Independent truckers in Mexico are calling for clearer, better rules on vehicle weights and dimensions, reports the Associated Press.

The drivers, who have launched national protests, say they want to do away with permits that let double-trailer vehicles carrying loads as heavy as 100 metric tons drive on two-lane roads.

These protests, organized by the Alliance of Mexican Transport Organizations, come on the heels of two deadly accidents in which the rear units of double trailers came unfastened and crashed into passenger buses, killing a total of 49 people.

The rule being protested bans semis hauling double trailers from Mexico's secondary roads unless the trucking company qualifies for and applies for a "connectivity permit."

Truckers say the fact that some double-trailers are allowed on back roads with these permits leads other drivers who don't qualify for permits to do the same. If caught without a permit, they simply pay bribes to corrupt police officials.

According to the The Communications Transport Department (SCT), in 2011, 5,710 trucks received fins for being overweight. In an interview with local media, SCT trucking director Miguel Elizalde said about half the 30,000 accidents recorded each year on Mexico's federal highways involved trucks, and roughly 900 of those were double trailers.

The U.S. federal commercial vehicle maximum weight standard on the Interstate Highway System is 80,000 pounds or 40 tons, reports AP. Mexico allows twice that much, and even that limit is often exceeded.

Many accidents are not registered, as motorists are simply run off the road by semis pulling double trailers.