WASHINGTON - Drivers could find themselves sharing the roads with longer double-trailer trucks under a proposal making its way through Congress.

Large trucking companies and a national industry group are urging lawmakers to allow longer trailers, arguing that doing so would increase efficiency and move more goods with fewer trucks.

Republicans in both chambers of Congress strongly support the truckers' effort, but it has sparked outrage among safety advocates.

Double trailers could stretch to 33 feet instead of the current 28 feet under a provision in a transportation spending package awaiting consideration by the full Senate. The House has passed a spending bill that also includes the provision.

The trailer debate comes as forecasters see major increases on the nation's transportation network in the coming decades - with few signs of a dramatic increase in infrastructure spending or major change in the way the country moves goods.

"Are there opportunities to reduce congestion and move goods more efficiently in a more cost-effective way?" asked Rep. Frank LoBiondo (R., N.J.), who ran a trucking business before coming to Washington. "I think that's what we're going to have to debate."

Highway safety advocates don't see truck length as an efficiency issue. They're calling for Congress to apply the brakes, saying safety questions are still unanswered.

"What we're talking about is people's lives," said Joan Claybrook, chairwoman of Citizens for Reliable and Safe Highways. "It's not just an economic issue. This is a life and death issue."

Big U.S. shipping companies signaled they would be jumping into the ring early to fight for longer trucks.

FedEx, UPS, Con-Way, Estes Express Lines, and other major shippers formed the Coalition for Efficient and Responsible Trucking and said in February they wanted to insert a trailer-extension provision into the next highway bill.

The group's pitch is simple: Trucks fill up before they hit the weight limit, wasting space, time and resources. Longer trucks are fuller trucks, but the group doesn't need any increase in the federal weight limit.

"Everyone - Republicans and Democrats - is looking for ways to stretch transportation dollars at a time when the nation's highways have fallen into disrepair," said Ed Patru, a spokesman for the group, in an interview earlier this year.

Patru said that extending double trailers would cut around 6.6 million truck trips a year by increasing truck capacity by 18 percent. That would reduce highway accidents attributable to congestion by more than 900 per year, he said.

One thing is for sure: More trucks are coming.

The Federal Highway Administration estimates freight traffic will increase to 29 billion tons in 2040, up from 20 billion tons in 2012. The agency sees the value of goods shipped rising even faster, to $39 trillion in 2040 from $18 trillion in 2012. Trucks handled about two-thirds of the tonnage and value in 2012, the FHWA said.

The American Society of Civil Engineers said in a 2013 report that 42 percent of the nation's major urban highways were already congested.