John Shelby Spong says in Jesus for the NonReligious, page 168, that in his research, he has been able to find no evidence that there was a custom of releasing a prisoner at the time of the Passover. He says the original crucifixion story could have been related to Yom Kippur because in this tradition, one lamb or goat was killed for our sins and one had the sins of the people symbolically transferred to him, after which he was chased away. Following this tradition, Barabbas was to be pardoned.
Historian Richard Carrier agrees with Bishop Spong that Barabbas took the role of the Yom Kippur scapegoat. His view on this is available online as part of a review he wrote for a book by Dennis R. McDonald:
[A] practice [of releasing a prisoner] could hardly have been approved by Rome, since any popular rebel leader who happened to be in custody during the festival would always escape justice.
Of course, Barabbas means "son of the father," and thus is an obvious pun on Christ himself. He also represents the violent revolutionary, as opposed to the very different kind of savior in Jesus (the real "Savior") . . . The Jews thus choose the wrong "son of the father" who represents the Old Covenant (symbolized by the rainbow, and represented by the ideal of the military messiah freeing Israel), as well as the scapegoat (the lamb) sent off, bearing the people's sins into the wilderness, while its twin is sacrificed (Lev. 16:8-10, 23:27-32, Heb. 8-9).
In the above extract, Carrier discusses the parallel passage in Mark's Gospel. New Testament scholars overwhelmingly regard this as the main source of information used by the author of Luke's Gospel, so it is equally relevant to most questions about Luke.