PLYMOUTH, MASS – The showroom at Pilgrim Power Sports is sparse if you're looking for an all-terrain vehicle.

Only three new ATVs are on the lot, one for almost two years. Pilgrim used to sell 50 ATVs annually. This year, 16 have been bought.

"ATVs were targeted by the state," said store manager Larry Liberti, who now relies on motorcycle and dirt bike sales to carry the business. "There is no doubt about it. And they've succeeded."

Massachusetts bears the nation's toughest law for protecting children from the dangers of operating ATVs. No child younger than 10 can ride one. Children younger than 17 are banned from vehicles designed for adults, and there are no exceptions based on training or adult supervision.

Other states enforce ATV rules on public lands, but Massachusetts' law is far more sweeping: It applies to all public and private property.

"It's the best piece of ATV legislation in the country," said Sue DeLoretto-Rabe, co-founder of Concerned Families for ATV Safety. "There are so many loopholes everywhere else."

Yet even the most ardent supporters of the Massachusetts restrictions say they are waiting to see a definitive effect on safety. Since the rules were approved, three children have died in off-road accidents — the same number of deaths during the previous three years. Hospitalizations fell from 27 in 2011 to 18 in 2013, but the state tally is incomplete.

And authorities say children are still hitting the trails on their parents' machines.

The problem is that the state needs at least 90 more off-road agents throughout the state, or twice the current roster, said David Gould, director of marine and environmental affairs for Plymouth.

"Massachusetts has this new law, which is great," he said. "But we all know the key to anything with a new law is enforcement."

Each year, thousands of children across the country are hospitalized or killed in ATV accidents, typically while operating adult-size vehicles. Safety advocates are hoping Massachusetts will serve as a model for others to follow.

Supporters of the law say they are seeing signs that it is beginning to have an effect. At Massachusetts General Hospital, trauma surgeon Peter Masiakos said he hasn't treated a child with an ATV-related injury in four years.

"Most people don't ignore the rules," Masiakos said.

State lawmakers stress that it has been almost two years since a child died in an off-road accident. "The bill we passed saved lives," said state Senate President Therese Murray.

A push for 'Sean's Law'

Sean Kearney was 8 years old in 2006 when he went over to a friend's house to play. His mother thought the boys would ride bicycles or play video games. Instead, the other child's parents let them ride an ATV built for adults.

While riding under the power lines in Plymouth, a popular spot for off-roading, the children flipped the ATV. The 600-pound vehicle landed on Sean, crushing his 65-pound body.

Masiakos tried to save Sean at Massachusetts General, but the boy died five days later. The doctor had never attended the wake of a patient, but he found himself at Sean's.

Katie Kearney, Sean's mother, approached Masiakos with a challenge. "You have to help us pass a law to change this," she told him.

Masiakos began probing medical records. He discovered that in one year, more than 930 children in the state had been injured in ATV accidents, including 35 with severe head injuries.

"That really spurred a lot of the naysayers" to act, he said.

But the first two attempts to pass a bill failed. ATV dealers and riders rallied to defeat the proposals, arguing instead to expand training programs for young riders.

The Kearneys spent two years telling their story to lawmakers and campaigning to change state law.

"There was a lot of ugliness," Kearney said. "I gained a lot of weight after my son died, and people said I should be worried about my obesity and not worry about other people's families."

By 2010, after Masiakos delivered endorsements from the state's major medical groups, the bill finally had the votes. But Murray, the Senate president, knew ATV supporters would be tough to beat. So she fast-tracked the legislation to a floor vote in a single day, bypassing a process that typically takes months.

Opponents were stunned. Dan D'Arcy, leader of the state's ATV dealership group, said he didn't have time to mobilize any resistance. "It was backdoor politics at its best," he said.

Murray is unapologetic. "This death happened in my district," she said. "I saw the pain that this caused."

One-man ATV patrol

In a subdivision of Plymouth lined with Cape Cod-style homes, the roar of ATVs can be heard every weekend in the fall.

"Plymouth is a mecca for ATVs. It's known for great trails and no enforcement," said Plymouth warden Nate Cristofori, the one man responsible for patrolling off-road activity across the town's 103 square miles.

Off-roading enthusiasts are attracted to Plymouth's gravel roads and the dirt trails used by the local utility to access its power lines.

Since he was hired by the town last year, Cristofori hasn't stopped a single driver younger than 16 during his once-a-week patrols. A good sign, perhaps, but he says he knows young riders are out there. When recently visiting an elementary school, he asked students whether they had driven an ATV under the power lines. Several hands went up.

Gould decided to create Cristofori's job last year when he realized the state wasn't going to expand its staff. He recently got approval to hire a second warden, based on the volume of off-road activity Cristofori encounters.

On a sunny day in September, Cristofori was still unshackling the ATV he uses for patrols when he heard the roar of a four-wheeler.

He waved over an 18-year-old rider, who said he didn't know it was illegal to ride there. His machine wasn't registered and he didn't know it was illegal for his girlfriend to be on the back of the machine.

"Just do me a favor," Cristofori told the teenager. "Just go back and don't come out here again, OK?"

"No problem," the rider said.

Fewer families buying four-wheelers

The state's ATV law didn't include much new funding for enforcement. No public awareness campaigns followed until this year — a 15-second message showing in 14 movie theaters.

State officials said they are doing the best they can, approving "targeted enforcement" of off-road rules by paying for overtime. The state has yet to formally assess the impact of Sean's Law. Murray, the state Senate president, said she wants to do more, but the budget is tight.

D'Arcy said families that visit his dealership have stopped buying four-wheelers because they know they can no longer ride together.

"When people find out they can't ride with their kids," D'Arcy said, "they get upset."

Next: Part 4: New push for changes to ATV design

Go to Part 1: ATV thrills drive child injuries, deaths

Go to Part 2: States ignore federal ATV age limits

Go to Part 5: Dangerous terrain: ATVs and open roads