At the beginning of the civil rights era in May 1961, a racially mixed group of men and women traveled by bus together from Washington, D.C., to New Orleans to test compliance with Supreme Court rulings that had outlawed segregated waiting rooms, lunch counters and restroom facilities for interstate passengers. By November 1961, more than 400 of these Freedom Riders had risked their lives, endured savage beatings and imprisonment.

A new PBS documentary titled “Freedom Riders” premiers on May 16, the anniversary of the first ride.

“I don’t think anyone knows about the Freedom Riders,” filmmaker Stanley Nelson told ABC News. “When I started the film, I didn’t know the story. I knew the term and the iconic photo of the bus burning, but if you had asked me, ‘Tell me the story of the freedom riders in two minutes’ the story I would have told you would have been so wrong and so mixed up it would have been crazy.”

Although the Supreme Court decision in the case of Boynton v. Virginia effectively outlawed segragation for interstate bus passengers, the ruling was not enforced south of the Mason-Dixon Line. It took the action of the Freedom Riders to force the the Interstate Commerce Commission to finally outlaw discriminatory seating practices and enforce the removal of “whites only” signs from interstate bus terminals on Nov. 1, 1961.

“The first group of Freedom Riders were 12 individuals — six blacks, six whites — who got on Greyhound buses and Trailways buses and decided they’re going to go from Washington, D.C., down to New Orleans,” Nelson told ABC News. “They’re going to sit together on the front of the buses. They’re going to eat together in the restaurants in the bus stations. The white people are going to use the ‘coloreds only’ restrooms, the African-Americans are going to use the ‘whites only’ restrooms, and they’re going to really test the law and see what happens.”

The first bus carrying the Freedom Riders was firebombed and the Riders beaten so savagely that none of them could finish the journey.

“I think that they knew that there would be violent resistance. They knew that there would be problems,” said Nelson. “I don’t think that they thought the resistance would be nearly as violent as it was. … Nobody would have guessed that somebody would firebomb the bus … and then hold the doors closed and try to burn you alive.”

Diane Nash, a college student in Nashville, Tenn., was 22 when she saw the headlines splashed on front-pages across the country. That very same day, she and other students got on a bus to Birmingham, Ala., determined to complete the journey.

“They had no connection to the original Freedom Riders, none,” Nelson told ABC News. “They just read in the paper that the other group has been beaten so badly that they can’t continue, and they said, ‘OK, well, we’ll continue it.'”

The group was conscious of the danger they faced. They signed their last will and testaments before getting on the bus to Alabama.