Quercus
I would love to see this tried out by a journalist to see how many random strangers actually responded, and if so how.
I saw some youngsters doing it on YouTube.
One girl responded by rushing back, throwing her arms around the girl and making a fuss of her as if she was a long lost friend. The 'abductor' started making a fuss and trying to pull the 'abductee' away then pretended to run off.
An older woman just brushed her shoulder where she had been bumped and gave the girl a glare. She probably didn't even recognise the sign and thought the girl had rudely bumped into her.
In another one was a girl being pushed into a car. A passing couple saw the help sign and both rushed to help - the man looked like he was going to beat the 'abductor' to a pulp before they owned up to what was happening.
I don't think people are expected to rush in and help - just take notice and report it to bring help.