Most school buses were painted yellow beginning in 1939. In April of that year, Dr. Frank W. Cyr, a professor at Teachers College at Columbia University in New York organized a conference that established national school bus construction standards, including the standard color of yellow for the school bus. It became known officially as National School Bus Chrome, later renamed "National School Bus Chrome Yellow". The color, which has come to be frequently called simply "school bus yellow", was selected because black lettering on that hue was easiest to see in the semi-darkness of early morning and late afternoon.
The conference met for seven days and the attendees created a total of 45 standards, including specifications regarding body length, ceiling height, and aisle width. Dr. Cyr's conference, funded by a US $5,000 grant from the Rockefeller Foundation, was also a landmark event inasmuch as it included transportation officials from each of the then 48 states, as well as specialists from school bus manufacturing and paint companies. The conference approach to school bus safety, as well as the yellow color, has endured into the 21st century.