Independent Mexican truckers launched nationwide protests Thursday calling for clearer, fairer rules on vehicle weights and dimensions and for an end to discretionary permits that they say let double-trailer vehicles carrying as much as 100 metric tons of freight operate on narrow, two-lane roads.

U.S. union and consumer groups have complained for years about dangerous Mexican trucks. Now independent Mexican truckers have grown fed up after two horrifying crashes in less than a month.

The protests by the Alliance of Mexican Transport Organizations follows two accidents in which the rear units of double trailers came loose and careened into passenger buses, killing a total of 49 bus passengers.

Mexico's Communications Transport Department, which oversees trucking, did not respond to requests for comment about the protest.

Alliance national coordinator Francisco Leyva said discretionary rules put pressure on truckers to carry bigger loads and allow corrupt police and officials to get more bribes when truckers are caught.

Leyva said his group set up roadside encampments with parked trucks carrying banners and placards denouncing the double-trailers, which also cost truckers jobs because one driver carries twice as much.

"As far as I know, there is not a country in the world that carries as much weight (in trucks) as we do," Leyva said.

In the United States, the federal commercial vehicle maximum weight standard on the Interstate Highway System is 80,000 pounds (40 tons). Mexican norms allow twice that much, and even that limit is frequently exceeded.

On April 13, a semi-truck pulling two trailers of grain allegedly lost its brakes while speeding along a highway west of Mexico City, then the rear trailer broke free and slammed into the back of a bus carrying university students. Five students and a teacher were killed.

Leyva said two full trailers of grain would easily break the 80-ton weight limit.

On Friday, a double-trailer truck traveling on a two-lane road in the Gulf coast state of Veracruz also lost its rear trailer, which slammed into a bus carrying farm workers, killing 43 people.

At issue is an obscure rule that normally bans semis hauling double trailers from using Mexico's secondary roads, unless the trucking company applies for a "connectivity permit" that allows such travel by trucks that meet certain requirements. The Communications Transport Department, known as the SCT, said in a statement that the truck involved in the most recent accident didn't have such a permit.

But truckers say the mere fact that some double-trailers are allowed on back roads encourages other drivers to do make similar trips, paying bribes if their 106-foot-long vehicles are caught without a permit.

The SCT says 5,710 trucks were fined for being overweight in 2011. In an interview with local media, SCT trucking director Miguel Elizalde said roughly half the 30,000 accidents registered each year on Mexico's federal highways involved trucks, and about 900 of those were double trailers.

But Leyva, the trucker alliance official, said "there are many, many accidents" that may never be registered, in which vehicles are simply run off the road by semis pulling double-trailers on tight curves.

"If a double trailer rides over into the oncoming traffic lane, the car in that lane is simply going to go off the road," Leyva said.

The United States was required under the North American Free Trade Agreement to grant Mexican trucks full access to U.S. highways by January 2000. But the Teamsters union, U.S. consumer groups and independent insurers warned that Mexican trucks were unsafe and lobbied Congress to keep them out. At the time, opponents circulated pictures of a Mexican truck so overloaded its wheels were sinking into the asphalt.

U.S. legislators succeeded in delaying the opening until a pilot program allowing some Mexican trucks was instituted in 2007.

In March 2009, Congress failed to renew the pilot program that let a limited number of Mexican trucking companies haul freight beyond a 25-mile (40-kilometer) border commercial zone. Mexico retaliated by placing tariffs on 99 U.S. agricultural products worth more than $2 billion annually.

Finally, last October, the first Mexican truck to deliver goods in the U.S. interior entered the United States under the long-delayed free-trade provision.