@alfiekohn
"it may not stop the detention"
So then pls don't spread this as a general advice. It may be useful in some situation, it may be harmful in another.
If a kidnapping is witnessed, I believe it's the duty to act against it based on the abilities each has. And an advice of what to do in such case needs to take this into consideration.
There are so many more possibilities that can be taken, and there are also simply many situations calling the police is just harmful.
You can reach the door of a car with a detained person? Go there an open it if you can.
You see kidnappers approaching a person, and you happen to drive by with a car? Use your vehicle to disrupt the situation, by force if necessary.
You see a kidnapping and others witnesses are around? Get them engaged and approach the kidnappers if possible. Try to figure out if the detained wish's to escape. If so help them do it. Be creative.