They would have an easier time shutting dow subways than buses.
I think about the aftermath of 911. I was in a doctor's office right after it happened and there was a tiny tv reporting the events. All the workers were calling loved ones, telling them to get home, making plans to go home ASAP. There was panic in the suburbs of Philly, a hundred miles from NYC and about the same from the pentagon. I can only imagine all the people flooding the streets in new york, trying to get home. Your scenario would be on a smaller scale, but if there were tens of thousands of people in a crowd where the prez was speaking, I can imagine chaos as folks rushed toward safety and to get home. Not sure if your setting is a sitting, but if so lots of folks would be heading toward public transportation, fighting to get on buses, squishing into subways. Even if there was a quick alert to bus drivers and train operators. It would be difficult, if he made a fast getaway, to alert them fast enough.