Final answer:
The question is about the segregation practices enforced on city buses in the southern United States during the period of racial segregation.
Step-by-step explanation:
In the context of the question, the phrase 'Bus operators are forbidden to (13-166)' refers to the segregation practices that were enforced on city buses in the southern United States during the period of racial segregation.
This specific reference indicates that African Americans were required to give up their seats to white passengers if there were no available seats for the white passengers. These discriminatory practices were part of the larger system of segregation that affected various aspects of life in the southern states at that time.