You missed the point entirely.
The rush across the street and up the knoll didn't occur immediately (as correctly pointed out by John Mytton). The photo you posted was taken well after every single vehicle in the motorcade disappeared (including the press bus at the end of the procession) and was well on it's way to Parkland.
Understand now?
I wish you understood how silly your suggestion is. You and Mytton fail to keep up with reality and somehow find a way to get lost. If "immediately" means getting run over crossing the street, maybe you could give a live demonstration to show what you are talking about. Like the people you see running towards the knoll, next, you'll say they were intending to climb a tree, right?