Grotug

I imagine they don't necessarily want to reach ridiculous speeds down hill anyway. Seems it would not be the most efficient use of their energy as the amount of aerodynamic drag you are overcoming at those speeds is enormous. Better to save some energy and go hard on the flat. I am an inexperienced roadie and I find at speeds over 40mph it's better to just hunker down, my chin near the handlebars. Many hills where I live allow me to reach 40mph+ (65kph) just by tucking low. The amount of energy it requires my skinny little legs to take me another 5mph faster isn't worth it! Better off just coasting until my speed is in the 30s and then pedal. I imagine it is a similar concept for the riders. They want to use the hills for recovery, so they are not going to pedal full out to see what kind of top speed they can reach on the downhills. My point is, they probably are capable of much higher speeds than what they clock in a race. So if they are only going 50mph down a hill, it's not because they can't go faster, it's because that's an efficient speed for them to recover their legs from the climb. Of course, I am a newbie and amateur to the world of racing, but this makes sense to me! Anyone feel free to correct me as I have never even watched the TDF for more than fleeting moments on my tv set many years ago.

Jinker

Grotug: None of these riders are pedaling when they hit their top speeds. At 60mph with a 54x11 top gear, they'd be pedaling 150 rpm.

Using the Kreusotter calculator (https://www.kreuzotter.de/english/espeed.htm) it seems like going 60mph will require about an 18% slope, which seems about right. Adding 400 watts pedaling would raise the speed by around 2mph. Not very much. Add to that the fact that you can't achieve as aerodynamic a position while pedaling, and you see why pedaling makes no sense on really fast descents.