In the US, public school buses transport 25 million children to and from schools every day. The vehicles are covered by federal legislation, have their own colour - School Bus Yellow - and occupy a hallowed place in the national psyche.

In the US, public school buses transport 25 million children to and from schools every day. The vehicles are covered by federal legislation, have their own colour - School Bus Yellow - and occupy a hallowed place in the national psyche.

The buses turned their now-recognisable shade in 1939 thanks to a conference held in Columbia University, in New York, aimed at establishing national school bus construction standards. Its organiser, Frank W Cyr, an educator, author and expert in rural schooling, became known as father of the yellow school bus.

The idea behind the colour was that black lettering would be more easily visible against it in the morning, so it would help other motorists see the buses.

After the Second World War, a move to larger, more centralised educational institutions meant pupils had to travel further to school. The demand for buses expanded accordingly. By the mid-1940s, most states had introduced laws forcing motorists to stop for school buses while they were loading and unloading. There followed a raft of motor vehicle safety standards for school buses in 1977, imposed by federal legislature.

The safety requirements have grown ever more sophisticated since, including placing bars by the bus exits to ensure that children cannot walk through the driver's blind spot when getting off. But there remains a debate over the introduction of safety belts, which opponents argue are unnecessary, given the slow speed at which the buses generally travel.

School buses became embroiled in America's conflict over racial segregation in the 1940s and 1950s when black pupils would be transported separately to all-black schools. It ended after a 1954 court decision ruled schooling and other forms of segregation violated rights inscribed in the US constitution.