LOL. Everything coming from LNers seem like 'excuses' to you lot. And that we are sheep-dipped lemmings.
Only the patsy gets sheep-dipped. You're just a shill with a twist of lemming.
You seem to be speaking for Greer. How do you know what he would or wouldn't 'need to know'?
I assume Greer's primary duty would have been to protect the POTUS at all cost, which meant fleeing the area after hearing shots fired. At least that's how it works in any movie I've ever seen about an assault on the POTUS.
My take on the Greer thing has always been based on what most of us would do naturally: look back to see what WTH is going on behind us. And in doing so, most of us would arguably pause our foreward movement... and in Greer's case there would be no way of knowing from which direction any shots were coming from, including the overpass.
The SS are trained to escort the POTUS away from any potential danger ASAP. That means Greer should have sped up as soon as he heard the 1st shot and certainly by the 2nd shot. Instead, after the 2nd shot he slowed the limo down to a near stop, turned back to see if JFK had been hit, watched JFK's head explode, then turned face front and waited for Hill to climb aboard and Jackie to climb onto the trunk before stepping on the gas.
Now see if you can turn face front in less than the blink of an eye like Greer did. Kellerman almost does a face plant in the dash from Greer braking. This could have been where a few alternate frames were removed to help speed up the limo. The following GIF is in real time at 18 fps.