This is very common in this exercise as well as in Park Bench. For Bus Stop, try telling students it's their stop and time to get off when you, the teacher, dings a bell. It will force them to leave and allow you more control. If you'd prefer, you could tell them they must make it clear why it's their stop (the purpose of the journey) somewhere in the improv.
Drama Menu had a similar game that I felt solved this problem too. I believe it's called Speed Date. Player 1 must come on as a terrible date to the more neutral player 2 (already seated) in a speed dating situation. The bell dings after 10 seconds (or 20 or 30, your choice) so player 2 leaves and player 1 switches to the player 2 spot, allowing for another person to come in which certainly expedites the process and allows for more students to take part. Best of luck!