Acadia National Park and supporters appear to be succeeding in their campaign to persuade visitors to leave behind their cars when they enter the park.

According to new federal statistics, the Island Explorer, the park’s fare-free shuttle system, carried a record 503,224 passengers in 2014. It was the first time the system cracked 500,000 passengers for its estimated 3.5-month season of operation.

“The bus ridership was way up this year,” said Stuart West, chief ranger for Acadia National Park, in an e-mail. He referred questions about Island Explorer numbers to Paul Murphy, general manager for Downeast Transportation, Inc., which runs the Acadia bus shuttle.

The numbers for the bus system came as the Maine national park is on pace to attract about 2.7 million visitors this calendar year, the most in nearly 20 years, the federal statistics said.

The bus passenger statistics, made available on Tuesday on a National Park Service web site, show that passengers on the shuttle system increased by about 15 percent from 438,737 in 2012.

Island Explorer operates from late June through Columbus Day. The propane-powered buses have run since 1999, or 16 years, carrying 141,000 riders the first year.

The shuttle system hit another major milestone on June 30 when it reached 5 million passengers since its inception. The bus numbers include local users of the system, including employees of area businesses. The statistics do not break out the park share of the riders using the bus system.

The 5 millionth passenger was Marisa Gray, an Otter Creek high school student, commuting to her summer job at Hannaford Supermarket in Bar Harbor.

The shuttle system this year ran eight routes, plus the Bicycle Express, through Acadia National Park and the surrounding communities.

Riders totaled 25,200 in October, 67,158 in September, 208,904 in August, 169,852 in July and 32,110 in June, according to the federal numbers.

Downeast Transportation, the operator of the Island Explorer, noted on its Facebook page that the Explorer enjoyed a record year.

Murphy, general manager of Downeast Transportation, said a public release was being prepared about the passenger numbers.

Leaders and supporters of the park said the shuttle system is critical for reducing traffic congestion and vehicle emissions in the national park.

The system started with eight buses and now boasts about 30.

Friends of Acadia, L.L. Bean, others support Island Explorer

The Island Explorer is financially backed by the park, the US and Maine Departments of Transportation, contributions from L.L. Bean and Friends of Acadia, local municipal dollars, fees from businesses that receive front-door service and passenger donations, a recent press release said.

A portion of every weekly and annual park entrance fee is dedicated to financing the operations of the Island Explorer.

Not only does the Island Explorer reduce the stress of trying to find a parking spot during the busy summer season, it also creates new one-way hiking opportunities, with the bus serving as the way back, or the way to the next destination.

In addition, in line with the pet-friendliness of Acadia National Park, the Island Explorer allows leashed pets on board.

Plus there is a direct bus from the Hulls Cove Visitor Center to Jordan Pond House, for visitors who don’t want to worry about finding parking in order to get their afternoon tea and popovers.

Finally, visitors can take a ferry from Bar Harbor to Winter Harbor (fees apply) during peak season, and then hop on the fare-free Island Explorer for a tour of Schoodic Peninsula, the only part of the park on the mainland.

That ferry-Island Explorer connection makes hiking and biking around Schoodic a lot easier for visitors staying on Mount Desert Island. And it opens up opportunities to stay on the quieter Schoodic Peninsula and do the reverse commute, to have lunch or shop in Bar Harbor, or hike the trails on MDI.