Flashing reds are based entirely off of how long it takes an "average" person to cross. As a relatively tall young adult, I walk much faster than the "average," so I can frequently make it across by walking even if it's been flashing red for a few seconds. Furthermore, the "Don't Walk" sign becoming solid is timed to correspond to the yellow lights appearing for the parallel traffic, so the opposing traffic won't start moving for a few seconds after the "Don't Walk" signs appear.
(Disclaimer: This is based on Raleigh, NC's traffic lights. I don't know what level this is standardized at.)
(Disclaimer: This is based on Raleigh, NC's traffic lights. I don't know what level this is standardized at.)