Officer Earle Brown (stationed on the T&R RR Overpass) said the limousine stopped on the access ramp for about 30 seconds, while the Curry car pulled up to it.
Officer James W. Courson was a motorcycle policeman assigned to the middle of the motorcade. He told Larry Snead ("No More Silence" 1998 ( Google Books )) that he was "stopped" at Main and Houston when he heard the shots. He was at the top of Elm and saw Mrs. Kennedy on the trunk. He caught up to the limousine as "they entered the Stemmons Freeway ramp." He was one of two rear escorts to Parkland.
The McIntire photo suggests Courson is wrong about the entrance to the ramp being where he caught up to the limousine. I think Courson caught up with the limousine on the ramp itself not far from the entry onto Stemmons. To cover that distance and catch up to the limousine would seem to mean the limousine had slowed considerably or, as Officer Brown claimed, momentarily stopped.
The only witness for a stop is Brown, who reported this in 1980 but not in 1964. From the films of the motorcade leaving Dealey Plaza, the Queen Mary and the VP car both took off after the limo. There were four guys in the lead car, six in the limo, seven in the Queen Mary, and seven more in the VP car, with a couple of motorcycle officers caught up with them. Of those, the only one who ever said anything close to describing stoppage on the Stemmons ramp is Curry, who had to be told by Gary Mack that the limo stopped. Even then, Curry never said it stopped. Put it all together, and it doesn't add up to a stop.
I'd bet the limo slowed down just before it hit that blind right hand curve on the ramp. Trying that at speed in a four ton automobile rolling around on four skinny bias-ply tires is one of those "watch this" sort of stunts. I can see Greer letting off the gas negotiating it, and a 7800lb car isn't going to accelerate all that well once it's around the bend. I can't see them stopping for any reason. After all, they had radio.