Calling 9-11 works:

People often warn others against relying on the police to protect you from criminals.
I agree that relying entirely on one thing to keep you safe is foolish, simply being a calm and safe driver doesn't provide absolute protection from a car accident, nor does simply wearing a seatbelt.

The police aren't perfect, but there are a lot of good cops out there, who have a lot of training and are otherwise properly equipped to handle dangerous criminals. If you ever have the choice, best if you let them handle an issue at hand.

When a pair of criminals, one armmed with a firearm, broke into a house, they were heard by four children living there. The oldest of the kids was only 15, clearly they weren't going to stop a pair of gun packing burglars themselves, but they did the smart thing, they avoided confrontation, called 911 and followed the instructions of the person on the other end of the line. They got out of the house undetected and waited for the police, who resolved the situation without anybody getting hurt.

(Read the entire story here)

So there you have it: calling 911 is no useless feat, if you witness a crime in progress, by all means try to call them, to help somebody else or when you yourself are the victim! But also, by any means: do not rely entirely on the police to protect your life. It may take some time for help to arrive, do whathever it takes to stay safe untill they do.
So if the police aren't there to protect you, make sure that you can protect you life, as well as the wellbeing of your loved ones.