The plan affects large, tour-style buses, not city buses or school buses, which are state-regulated.
The motorcoach industry, which transports 750 million passengers a year, has 90 days to respond to the proposal. It would take effect three years after it's made final.
The National Highway Traffic Safety Administration indicated in the proposal that it is also considering requiring existing buses be retrofitted with belts, which is more expensive than incorporating belts into new buses. The proposal solicits comments on how that might best be done and whether lap-shoulder or lap-only belts should be required.
Between 1999 and 2008, there were 54 fatal motorcoach crashes resulting in 186 fatalities, most of them passengers ejected from buses, according to NHTSA.
The majority of motorcoach trips 65 percent are made by children and senior citizens.
The National Transportation Safety Board has been urging NHTSA for 11 years to strengthen motorcoach safety regulations, including devising a means to keep passengers from being ejected from their seats during accidents, either through seat belts or some other safety system.
In a January 2008 accident near Mexican Hat, Utah, nine passengers were killed and 43 injured when their motorcoach took a turn too fast at night as they returned from a ski trip. The bus tumbled down an embankment, its roof was sheared off and everyone aboard ejected except for the driver, who was wearing the only seat belt on the bus, and one man who was pinned between two seats.
Last year, the board said NHTSA's slow progress on motorcoach safety regulations contributed to the severity of the Utah accident.
Wearing lap-shoulder belts on motorcoaches could reduce the risk for passengers of being killed in a rollover crash by 77 percent, according to NHTSA.
"Seat belts save lives, and putting them in motorcoaches just makes sense," LaHood said in a statement.
In an accident in March 2007, five members of Ohio's Bluffton University baseball team were killed along with their driver and his wife when their bus hurtled over an Atlanta highway overpass onto an interstate below. Twenty-eight people were injured.
John Betts, whose 20-year-old son was among the Bluffton baseball players who died, said the proposal doesn't go far enough.
Limiting seat belts to new buses would mean motorcoaches built in recent years will be on the road for decades without safety restraints, Betts said. He also wants the government to require stronger bus roofs and shatterproof windows, as recommended by NTSB, to help prevent passengers from being thrown out.
Betts backs a bill introduced in Congress two years ago that would require those changes.
"It has been very frustrating for a lot of us and a long haul for something that seems so simple," he said. "It still seems to be falling short."
Industry officials said they support the proposal for new buses because NHTSA has been able to show through crash-testing and other research that it will improve safety. However, they are leery of requiring existing buses be retrofitted with belts.
Victor Parra, president and CEO of the United Motorcoach Association, said it's not clear belts can be added to older buses.
"There are a lot of questions that have to be answered before we can say yes or no to that question," Parra said.
LaHood responded to NTSB's findings in the Utah accident by ordering an overhaul of motorcoach safety regulations. In recent months NHTSA has announced steps to address driver fatigue or inattention and improve operator maintenance. Research on improving motorcoach structure, fire safety protection and exiting in an emergency is also under way and may lead to new federal standards, the department said.