What is considered the maximum safe downhill speed on a road bike? I've heard between 30-40mph, but I am pretty sure I have hit 45 and felt safe and in solid control of my bike. If you look down and see you are above XXmph, you decide it is time to put on the brakes...what is that speed?

Edit: In summary, how can one determine what a safe speed is for a particular bike in a particular situation?
I think it depends both on the rider (as @moz pointed out) and on the bike. And of course, you should have a clean road also.

If you have a high end road bike in good shape, you can get to very high speeds if you've got the skills and the clear road ahead. 

In Tour de France downhill sections, they can go at speeds as high as 65 mph / 110 Km/h, even losing the motorbike reporters.

As a counter-example, I had to brake at less than 30 mph / 45 Km/h on an old road bike, as the wheels were shaking due to bad balancing/tired axles/etc...

On a side note, I think going over the cars legal limit is a very bad idea on open roads (this refers to the Tour de France-class of speeds).