PROVIDENCE — The state has proposed banning vehicles with more than two axles from two key bridges that are weakened by deterioration, the Route 24 bridge over the Sakonnet River and the Route 95 bridge over the Pawtucket River.

The state says that the weight limits it has imposed on both bridges aren’t working well enough, while the trucking industry says that the state’s plan would fall most heavily on local businesses and force thousands of trucks that aren’t heavily loaded to take detours.

The clash of interests the plan raises — the cost and inconvenience of detours versus the need to protect two major bridges that are failing — points to the difficulty the state is having maintaining its transportation system.

In the case of the Sakonnet River Bridge, the detour means a trip to Bristol and the Mount Hope Bridge, the closest other way to get from the mainland to Aquidneck Island. The detours around the Pawtucket River Bridge are much easier to use. Route 146 and Route 295 let truckers avoid the bridge while staying on main highways. But trucks taking local detours must use Pawtucket city streets.

The steel beams supporting both bridges have been damaged by years of rust. The Department of Transportation has imposed a series of weight limits on vehicles crossing the Sakonnet River Bridge, most recently setting a 22-ton limit last June. In November, a similar weight limit was set for vehicles crossing the Pawtucket River Bridge.

The DOT plans to replace both bridges and hopes to do it within three to five years. In the meantime, DOT engineer Robert Rocchio said, it’s critical to protect the bridges from being damaged further, possibly to the point of having to close them.

That would cause enormous problems in either case.

The state police have been enforcing the Pawtucket River Bridge weight ban intensively, Rocchio said, but the weight limit lacks teeth — thousands of dollars in fines per violation — unless troopers prove their case by weighing the trucks.

State police Maj. Steven G. O’Donnell said that between Nov. 7 and Feb. 18, the state police Commercial Enforcement Unit stopped 3,074 vehicles and weighed 323 of them, finding 225 of them overweight.

But as one state trooper put it, “We don’t have x-ray vision.” It’s impossible to tell which trucks to stop.

Even with the portable scales that have supplemented roadside weigh stations, O’Donnell said, “We can’t weigh every truck.”

Rocchio said the result is that “There are a sizable number of vehicles that are overweight that are crossing those bridges.”

He said that when it imposed the weight limit in Pawtucket, his agency expected that a period of strict enforcement would be followed by compliance by truckers.

However, he said, “the violations aren’t dropping off like we expected. It’s just not happening,” as truckers continue to risk getting caught.

The proposed ban includes exceptions for emergency, state and municipal vehicles. The fine would be $3,000 for the first offense and up to $5,000 for subsequent violations.

However, the broader ban would force a whole new category of trucks, those with three axles but weighing less than 22 tons, off the bridges.

Trying to learn how many trucks would be affected, Rocchio said, the DOT counted between 2,100 and 2,200 trucks with more than two axles crossing the Pawtucket River Bridge in one 24-hour period early this month. He said some of those trucks were probably also overweight.

John Atwood, the president of the Rhode Island Trucking Association, said the proposed ban based on axles could have unintended, negative side effects.

Atwood said his industry group supported the weight limit, and it credited the DOT with setting up an “excellent” array of signs warning truckers about the weight limit and urging them to avoid the bridge.

However, he opposes the new ban, and argues that the burden would be felt mostly by local businesses, the ones that make pickups or deliveries and then finish their trip empty.

Over-the-road truckers passing through Rhode Island work hard to avoid “dead-heading,” or traveling one way empty or close too it, he said — fuel is just too expensive. Those truckers, usually traveling on Route 95, can also use the relatively convenient highway detours around the Pawtucket bridge.

He said the ban would fall instead on the fleets of trucks belonging to local businesses, from oil delivery trucks to beer trucks, that go out fully loaded, make their deliveries, and come home empty. With three or more axles, they would still have to take detours even though they would be under the 22-ton limit.

The ban based on axles is attractive to government officials: It’s easy to see which vehicles are breaking the rules. Nobody has to weigh the trucks. And any police officer, state or local, can enforce the rules, not just the state police truck squad.

But Rocchio said the DOT doesn’t really have a choice.

“We have to preserve the life of these bridges,” Rocchio said. “We can’t think of any other practical way of doing it.”

blandis@projo.com