During the trial before Pontius Pilate, which Roman custom allowed the release of one prisoner during Passover?

At Passover the Roman governor customarily freed one prisoner chosen by the crowd. In the trial of Jesus, Pilate offered the crowd a choice, and they asked for Barabbas to be released. Of the options given, Barabbas is the one tied to this event.

Pax Romana means the long period of Roman peace across the empire. It has nothing to do with freeing prisoners.