Oh phooey. It's a longstanding joke that the "Don't Walk" lights don't change to "Walk" any sooner if you press the button. Just like the "close door" button in elevators doesn't make the doors close any sooner than they normally would.
I recently read an article (maybe even here on HN) that explained this fully.
Basically, in the US, under the Americans with Disabilities Act, the doors must stay open for a minimum time (which depends on some variables). The door close button can only have an effect after that time is elapsed, but in many (if not most bases) the time after which the door automatically closes is set to that same minimum anyway.
There are, of course, always exceptions. Some reading I did earlier suggests that there is some variation based on elevator brand, and possibly even then whims of the building owner (in terms of what instructions they gave the elevator installer about how they wanted things to work). Curiously though the "door open" button does usually pre-empt the (automatic) closing of the door, even in "normal" mode. At least IME.
I had never thought about this. In Thailand they definitely do close the door sooner, or straight away, and everybody does it all the time.