The bus hugs the edge of the road so closely at times that tree branches caress the windows. The arched bridges of the carriage roads are high enough only in the middle to pass under safely, so the bus slows down each time. Craggy rocks and tall pines coat the landscape on the right, and the ocean swims far below on the left.

That’s the view from the window of an Island Explorer bus as it twists its way around Acadia National Park one morning this week.

The passengers are quiet, even the children, as they watch the scenery trundle past. Someone pulls the rope overhead, and the driver stops to let him off. Most were coming or going to Sand Beach, or the Jordan Pond House, or Northeast Harbor.

Some just want a lift from a village to Bar Harbor to shop or eat. Mostly to shop.

A few riders need their bikes hooked on. The driver makes room inside the back of the bus for the ones that don’t fit on the bike racks outside.

“It’s so relaxing,” says Bob Byrnes of Hopkinton, Mass., who has his arm hooked around his wife, Mary, as they ride into Bar Harbor and talk about maybe retiring to Maine some day.

The Byrneses are spending their second vacation on Mount Desert Island, and since discovering the bus last year they haven’t used any

other means to get around the island and park.

“You don’t have to drive,” he says. “It goes everywhere you want to go. And you can look. When you drive, you can’t look.”

The Byrneses are two of the 220,000 reasons the Island Explorer bus service has achieved such success in four short summer seasons.

Started as a way to reduce some of the summertime congestion and pollution on the island, the bus service now is trying to expand its operating season into the fall, buy eight more propane-fueled buses and plan for a major off-island hub that could further reduce the number of vehicles on the island.

All that because people not only tried the bus, but liked it.

“It was perfect timing. We took the bus to get some lobster dinners last night,” says Alan Gonzalez of Madison Heights, Mich., who planned to stay one night at Hadley Point Campground and ended up staying three nights.

Gonzalez and his family vacation every summer with their brother- and sister-in-law and their children. They’re doing the East Coast swing this year, starting on MDI and working their way down the coast to Portland, Boston and New York.

The seven cousins slouch against their parents and each other in the back of the 28-seat Blue Bird bus. They are content to watch the forest and ocean out the window while their parents rave about the bus service and the beauty of the park and the island.

Gonzalez and his brother-in-law, Bill Broomfield, rode the Explorer to Bar Harbor the evening before to pick up their meals and lucked out and caught another bus back while the food was still steaming.

“It was still warm when they got back,” says Debbie Gonzalez.

The driver uses a mike to tell passengers about The Wild Gardens of Acadia as the bus rolls past, or the legendary Thunder Hole near Sand Beach, or the mansion on a point in Bar Harbor built for a bride who perished on the Titanic.

There are no cell phones on the bus, or even a Walkman. Three-year-old Alec Gonzalez sleeps on his mother’s lap in the back.

On an annual basis, the Island Explorer carries more riders than the Portland Metro, according to company officials. The late-morning bus to Sand Beach was filled to capacity Tuesday, but in a few more weeks, the blue, green and white buses will carry standing-room-only crowds – up to 43 people in each bus.

The buses this summer are sporting a new logo, the famous L.L. Bean name on the back and sides, over the motto: “Protect our national parks.”

The nonprofit membership group Friends of Acadia, whose sole mission is to preserve and protect the nation’s second-busiest national park, secured a $1 million grant from the retail giant.

The money will be used to expand and improve the Island Explorer bus system as concerns continue to mount that the park is nearing its capacity – if not for people, then certainly for their vehicles.

The Island Explorer uses the Village Green in Bar Harbor as a hub; buses run from early morning until late in the evening for the most popular campground routes. Riders also are picked up regularly off the island at Carroll’s IGA and the Bar Harbor Airport, both in Trenton.

Drivers pick up and drop off riders anywhere along their routes where it is safe to stop.

“They’re wonderful,” Kathi Anderson of Bar Harbor says of the bus drivers. “We can just call them up and say, ‘Pick us up,’ and they will. We’re just so fortunate to live here.”

Anderson is riding the bus home after taking her two toddlers, Samantha and Garth, to Sand Beach for the morning. They are tired, hungry and sandy, but they love the ride and the scenery.

“They’re in the bus stage,” Anderson explains as she unwraps a peanut butter sandwich – the “picnic on the bus” she promised – and hands Garth a sippy cup filled with orange juice.

“Who needs a roller coaster?” she asks.

Anderson and her husband, Mike, a researcher at The Jackson Laboratory, moved to Bar Harbor four summers ago. They still miss their family back in Iowa, but their island life lures their relatives east.

And they all ride the bus.

“It’s hard to leave family, but they sure visit a lot,” Anderson says as she settles both children on her knees.

“I can’t hold my kids on my lap in the car, so this is great,” she says. “A little quality time, right guys?”

The bus service is offering a new, free guide that shows in detail all of its routes and scenic sights. It is available at park campgrounds, the visitors center and also at the information hut at the Bar Harbor village green.

The buses are equipped for wheelchairs, and all rides are free to encourage more use. Schedules are available at the dispatch center on the green or by calling Downeast Transportation at 288-4573.