I've also found pictures of motorcades with:
Adlai Stevenson,
Ike with Khruschev, Ike by himself I believe, maybe one with Truman as well.
VP Richard Nixon.
It sure seems ill advised for the head of the state to be travelling around in the open.
Check JFK's motorcades for his Berlin visit. There too are some interesting photos and I opine, much better security. Somewhere in the Far East too, I believe.
As they say, hindsight is 20/20 but honestly, at the height of the cold war? And besides that, all of the civil rights unrest as well not just JFK but a few of these people were too wide open for comfort. That's all I'll say.
That up in Nashville per the prior post, that was probably 4 dignitaries in one car, even if one had no more than a knife, it looks like one could have inflicted some damage.