Which U.S. airport is offering free public transportation?

— -- Want a fast, free ride from the airport into town? In Boston this summer, you'll have your wish.

In what is apparently a first for a North American airport, Boston Logan International is waiving the $2 fare for all passengers who ride the inbound Silver Line bus from the airport to South Station in downtown Boston. From there, the transfer to the subway - known as the "T" - is free.

The clean-fuel Silver Line buses have been in operation since 2005, have room for luggage and make the trip in from the airport in 20-40 minutes (depending on which terminal you board at), a bit longer than it takes to drive.

The 90-day, free-ride pilot program, called "On Us," should be a hit with many of the city's summer visitors, who might otherwise pay $25 or more for a cab ride into the city. But a major goal of the program is to convince locals to switch to the bus and stop paying to park their cars in the airport's central garage.

At first glance, that part of the program seems a bit counter-intuitive. "Parking is a critical revenue source for North American airports," said Debby McElroy of Airports Council International-North America (ACI-NA), an airport trade group. In fact, according to the organization's 2011 Airport Concessions Benchmarking Survey, parking accounts for 42% of non-aeronautical revenue at airports. "These funds allow airports to modernize facilities, which ultimately benefits travelers," said McElroy.

So why would Logan Airport decide to spend close to $300,000 to subsidize free bus rides for three months and encourage customers to leave their cars - and their cash - at home?

The answer is space, and not enough of it.

Sprawling airports in Denver and Dallas/Fort Worth sit on vast tracts of land beyond or between urban centers, but Logan has a very small footprint - just 1,700 acres - and is only three miles from downtown Boston. And while the airport's passenger numbers keep growing (BOS expects to serve close to 30 million passengers this year), due to a regulatory cap limiting commercial parking, space at the airport's central garages cannot.

The bottom line, is that "Logan cannot build new garages to meet demand," David S. Mackey, Massport's Interim CEO & Executive Director said in a statement. (Massport, the Massachusetts Port Authority, owns and operates Boston Logan Airport).

Instead, Logan has been trying to wean passengers from driving to the airport. In March, the daily parking rate in the central garage was raised $3, to $27. At the same time, the daily parking rate at Massport's suburban lots was lowered to $7, down $4.

The Silver Line "On Us," pilot program is the next step in that weaning. As part of the effort to reduce curb congestion and bulk up high occupancy vehicle (HOV) usage, the airport has hired and placed extra staff outside the terminals to explain the program to passengers. "We're collecting information and counting the boardings and the loads at the bus stops at each terminal," said Carl Leiner, Massport's deputy director of economic planning and development. "There are people with stop watches calculating the dwell times, which is the amount of time people spend waiting for the bus."

While no other airports are jumping to match Logan's free-ride offer just yet, many are watching closely.