Many students returning this week after winter vacation got a surprise when they used the T to get back to campus: a fare increase.

The Massachusetts Bay Transit Authority (MBTA) raised fares for its subway, bus and commuter rail systems on Jan. 3, leaving some riders inconvenienced and annoyed.

The fee for riding the T, previously one dollar, has been raised to $1.25, and riding the bus now costs 90 cents, up from 75 cents.

“We felt that the fare increase was absolutely essential to closing a hole in the T’s operating budget,” said Lydia Rivera, a spokesperson for the MBTA. “The T’s fiscal stability was threatened, and we needed more revenue to pay a $4 billion debt; this was a last resort.”

The promise of improved service may make the higher fares easier to swallow.

The MBTA will use the additional revenue generated by the fare increase to maintain, as well as upgrade, existing services and systems. Potential upgrades include the purchase of new buses that utilize compressed natural gas, new cars and new signals for the Green Line and new double-deck trains and signals for the commuter rail.

“We have to continue to advance systems and maintain what we already have,” said Rivera.

The MBTA spokesperson said she did not anticipate any lost ridership due to the fee increase.

However, studies have shown that fare increases in public transportation are often accompanied by slightly reduced ridership, according to David E. Luberoff, associate director of the Taubman Center for State and Local Government at Harvard’s Kennedy School of Government.

Luberoff cited a 1992 article in the Journal of Transport Economics and Policy that found a 10 percent fare increase typically leads to a 5 percent decrease in patronage.

“What usually happens is ridership declines slightly, but the overall revenue usually goes up a little more than ridership decline,” said Luberoff, who co-authored the 2003 book Mega-Projects: The Changing Politics of Urban Public Investment.

The reduced ridership can be attributed to people resorting to other modes of transportation or carpooling.

Even if Boston commuters do seek other transportation options in protest, the fare hike is here to stay.

In late August, Massachusetts lawmakers passed a piece of legislation which undid a part of state law that required fares to return to their previous level if ridership dropped by more than 4 percent.

However, Luberoff said he finds it hard to imagine that a fare increase of 25 cents would drive a person to use a car instead of using the T.