How long is a yellow light at the intersection traffic light?

This question came up today at a safe driver training course at work today, no one know the answer yet... but on the web I came across ...

How long should a traffic light stay yellow?

The length of time that a traffic light stays yellow will vary from one intersection to another. The timing depends mostly on the speed limit and the width of the intersection.

From: http://my.uen.org/documentsDisplay/downloadfile?userid=dhendricks&documentid=3061097

Doesn't answer the question specifically other than to tell the derivation factors. One co-worker states that the 1/10th of speed of the street at the intersection, is the time the yellow light will be on.
So 55mph zone will have a 5.5 second yellow. Maybe this is right, I'll look into it farther.

By the way the safe distance to stop in traffic is the distance that allows you to see over your hood and look at the bottom of the tires of the vehicle in front of you. Liberty Mutual insurance company calls this the "Liberty Wedge"