But presumably, the people at the Main Street were just the general public, and the only thing they had to go on was what the police said they were going to do. And, the Dallas Police said that they were going to drive Oswald out thru the Commerce Street ramp.
This is what the New York Times published the next day:
"A ramp dips through the basement garage of the municipal building, running from Main Street to Commerce Street. Patrol wagons drive down this ramp and discharge prisoners at a basement booking office. The garage ceiling was too low for the armored car, so the van was backed up in the Commerce Street portal of the ramp. The plan was to lead Oswald out the doorway in the center of the basement and about 75 feet up the ramp to the back of the armored car."
The general public in Dallas had no reason to expect anything different. What could they do except go by what the police were telling them? And that is what the police were telling them. So, there was no basis for anyone to congregate at the Main Street ramp.