PORTLAND – Officially, the national weight limit for freight trucks on interstate highways is 40 tons. In reality, trucks are getting heavier in more states – legally – and advocates for highway safety and the trucking industry are sharply at odds about it.

Trucks heavier than 80,000 pounds are allowed to operate on federal highways in at least 20 states. Congress added Maine and Vermont to the list earlier this month, granting exceptions to allow trucks up to 100,000 pounds on interstates there for the next 20 years. The change went into effect Friday when President Obama signed it.

Critics say heavier trucks make highways less safe because they’re harder to control and stop, and that they leave taxpayers on the hook for damage to roads and bridges. Furthermore, they claim, the latest increases will spur the trucking industry to seek higher limits in other states.

“The trucking industry is energized by what’s happened in Vermont and Maine,” said Jackie Gillan, president of the Washington-based Advocates for Highway and Auto Safety organization. “The American public is going to pay with their lives and their wallets.”

But supporters of higher weight limits argue that allowing heavier trucks will actually make highways safer because fewer trucks will be able to move the same amount of goods. With fewer big rigs rumbling around, it’ll cut pollution and reduce the cost of doing business, they say. And concerns about road and bridge damage are overblown, they claim.

“Whatever arguments the opposition puts out there, if you look at the research, their arguments don’t hold water,” said Darrin Roth, director of highway operations at the American Trucking Associations.

Before the new law raising the weight limit in Maine went into effect, Douglas Haskell, a truck driver from Palermo, had to drive loads of cement powder along two-lane state highways — even with Interstate 95 nearby — for delivery to northern Maine, New Brunswick and Quebec.

He drove through school zones, over railroad crossings and in small towns, while dealing with cars in breakdown lanes, moose and pedestrians. Allowing larger trucks cuts emissions, saves on fuel and cuts down on driver stress, he said.

“If we all cut back to 80,000 pounds, we’d probably have twice as many trucks on the road, so what are you accomplishing there?” said Haskell, who’s been a trucker for 38 years. “You’re going to have twice as many trucks out there creating havoc with the public.”

The congressional vote earlier this month adds to a jumble of inconsistent highway weight laws around the country that’s been around since the interstate network was created in 1956. Back then, Congress set a limit of 73,280 pounds on federal highways, but at the same time authorized states to allow heavier trucks if they already did so on state highways.

In 1974, Congress gave states the option of raising the interstate limits to 80,000 pounds, and in 1982 required all states to adhere to that limit.

Congress has generally allowed states to grandfather existing laws. Several states allow some trucks weighing more than 100,000 pounds or make exceptions for specific products, such as sugarcane, milk, logs or coal, according to the Federal Highway Administration.

Over the years, the trucking industry and other groups have argued for higher weight limits. The railroad industry, safety groups and others have argued against them. Both sides cite report after report they say support their positions.

A bill submitted by U.S. Rep. Jim McGovern, D-Mass., called the Safe Highway and Infrastructure Preservation Act, would freeze the 80,000-pound limit on federal highways.

A competing bill submitted by Rep. Mike Michaud called the Safe and Efficient Transportation Act, would allow six-axle trucks weighing up to 97,000 pounds on federal highways, with states having the option of increasing the weight limits.

The Maine Democrat said he was impressed by an Alabama business owner’s testimony in Congress a few years ago that allowing heavier trucks on the roads would save him $73,000 a week in fuel costs, reduce carbon monoxide emissions by 130,000 pounds a week and cut the number of his trucks on the road from 600 to 450.

In states such as Maine and Vermont, he said, higher weight limits get the biggest trucks off rural two-lane highways and onto the interstates, where they pose less danger.

“When you look at economic impact, environmental impact, safety impact, it’s positive in all three areas,” Michaud said.