More government investment is needed to make school buses safer, experts say.

Concerns about the safety of buses arose after an overloaded van carrying students collided with a coal truck on Wednesday. The crash took 21 lives, including those of 19 preschoolers, and left another 44 people injured.

The overloaded van was not technically a school bus, nor are many of the vehicles used by schools in China, said Li Tao, an expert with the Chongqing Vehicle Test and Research Institute who drafted the 2010 national safety standard for primary schools' buses.

Li said the best way to ensure that schools begin using standard buses is to have the government put money toward that end. He said the cost of maintaining such vehicles is beyond what most schools can afford.

"School buses are different from buses used for public transport since they barely generate profits and only run for 10 months of the year," Li said.

Guo Xiamei, an education scholar from Ohio State University, said money for school buses can also come from NGOs and local governments. Local governments and the central government both play an essential role in ensuring the safety of the vehicles.

"In the United States, money for school buses comes from many places, school districts, state governments and the federal government," Guo said.

"In China, local governments' financial situations vary a lot and some have heavy debts. But as the experiences of countries abroad show, government support is indispensable toward establishing a sound school-bus system."

Li Tao said vehicles designed to be school buses usually cost more than other types of vehicles. He said school buses' prices will decrease only after suppliers receive more orders for them from schools.

Statistics provided by suppliers of school bus, such as Yutong Group in Henan province and the Huanghai Bus Company in Liaoning province, show that far fewer buses are being built than could be.

In 2010, Yutong Group sold 700 vehicles. The Huanghai Bus Company, meanwhile, says it sells from 300 to 500 vehicles a year.

"The same situation also goes for other suppliers of school buses," said Lin Chongfu, head of the Huanghai Bus Company's sales department.

In Beijing, schools generally prefer to hire professional companies to provide bus services to students.

"It would put much less pressure on schools if they entrusted bus services to professional rental companies, which have much more experience in being supervisors, whether it be in supervising bus drivers or the maintenance of buses," said Wang Jin, bus coordinator for an international school in Beijing.

Zhang Yuan, bus coordinator for another international school, said his employer chose the vehicles it uses - which range in type from SUVs to buses that can hold up to 48 people - according to the places where they pick up students. Those that go to the most populated areas, and are thus likely to carry the greatest number of passengers, are the largest.

For a vehicle, the school pays from about 7,000 yuan ($1,100) to about 16,000 yuan to the rental company in monthly fees, depending on the size of the vehicle that is being rented.

Zhang said vehicles built specifically to be school buses have disadvantages.

"Some of the seats are too narrow and some children can barely fit in them," Zhang said.

And rental companies also think more versatile buses give them better prospects for making money.

"We could rent the buses to other companies on weekends," said Zhang Hua, the school-bus manager for Wanjiabang Car Rental Company in Beijing.

"The school buses we provide now can also be used as ordinary buses and not be restricted to just taking students," he said.