Knowing for a Fact that the SS had an AR-15 locked and loaded on the floorboard of the Queen Mary, do you really believe, (1) Should the POTUS come under fire that his only protection inside the JFK Limo would be the Service Revolvers of SA Greer and ASAIC Kellerman?, (2) Should the POTUS come under fire & the Limo and the Queen Mary become separated that the Only weapons available to return fire and protect the POTUS would be the service revolvers of SA Greer & ASAIC Kellerman? and (3) Should the POTUS come under fire that there would Not be a SS Protocol for the SS Agents in the motorcade to adhere to? It is a common military practice when under surprise enemy attack/fire that when possible to Regroup and Reaccess your current position and possible options going forward. ALL of this would require a brief STOP. This is what unfolded West of the Triple Underpass BEFORE the Limo boarded the actual Stemmons Fwy.
This was a protective detail, not an infantry platoon. And they had enough agents in place in the limo to figure out what was going on even with someone driving, and radios to keep them in touch with other key players in the motorcade. For that matter, the blood, CSF, and brain matter that pelted Greer, Kellerman, and the windshield would have been an obvious indicator that someone was badly hurt, even without Hill in the back to advise. For reference, once Ronald Reagan was in the limo after Hinckley shot him, the limo didn't stop until it reached the hospital, even though the agents covering RR didn't at first realize he'd been shot, and diverted from the original destination (the White House) to GWU hospital.