Cite for your claim of 15 m.p.h. being "parade speed."
From Greer:
"After we left the airport, we drove several miles at speeds ranging from 15 to 30 miles per hour depending on the crowds. When we reached the business section of Dallas the crowds were very large and the motorcycle Police along side the President's automobile had a hard time keeping the people back.
"When we came to a point where the crowd had thinned out, there was a right turn for about half a block and then a left turn. At this point, I would say the President's automobile was traveling about 12 to 15 miles per hour."
The "business section of Dallas" where the limo had to slow down was, of course, Downtown.