A straphangers' advocacy group wants the MTA to turn its connecting bus service to LaGuardia Airport into a free shuttle.

The Riders Alliance released a report on Monday suggesting that the state transit agency can convert its Q70 bus line into the "Free LaGuardia Subway Shuttle" at zero net cost to taxpayers.

RELATED: Joe Biden compares LaGuardia Airport to 'Third World country'

"It’s not a billion-dollar project," said alliance Executive Director John Raskin. "It's a free project with billion-dollar returns."

The alliance argued that the city can do more to promote public transit options with signage alone, and that directions are unclear for non-New Yorkers. It also suggests rethinking the actual bus itself to make it unique for riders specifically using it to get to LaGuardia.

"A clearly branded, free airport subway shuttle … is a low cost solution that would provide LaGuardia's 27 million passengers with a 21st century access link," said Joe Sitt of the Global Gateway Alliance, a local airport advocacy group. 

But the MTA has its doubts.

RELATED: Biden, former critic of NY's LaGuardia Airport, upbeat on redesign

Agency spokesman Kevin Ortiz threw cold water on the proposal coming at no cost to the state. One out of every four riders on the line, he said, don't come from the subway and don't use the existing free transfer the MTA already offers.

"If ridership would continue to grow on the route to the level they claim, we would have to add service, and that costs money," Ortiz added. "And where would we find the buses?"

The MTA launched the Q70 in 2013, offering limited-stop bus service, connecting the airport with the E, F, M, R and 7 trains.

Last year, the Q70 saw an average weekday ridership of more than 3,400 people.

RELATED: Cuomo pledges to fund MTA gap; outlines plan for new LaGuardia   Airport

But in early 2014, the MTA also upgraded the M60 bus to a offer select bus service, meaning faster access to LaGuardia from the line's furthest point at 106th Street and Broadway in northern Manhattan.

"At the end of the day, there is simply zero evidence that making it a free shuttle would increase ridership on subways to the point it would make the shuttle self-sustaining," Ortiz of the MTA said.

The Riders Alliance proposal comes amid a series of concerted efforts to improve services and access to LaGuardia, which has some 11,000 employees. The airport saw more than 2.3 million passengers in September 2015 alone, adding up to more than 21 million so far this year.

In January, Gov. Andrew Cuomo announced a $450,000 proposal to create a new AirTrain connecting the airport with the subway and commuter rail services at the Mets-Willets Point Station. 

The Cuomo administration previously said the AirTrain could open as early as 2020.