be shut off to any traffic. That means some bus lines will be detoured, while others will turn around at the Secret Service’s security perimeter.

Thus, Metro officials caution that even if people can get downtown, they need to be prepared to walk several miles to get around security closures.