FYI
I looked up Hincapie's 2006 crash and found.
Answer from Trek:
You are correct - Hincapie was riding an aluminum steerer tube in Paris-Roubaix. It was a 6061 aluminum steerer, the same one that we have been using on team and production bikes for over 8 years. Although we are experts in carbon fiber, we have always leaned to the conservative (and, hopefully, safe) side of fork technology. This has prompted us to remain with aluminum steerers, even for our team bikes.
Wow...I am getting schooled because I thought almost for sure that the fracture resistance of CF was at the root of steerer breakage and why for example the 40mm ceiling on spacer stack height under the stem as spec'ed by many manufacturers of full CF forks. I had no idea that Hipcapie was riding an Al steerer CF fork.
Thanks.