Six or more, and the call goes out: possible flash mob in progress.
With the warm weather approaching, flash mobs are a concern for local officials. The police presence in Center City is apparent each afternoon.
So how does SEPTA handle the situation?
According to a 2011 notice from SEPTA, cashiers had been instructed to be on the lookout for indicators of a possible flash mob and report it immediately.
The indicators are: The presence of a large crowd of juveniles gathered at a station; the presence of unusual activities in the stations; the presence of six or more juveniles entering the station at one time.
"All personnel must be alert and immediately report to the Cashier Dispatcher any unusual activities within the Sub/El system," according to the August 2011 memo.
SEPTA Spokeswoman Jerri Williams said the policy, which was instituted the same time the city issued a curfew for young people, has not been rescinded but it's not necessarily in effect right now.
"If we do see a recurrence of flash mobs in the city, then we will definitely ask the cashiers to be on the lookout," she said.