When you are supposed to get 16 people into an area within two minutes, every second counts. Yes, you can go up and down the line to find single riders but it’s much quicker if they are right there. You can save the going up and down the line looking for riders when you need 2,3,4 or 5 riders to make the correct ride count. Slinky Dog Roller Coaster splits us into a separate line because we had an even number of rides. The odd number of riders went into the other line. It was an interesting way of doing it but it seemed to work well.
FoP makes it less of an issue by having a pre-show that can lengthen or shorten as needed.