Final answer:
The Supreme Court's decision in Morgan v. Commonwealth of Virginia, which stated that Virginia's law enforcing segregation on interstate buses violated the interstate commerce clause, made segregated bus travel unconstitutional.
Step-by-step explanation:
Segregated interstate bus travel was made unconstitutional primarily by the 1946 U.S. Supreme Court decision in Morgan v. Commonwealth of Vir-ginia.
The Court ruled that Virginia's state law enforcing segregation on interstate buses was unconstitutional as it violated the interstate commerce clause of the Constitution.
This paved the way for further civil rights actions, such as the Freedom Rides organized by CORE (the Congress of Racial Equality) to challenge the enforcement of desegregation laws, leading to increased pressure on the federal government.
The Interstate Commerce Commission, with support from the attorney general, eventually issued an order banning all segregation in interstate travel facilities, further cementing the unconstitutionality of segregated interstate bus travel.