Medical evidence
The Romans choice of using a cross over a stake was apparently due to the cross being able to extend the time it takes a person to die. By staying alive for hours or days the crucified person served as a warning example to others. The site centuryone.org/crucifixion2.html (February 15 2006) looks at some of these issues. Quoting research contained at F.T. Zugibe, 1984 Death by Crucifixion, Canadian Society of Forensic Science 17(1):1-13.6 it shows that on a cross, rather than a rapid death from asphyxiation death it can take hours or days to die from hypovolemic shock. On the other hand, death on a stake is rapid. Summarising research by P. Barbet 1953 Les Cinq Plaies du Christ 2nd ed. Paris: Procure du Carmel de l' Action de Graces;
"Eye Witness accounts by prisoners of war in Dacchu during WWII reported that victims suspended from beams by their wrist, which were tied, expired within ten minutes if their feet were weighted or tied down and within one hour if their feet were unweighted and the victim was able to raise and lower himself to permit respiration. Death in this manner, which is one form of crucifixion, was the result of suffocation."
The length of time Jesus and the other two survived after impalement shows they were likely supported on a cross by a sedile.