Have you ever called the police on a reckless driver?

I was on my way to an appointment last night, and there was a car in front of me going 20 MPH in a 35 MPH zone. The driver was riding the brakes, and was swerving. We arrived at a traffic light and the driver was taking a left. She appeared to be in her 70s – not too old.

Anyway, I got to my appointment and was about to call the police, but realized that I had never done so before and didn’t know how long it would take. Also, is this kind of thing helpful? They would be unlikely to know where this person went or if she is even driving any more.

Have you ever called and reported a driver like this? What was the response?