By Mike DeNardo

UPPER DARBY, Pa.  (CBS) — With the Super Bowl just days away, Septa is posting billboards in its stations alerting travelers to look for signs of human trafficking.

“We’re trying to look for that predator that’s preying on the vulnerable,” says Septa police chief Tom Nestel (in photo).

He says commuters at transit hubs should keep an eye out for pimps who may be trying to offer teenaged girls shelter and “love” — but who really want to control their lives.

“We’re looking for people who are talking to young women; young women who are by themselves and seem maybe apprehensive about the conversation,” Nestel said today at 69th Street Terminal.

Ann Marie Jones, a former prostitute now working at Dawn’s Place, a shelter for exploited women, described one signal that a woman may be vulnerable:

“When a woman is walking down, say, Kensington Avenue, and she’s walking with her head down and she’s trying not to look at anybody, that’s a sign.”

Septa’s billboards urge anyone seeing evidence of exploitation to call the National Human Trafficking Hotline, 1-888-373-7888.