“”Couldn’t he have sat down, belted or not?””
Or stayed on the floor.
This was self inflicted, not intentional by the van driver.
I disagree that it was self inflicted. He was shackled, so he didn’t have as much control over the centrifugal forces that he would be subject to when the van made the various stops and turns.
It was likely that he didn’t stay seated and was thrown at an inopportune moment and hit one of the walls.
The more detailed report I read said it was noted several times that Gray was intentionally thrashing around and banging on the walls.. If he had remained, prone, he would not have been injured.
The M.E. also mentioned that he could have had convulsions. That would be consistent with heroin overdose.