School buses weren’t always yellow. But now, the color, known as national school bus chrome, is the universal language for “be careful – children on board!”

In fact, the color is designated by federal law for its attention-grabbing properties, according to the American School Bus Council.

Before the 1930s, school districts that provided transportation to students used their own discretion in the design and coloring of their buses, which probably bore little resemblance to the ubiquitous lumbering, rounded-top, yellow buses with black lettering of today.

A Columbia University professor is credited with conceiving the color and other standardization of the modern school bus, according to the New York Times. Frank W. Cyr traveled the country studying public school transportation in the 1930s before convening a conference of educators, bus manufacturers, and paint specialists in 1939 to create the nation’s first school bus safety manual.

Cyr wanted the color to be standard along with 43 other safety features, according to the website Today I Found Out. The color was chosen for maximum visibility, and the general idea was that it would signal, ‘slow down, there are children nearby.”

Thirty-five of the United States and some parts of Canada adopted the color soon after the conference, but it wasn’t until 1974 that all U.S. school got on board the yellow school bus.

The “chrome” of the name derives from the yellow-hued lead chromite.