This is an image of the Main Street entrance to the garage, which Ruby used.
What I observe is how narrow it is. So, Officer Sam Pierce was sitting there in his police car when Ruby walked by. And let's be clear about something: Ruby identified Pierce; he recognized him. So, Ruby must have been there. This is like Oswald citing Bill Shelley out front. How did he know Shelley was there unless he saw him? Oswald had to be in the doorway to know that Shelley was in the doorway. Likewise, Ruby had to enter via the Main Street ramp to know that Pierce was there. So, it confirms that Ruby was telling the truth when he said he got to the garage that way. But, if he saw Sam Pierce, how could Sam Pierce not see him? It's impossible. He had to see him. He was waiting for him. And he probably signaled the others with his police phone that Ruby was coming. Then they had their little bout with Ruby and then got rid of him, taking him up to the 3rd floor. And then they did it again with Bookhout for the cameras. Then, they craftily brought Ruby back down so that he would be seen and filmed milling around as part of the shooting aftermath: a classic bait and switch.