Expanding the Silver Line and developing bus routes are the MBTA’s top priorities, MBTA General Manager Michael Mulhern said Tuesday at a lecture at Harvard University’s John F. Kennedy School of Government.

Though it will focus on buses, the Massachusetts Bay Transportation Authority also plans to use advertising to improve the Green Line by having companies pay to rebuild stations in exchange for ad space. Mulhern said the renovations will begin in 18 months though he does not know which companies will fund the renovations.

Because of the size of the MBTA’s budget, “we simply can’t expand [the] subway system to every region” of Boston, Mulhern said. Instead of subway cars, he said the agency will expand the bus system and make the Silver Line – a bus route that goes from downtown Boston to Roxbury and will eventually connect to Logan Airport – one of the most vital transit routes in the city.

“The Silver Line is our number one priority,” he said. “If Boston is going to be a world-class city,” there should be a “connection between downtown and the airport.”

He added that the Silver Line is essential for transporting “inner-city residents” to jobs downtown.

Mulhern said the MBTA is buying 578 new buses and replacing two-thirds of its fleet. The Silver Line buses, which have lower floors that provide easy access for the disabled and use low-emission compressed natural gas, are the “first of their type in North America,” he said.

The compressed natural gas buses are far less harmful than the MBTA’s old buses, Mulhern said. In 2001, the MBTA measured the soot that leaves bus tailpipes and found that 90 tons of particulate matter, which causes air pollution, were released. After the MBTA began replacing the buses with the new Silver Line vehicles, only 11 tons of particulate matter were released, he said.

Mulhern said the time is right to invest in the Silver Line after years of neglecting Boston’s bus system.

In the 1980s, there was strong investment in the “core transit system,” as the MBTA developed subway routes, Mulhern said. In the 1990s, the MBTA refocused its efforts on developing the commuter rail. Though the investments drew criticism, he said they were beneficial in the long run.

“If the system remained stagnant and confined to the city, there would be no political will to earmark 20 percent of state sales tax” to the MBTA, as is the policy now, he said.

After the investments, Mulhern said, “people throughout the commonwealth looked at the system as an asset, not a tax burden.”

But Mulhern said that while subway cars and trains were expanded, buses were left behind.

“During the ’80s and ’90s, we virtually ignored the bus system,” he said. “We found ourselves maintaining the worst bus system in the United States.”

Revitalizing bus routes will especially help residents of poor neighborhoods such as Roxbury, which is on the Silver Line, he said. “Folks in inner-city neighborhoods would receive a better quality of life from our investments.”

But Harry Beck, a Metropolitan College sophomore who attended the lecture, said he was skeptical of the MBTA’s plans.

“I think the MBTA is inept,” he said, adding that its “poor management” will slow improvements in public transit.

He said the Silver Line would be useful, but inefficiencies in the MBTA will make it more costly than necessary.

“Boston has an obsession with tunnels,” Beck said, referring to proposals to create underground tunnels for Silver Line buses. “If police actually enforced bus-only lanes, operating expenses” would be reduced.