John Wood wrote: "And finally, if modern bikes are so clearly superior in every way, why does Peter Post's record for average speed still stands for Paris-Roubiax?"
There were two factors: first, a strong tailwind that year. And second, the number and length of the pave sections was shorter than the current race course. There was a loss of pave in northern France due to modernization, and the race had become more of a flat road race. The race directors started looking for more pave to bring back the character of the race.
This, of course, slowed the race speeds down and made Post's record almost untouchable. I say almost because you never know...
Rainy Arcata, California, USA