No matter how fast you think you're going - nothing compared to the pros (they do have the advantage off being able to use the full road and know there's not cars etc.). My fastest ever was on Tourmalet and is still around 20kph slower than the pros can go down mountains.
This seems a rather odd assertion to make - that non-pros cannot achieve the same high maximum speeds as the pros. I've ridden extensively in the French Alps, and know very well that it is perfectly easy to attain the same high speeds as pros if you've got the right qualities. These high maximum speeds are dependent on:
- long straightish stretches of steeper road
- reasonable or better road surface
- wind not against
- having the balls to 'let the bike go' - admittedly doing this on open roads takes more balls and concentration than on closed roads, but the full racing line is frequently possible on open roads, and even the pros don't achieve their highest speeds on sections where the sightlines aren't good
- good judgment and ability to assess risk (if you want to survive the exploit without crashing)
None of this is dependent on being a professional, or even a racing cyclist. The rider qualities needed may be honed slightly by a professional career, but they tend to be innate in the people who are fast.
However, where the pros do have an advantage is on the slower parts of descents where pedalling effort is required to maintain a fast
average speed. Here, the additional strength and fitness of the pros makes a big difference sprinting out of the many hairpins, and on the many shallower or against-the-wind sections. What some people don't realise is that racing hard down a descent in the Alps is bloody knackering too.
Funnily enough, though, in my many trips to the Alps, only a small proportion of riders encountered seemed prepared to put the effort in on descents....most toddled down on the brakes!