The speed where aerodynamics (air resistance) becomes a big factor is
about 40mph and it takes much steeper hills to break the barrier.
The average speed descending the major hills in the Gatineau Park,
Canada in a full tuck is near 45mph. So 40 mph is routine. In a
group, the fastest has been about 55mph. The road in this case is
closed to traffic and has grass siding.
Incidently, I saw some good point-of-view footage of a 50+mph descent
with roller skiers and in-line racers on a recent nordic skiing movie.
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