Actually, decapitated doesn't really apply here. This is an avulsion case.
Case: man running from the police on foot, on an elevated highway. They get close, and he hops over the concrete guardwall.
Last thing he ever did.
Warning! The photos below the fold are quite graphic.
Judging by the damage shown in the photos, I would say that he had the misfortune to have a wrought-iron fence stake penetrate his neck and sever the spinal column. The remaining tissue was then unable to take the force required to stop his body, so his head avulsed from the neck. Avulsion is a tearing of the flesh, decapitation implies cutting the head off.
This is a very unusual incident, both from the standpoint of the law of unintended consequences (sometimes known as Murphy's Law), and from a medical standpoint.
Don't even bother to jack me up because the person appears to be of color. I would have published the photos regardless of his race.
Kudos to Catfish for bringing these to my attention.
Now where's the fire truck for the washdown?