Christ Appearing to St. Peter on the Appian Way
National Gallery, London
In the Golden Legend St. Peter attempts to flee from Rome because Nero has ordered his arrest, but he soon encounters Jesus coming in the other direction. Peter asks, "where are you going?" and Jesus replies, "I go to Rome for to be crucified again." Thus Peter resolves to "return again then for to be crucified with thee." In the Legend the meeting is at Rome's Sancta Maria ad Passus gate, but Carracci follows other versions that put the encounter on the Appian Way. In the background are some of the tombs that line that road even today.
The crucifix in the painting is a way of making visual Jesus' statement that he is going to Rome to be "crucified again." It is not mentioned in the Legend.
Read more about images of St. Peter.
Source: this page at Wikimedia Commons.